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\documentclass{gqtekspec}

\usepackage{import}

\usepackage{bytefield}

\project{CMod S6 SoC}

\title{Specification}

\author{Dan Gisselquist, Ph.D.}

\email{dgisselq (at) opencores.org}

\revision{Rev.~0.3}

\begin{document}

\pagestyle{gqtekspecplain}

\titlepage

\begin{license}

Copyright (C) \theyear\today, Gisselquist Technology, LLC

This project is free software (firmware): you can redistribute it and/or

modify it under the terms of  the GNU General Public License as published

by the Free Software Foundation, either version 3 of the License, or (at

your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT

ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or

FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

for more details.

You should have received a copy of the GNU General Public License along

with this program.  If not, see \texttt{http://www.gnu.org/licenses/} for a copy.

\end{license}

\begin{revisionhistory}

0.3 & 5/23/2016 & Gisselquist & Draft for comment, includes ZipOS and PMod

                pin mapping\\\hline

0.2 & 5/14/2016 & Gisselquist & Updated Draft, still not complete \\\hline

0.1 & 4/22/2016 & Gisselquist & First Draft \\\hline

\end{revisionhistory}

% Revision History

% Table of Contents, named Contents

\tableofcontents

\listoffigures

\listoftables

\begin{preface}

The Zip CPU was built with the express purpose of being an area optimized,

32--bit FPGA soft processor.

The S6~SoC is designed to prove that the Zip~CPU has met this goal.

There are two side--effects to this.  First, the project proves how capable a

CMod--S6 is, and second, this project provides demonstration implementations

of how to interact with a variety of PMod devices: the audio amplifier,

the serial 2--line LCD display, the USBUART, and the 16--character numeric

keypad.

\end{preface}

\chapter{Introduction}

\pagenumbering{arabic}

\setcounter{page}{1}

% What is old

The Zip~CPU is a soft core CPU designed to fit within an FPGA, to use a minimum

amount of the FPGA's resources, and yet to provide the services of a fully

capable 32--bit computer.  It is based upon a Von~Neumann architecure and so a

single 32--bit wishbone bus provides it with access to both peripherals and

memory.

% What does the old lack?

Previous demonstrations of the Zip~CPU have focused on larger FPGAs, such as

the Spartan--6 LX9 and LX25 and the Artix--7 35T.  On these FPGA's,

the Zip~CPU runs in a pipelined configuration where it is tightly integrated

with a prefetch/instruction cache.  While these demonstrations have shown that

the Zip~CPU can act as a very simple CPU in these environments, they really

haven't demonstrated the ability of the Zip~CPU to use only a minimum amount

of the FPGA's resources.

% What is new

The CMod~S6 board provides the opportunity for that demonstration rather nicely.

% What does the new have that the old lacks

\begin{enumerate}

\item The Spartan--6 LX4 FPGA is very limited in its resources:

        It only has 2,400 look--up tables (LUTs), and can only support

        a 4,096~Word RAM memory (16 kB).

\item With only 4kW RAM, the majority of any program will need to be placed into

        and run from flash.

\item While the chip has enough area for the CPU, it doesn't have enough area

        to include the CPU and \ldots write access to the flash, debug access,

        wishbone command access from the UART, pipelined CPU operations,

        a prefetch cache and more.  Other solutions will need to be found.

\end{enumerate}

Of course, if someone just wants the functionality of a small, cheap, CPU,

this project does not fit that role very well.  While the S6 is not very

expensive at nearly \$70, it is still an order of magnitude greater than it's

CPU competitors in price.  This includes such CPU's as the Raspberry Pi Zero

(\$5), or even the TeensyLC (\$12).

% What performance gain can be expected?

If, on the other hand, what you want is a small, cheap, CPU that can be

embedded within an FPGA without using too much of the FPGA's resources, this

project will demonstrate that possibility as well as some utility.

Alternatively, if you wish to study how to get a CPU to work in a small,

constrained environment, this project may be what you are looking for.

Finally, because the Zip~CPU and the included ZipOS are as small and simple as

they are, the code base found here will be simpler to understand than the

code bases for some of these other projects.  For example, the Zip~CPU

instruction set is very simple.  With less than 40 instructions, it is much

easier to understand and learn than the ARM instruction set.  Further, unlike

the ARM, the entire specification for and description of the Zip~CPU is

publicly available.  Likewise, an operating system such as the ZipOS that has

less than \hbox{3,000} lines of code will be much easier to understand than any

Linux operating system--even if it has much less functionality.

\chapter{Architecture}

Fig.~\ref{fig:architecture}

\begin{figure}\begin{center}

\includegraphics[width=5in]{../gfx/s6bones.eps}

\caption{CMod S6 SoC Architecture: Zip~CPU and

        Peripherals}\label{fig:architecture}

\end{center}\end{figure}

shows the basic internal architecture of the S6~SoC.  In summary, it consists

of a CPU

coupled with a variety of peripherals for the purpose of controlling the

external peripherals of the S6~SoC: flash, LEDs, buttons, and GPIO.  External

devices may also be added on, and have been added on as part of this project,

such as an audio device, an external serial port, an external keypad, and an

external display.  All of these devices are then available for the CPU to

interact with.

If you are familiar with the Zip CPU, you'll notice this architecture provides

no access to the Zip CPU debug port.  There simply wasn't enough room on the

device.  Debugging the Zip~CPU will instead need to take place via other means,

such as dumping all registers and/or memory to the serial port on any reboot.

Further, the Zip~CPU has no ability to write to flash memory.  For this reason,

there exists an alternate CMod S6~SoC architecture, shown in

Fig.~\ref{fig:altarchitecture}.

\begin{figure}\begin{center}

\includegraphics[width=5in]{../gfx/altbones.eps}

\caption{Alternate CMod S6 SoC Architecture: Peripherals, with no

        CPU}\label{fig:altarchitecture}

\end{center}\end{figure}

Using this alternate architecture, it should be possible to test the peripherals

and program the flash memory.  Both architectures may be loaded into the flash,

together with the programming code for the Zip CPU.

The basic approach to loading the board is actually quite simple.  Using the

Digilent ADEPT JTAG configuration program, {\tt djtgcfg}, the alternate

configuration may be written directly to the device.  Once this alternate

configuration has been loaded, the flash may be examined and programmed using

the {\tt zipload} utility.  This utility uses Digilent's Asynchronous Parallel

Port interface (DEPP) to communicate with the device, and in particular to

tell the device what to write to the flash.  When writing to the flash,

the {\tt zipload} utility will program both a primary and an alternate

FPGA configuration into the configuration section of the flash, as well as

computer code into the rest of the flash.  Once complete, the system may then be

reloaded with the primary configuration file which will contain an image of

the CPU.  The CPU will then begin following the instructions found in flash

memory.

\chapter{Software}

This chapter provides an overview of the software that is available to support

the S6~SoC.  This includes not only the RTL, the Makefiles, and the software

that will run on the Zip~CPU within the S6~SoC, but also the support software

necessary for communicating with the S6~SoC in its alternate configuration.

\section{Directory Structure}

\begin{itemize}

\item[{\tt trunk/bench}] Contains software for emulating the S6 without the S6

        present.

  \begin{itemize}

        \item[{\tt trunk/bench/cpp}]  All of the bench testing software is

                written in C++, so it is found in this directory.  Primary

                among these programs is the {\tt zip\_sim} program which will

                simulate the Zip~CPU within the S6~SoC.  Specifically, it

                simulates everything at or below the {\tt busmaster.v} level.

                Some, although not all, of the peripherals have been simulated

                and made a part of this simulation.  These include the

                Quad--SPI flash, the UART, and the LED's.

  \end{itemize}

\item[{\tt trunk/doc}] All of the documentation for the S6~SoC project may be

        found in this documentation directory.  Specifically, I would commend

        your attention to anything with a {\tt .pdf} extension, as these

        are the completed documents.  Among these you should find a copy of the

        GPL copyright under which this software is released, as well as a

        pre--built copy of this document.

  \begin{itemize}

        \item[{\tt trunk/doc/gfx}] Here is where the graphics are located in

                support of this specification document.

        \item[{\tt trunk/doc/src}] And here is where the \LaTeX files are

                kept that were used in building both this document as well as

                the GPL copyright.

  \end{itemize}

\item[{\tt trunk/rtl}] Verilog files.  The two top--level files are

        {\tt toplevel.v} for the primary top level module, and

        {\tt alttop.v} for the alternate load.

  \begin{itemize}

        \item[{\tt trunk/rtl/cpu}]      Verilog files containing the Zip~CPU

                core and peripherals.  The toplevel file here is the

                {\tt zipbones.v} file, although some of the peripherals, such

                as the {\tt ziptimer.v} are referenced independently.

  \end{itemize}

\item[{\tt trunk/sw}] The main software directory, primarily a repository

        for software subdirectories beneath it.

  \begin{itemize}

        \item[{\tt trunk/sw/dev}]  This directory holds a variety of

                simple programs for the Zip~CPU, such as {\tt helloworld},

                {\tt doorbell} and {\tt doorbell2}, as well as software drivers

                for various peripherals, such as the real--time clock simulator,

                and the keypad and display device drivers.

        \item[{\tt trunk/sw/host}]  This directory holds support software which

                can be built on and run on the host machine.  Building this

                software will involve adjusting the Makefile so that it knows

                where your local ADEPT installation directory is.\footnote{Many

                of the programs also depend upon the serial number of my CMod

                S6 device.  This will need to be adjusted in any new install.}

                Once built, you will find a variety of very useful programs

                within here.

        \item[{\tt trunk/sw/zipos}]  This directory contains the source code for

                        a rudimentary, very basic, operating system that I

                        call the ZipOS.

  \end{itemize}

\end{itemize}

\section{Zip CPU Tool Chain}

To build programs for the Zip~CPU, you will need the Zip~CPU toolchain.  You

can find this as part of the Zip~CPU project, available at OpenCores.  Building

the Zip~CPU project should result in a set of binaries in the

\hbox{\tt zipcpu/trunk/sw/install/cross-tools/bin} directory.  Make this

directory a part of your path, and you should be able to build the CMod S6

Zip~CPU software.  Specifically, you will need to use {\tt zip-gcc},

{\tt zip-as}, {\tt zip-ld}, and {\tt zip-cpp}.  Other tools, such as

{\tt zip-objdump} and {\tt zip-readelf}, may also prove to be very useful when

trying to figure out what is going on within the SoC.

\section{Bench Test Software}

Bench testing software currently consists of the {\tt zip\_sim} program found

within {\tt trunk/bench/cpp}.  This program requires Verilator to run, and

simulates in a cycle accurate fashion, the entire S6~SoC from {\tt busmaster.v}

on down.  Further, the external Quad--SPI flash memory, UART, and LED's are

also simulated, although the 2--line display, audio, and keypad are not.

\section{Host Software}

Several software programs have been built to support the S6~SoC from a nearby

host.  These programs include:

\begin{itemize}

\item {\tt dumpuart}: My current approach to debugging involves

        dumping the state of the registers and memory to the

        UART upon reboot.  The dumpuart command found here is

        designed to make certain that the UART is first set

        up correctly at 9600~Baud, and second that everything

        read from the UART is directly sent to both a file and

        to the screen.  In this fashion, it is similar to the

        UNIX {\tt tee} program, save for its serial port

        attachment.

\item {\tt readflash}: As I am loathe to remove anything from

        a device that came factory installed, the

        {\tt readflash} program reads the original installed

        configuration from the flash and dumps it to a file.

        This program is only useful when the alternate configuration is loaded.

\item {\tt wbregs}: This program offers a capability very similar to the

        PEEK and POKE capability Apple user's may remember from before the

        days of Macintosh.  {\tt wbregs <address>} will read from the

        Wishbone bus the value at the given address.  Likewise

        {\tt wbregs <address> <value>} will write the given value into the

        given address.  While both address and value have the semantics of

        numbers acceptable to {\tt strtoul()}, the address can also be a named

        address.  Supported names can be found in {\tt regdefs.cpp}, and their

        register mapping in {\tt regdefs.h}.

        As examples, {\tt wbregs version}, will return the build date, or

        version of the RTL.  {\tt wbregs spio} reads the special purpose

        I/O register, and {\tt wbregs gpio 0xffffffff} will set all output

        GPIO ports high while {\tt wbregs gpio 0xffff0000} will set all

        output GPIO ports to ground.

        This program is only useful when the alternate configuration is loaded.

\item {\tt zipload}: This is the primary program you will need to get your

        software loaded on the CMod.  It takes three arguments.  The first is

        the name of the primary Xilinx configuration file, the second is the

        name of an alternate Xilinx configuration file, and the third is the

        name of the Zip~CPU program you wish to write to Flash memory.

        Each of these arguments is optional.  For example, if only one

        configuration file is given, the loader will load the primary

        configuration.  If only one Zip~CPU program is given, the program will

        be loaded into the program memory area and the configuration file areas

        will be left untouched.

        This program is only useful when the alternate configuration is loaded.

\end{itemize}

\section{Zip CPU Programs}

The following are a list of simple, independent, single-task example programs

that will run on the S6~SoC:

\begin{itemize}

\item {\tt helloworld}: The first program any programmer should build,

        ``Hello, world!''  This program sends the string, ``Hello, world!''

        over the UART connection once per second.  It is a very valuable

        program because, if you can get this program running, you know you have

        a lot of things working and working correctly.  For example, running

        this program means you can run the {\tt zip-gcc} compiler, load

        the auxiliar configuration, load the program info flash memory, load

        the primary configuration, and read from the UART port.  It also means

        that you must have the UART port properly configured and wired to your

        CMod board.

\item {\tt doorbell}: This annoying program verifies the functionality of the

        audio device by playing a doorbell sound to the audio port.  It will

        then wait ten seconds, and play the doorbell sound again (and again,

        and again).  (It gets old after a while ...)

\item {\tt doorbell2}: This adds to the functionality of the {\tt doorbell}

        program a wait for keypress, and a display of the current time on the

        2--line display.  While almost our fully functional program, this

        does not include any menus to configure the device or set time, since

        it doesn't include any keypad functionality.

\item {\tt kptest}: A test of whether or not they keypad driver works.  When

        run, anytime a key is pressed, the key's associated printable character

        will be sent to the UART.  Further, pressing an `F' on the keypad will

        also send a newline over the UART, in case you wish to keep your lines

        from getting too long.

\end{itemize}

\section{ZipOS}

The ZipOS is a brand new operating system, specifically designed to run on the

ZipCPU.  It is both pre--emptive and multitasking, although with many

limitations.  Those familiar with the internals of other operating systems, such

as Linux, may laugh that I call this an Operating System at all: it has no

memory management unit, no paging, no virtual memory, no file I/O access, no

network stack, no ability to dynamically add or remove tasks, indeed it hardly

has any of the things most often associated with an Operating System.  It does,

however, handle interrupts, support multiple pre--emptive tasks in a

multitasking, timesharing fashion, and it supports some very basic and

rudimentary system calls.  In a similar fashion, it does contain just about all

of the functionality necessary for a multi--tasking microcontroller built

around a do--forever loop.  For its size, I consider it an impressive

achievement.  You are welcome to disagree with me, however.

This version of the ZipOS starts in the {\tt resetdump.s} code, so that upon

any startup the ZipOS will dump register contents, the BusError register, and

any scope contents to the UART.  This can take some time, so you may wish to

configure what you really wish to send--if anything.  If desired,

{\tt resetdump} can also be configured to also dump the entire memory as well

while only using 9~memory locations.  All of this is quite useful in case the

Zip~CPU encounters a bus error or other sort of error that causes it to hang,

stall, or reboot, as the CPU registers are very carefully not touched prior to

being sent to the UART output port.  This extends to all registers save the

supervisor PC and CC registers, which would've been reset by a reboot anyway.

{\tt resetdump.s} then calls a rudimentary bootloader, to load the parts of

the ZipOS that need to run faster into Block RAM.  The choice of what parts

to load into Block RAM is made on a file by file basis, and found within

the linker script, {\tt cmodram.ld}.

Upon completion, {\tt resetdump.s} calls the entry routine for the O/S,

{\tt kernel\_entry()} found in {\tt kernel.c}.  This is the main task loop for

the entire O/S, and worth studying if you are interested in understanding how

the O/S works.

The user tasks are found (mostly) within {\tt doorbell.c}, also found in the

ZipOS directory.  This file contains two kernel entry points, {\tt kntasks()},

which returns the number of user tasks the kernel needs to know about, and

{\tt kinit()}, which builds each of the tasks and connects their file

descriptors to the various devices they will be referencing.

\subsection{System Calls}

The ZipOS supports a variety of system calls, listed here:

\begin{itemize}

\item {\tt int wait(unsigned event\_mask, int timeout)}

        Halts the execution of a process until an event matching the

        {\tt event\_mask} takes place, or a {\tt timeout} (in milliseconds)

        has been reached.  The events that can take place are a

        bitmask of the various interrupts the CPU supports, together with a

        bitmask of the software interrupt values found in {\tt swint.h}.

        The {\tt timeout} value can either be zero, to return immediately with

        the list of events that have taken place, negative, to wait

        indefinitely, or a positive number of milliseconds in order to wait at

        least that time for the event of interest to take place.

        Waiting on a zero event mask allows a process to sleep for any number

        of requested milliseconds.

        When wait returns, any events returned by the wait have been cleared.

        The other thing to be aware of is that events may accumulate before the

        wait system call.  Nothing within the wait system call clears prior

        events.  These prior events be returned and cleared, though, if

        the wait call indicates an interest in those events.

        Upon return, the a bitmask of events that have taken place will be

        returned to the process.

\item {\tt int clear(unsigned event\_mask, int timeout)}

        This system call works closely with the wait system call.  Indeed,

        when the timeout given is zero, the two function nearly identically.

        It clears any of the requested events which may be pending, and returns

        a bit mask of the events that were pending and cleared.

        However, if the timeout is given (and is positive), then {\tt clear()}

        starts a timer.  Once the timer has completed, a timeout event,

        {\tt SWINT\_TIMEOUT}, will be generated and made available to the task

        to wait upon it.

        In a similar fashion, if the timeout is negative, then any pending

        timeout is cleared.

\item {\tt void post(unsigned event\_mask)}

        Certain devices, such as the real--time clock and the doorbell

        reader, need the ability of being able to post events to any listener

        within the O/S.  The POST system call allows them to POST events in

        this manner.

        Were the ZipOS to be closer to a secure O/S, it might restrict what

        events each process could post.  Right now, however, any process can

        post any event--whether it be a hardware or a software generated event.

\item {\tt void yield(void) }

        This is simply a way of being nice to other processes.  This system

        call takes no arguments and simply asks the scheduler to schedule the

        next process.  It does not take this process off of the ready to run

        list, so the next process may be this one.  However, since the scheduler

        is a round--robin scheduler, it will only return to this process once

        it has gone through all other available processes checking whether or

        not they are available to be run.

\item {\tt int read(int fid, void *buf, int len)}

        This is roughly the same system call as the POSIX read() system

        call.  It reads some number of words (not octets) from the file

        descriptor (device) specified by {\tt fid} into the memory address

        range starting at {\tt buf} and {\tt len} words long.  If the memory

        requested is unavailable,

        the read will wait until it is available, possibly indefinitely.

        Upon return, the {\tt read()} function call returns the number of

        words actually read, or a negative value on error.

        As a future feature, a {\tt read()} system call should be able to be

        interrupted by a timeout.  This feature has not (yet) been implemented,

        but will likely be implemented via a combination of the {\tt clear()}

        system calls ability to set timeouts together with the {\tt read()}

        functions ability to wait for available data.

\item {\tt int write(int fid, void *buf, int len)}

        This is roughly the same system call as the POSIX {\tt write()} system

        call.  It writes some number of memory addresses (words, not octets),

        to the given file descriptor.  If there is no reader task or device

        associated with the file descriptor, then the {\tt write()} system

        call will block forever once the internal pipe fills up.  Otherwise,

        if something is reading from the file descriptor's pipe, the writing

        task will only stall until the data is either written to the receiving

        task, or copied into a memory buffer.

        Upon return, the {\tt write()} system call returns the number of words

        actually written to the system pipe (not necessarily the number that

        have been read), or a negative value on error.

\item {\tt unsigned time(void) }

        Returns the number of seconds since startup.  Eventually, this will

        return the number of seconds since January 1, 1970, and be identical

        to the UNIX system time() command, but that may not happen on this

        project.

% \item SEMGET

% \item SEMPUT

\item {\tt void *malloc(void)}

        Allocates memory from the system/kernel heap.  This is a very low

        overhead memory allocator that, while it does allocate memory, cannot

        free it later.  It is nearly 100\% efficient since only one memory

        address, the top of the heap, is used to determine what memory has

        been allocated.

\item {\tt void free(void *buf)}

        This function is a do--nothing stub.

\end{itemize}

\subsection{Scheduler}

The ZipOS currently supports only a round--robin scheduler.  Tasks are executed

in the order they were created, as long as they are available to be executed.

If no tasks are available to be run, the Scheduler will run the idle task which

puts the CPU to sleep while waiting for an interrupt.

\chapter{Operation}

The {\tt doorbell} program has been built to illustrate the operation of both

the Zip~CPU, the ZipOS, as well as showing off how all of the various

peripherals work.  It was envisioned after my family and I experienced an

unexpected visitor during the wee hours of the morning.  The {\tt doorbell}

program is designed to couple the doorbell and the exterior lights to a

single button.  Hence, when the doorbell to the house is pressed, the exterior

light is turned on for a half an hour.  This, then, would make it difficult

for someone to see inside during this time.

This chapter will present an example of how the {\tt doorbell} program works.

To run the {\tt doorbell} program, you will first need to build the various

RTL and software support programs just to get the {\tt doorbell} program on

the device:

\begin{enumerate}

\item First build the primary and alternate .bit files by building

        with {\tt toplevel.v} and then {\tt alttop.v} as your top--level

        RTL files.  I like to place my Xilinx work directoy into

        {\tt trunk/xilinx}, and if you do the same the load scripts that

        are referenced next will work.

        Before going on, double check that both configuration .bit files were

        created, that they each fit within the device (there would be errors

        if they did not), and that they met their respective timing

        requirements.

\item Then, load the alternate bit file into the S6~SoC.  You will need

        the Digilent tools installed in order to do this.  Having done so,

        you may run {\tt make axload} from the {\tt trunk/} directory.

        If you didn't run the Xilinx ISE from within {\tt tunk/xilinx},

        you may need to find your .bit files and adjust where they load

        from, but this should be fairly straight--forward from the instructions

        within the Makefile.

\item Build the software found in the host directory.  This sotware depends

        upon Digilent's ADEPT toolsuite, so you will need to adjust the

        Makefile so that it references the toolsuite.

        {\em Note:} Currently, the host software is serial--number locked to

        my own CMod--S6.  Until I fix this and make it more user friendly,

        you'll want to switch your software to reference your own CMod.

        To do this, look for the {\tt "SN:\ldots"} lines within the `*.cpp'

        files.  Currently, there is one such line within wbregs, another

        within readflash, and the last within zipload.  To find our your own

        device's serial number, type {\tt djtgcfg enum}.  It should find one

        (or more) CMod devices connected to your system, and list each of

        their serial numbers.

        Once you've done this, you should have a working set of host support

        programs: readflash, wbregs, and zipload.

\item Using {\tt wbregs}, you may now test your configuration.  {\tt wbregs}

        works like the peek and poke programs from a generation ago.

        {\tt wbregs <address>} will return the value of the memory (or

        peripheral) found at the {\tt <address>}.  Some addresses have names,

        such as {\tt UART}, {\tt SPIO}, {\tt GPIO}, {\tt PIC}, and so forth.

        These names are found in {\tt trunk/sw/host/regdefs.cpp}, and their

        mappings in {\tt trunk/sw/host/regdefs.h}.

        As examples, if you type {\tt wbregs version} you should be able

        to read the version (a.k.a. build date) from the currently installed

        .bit file.  Likewise if you type {\tt wbregs uart 65}, you should see

        an `A' (i.e. a 65) sent from the S6~SoC over the serial port.

        {\tt wbregs uart} by itself will read a single character from the

        serial port and so on.

        You should be able to test all of your peripherals by hand using

        {\tt wbregs}: GPIO, Flash, UART, keypad, buttons, LEDs, Interrupt

        controller, timer, etc.\footnote{The display and audio devices may be

        more difficult since these require multiple interactions over the

        course of a short period of time to work.}  This should give you some

        confidence in how these peripherals work, should you need it.  You

        may also use this time to verify that your wiring is properly set up.

\item If you wish to make certain you keep track of the original Flash bitfile

        that came with your device, you may read it out using {\tt readflash}.

        This will dump the contents of you flash onto {\tt qspiflash.bin}.

        You may then wish to change the name of this file, lest you overwrite

        it by running {\tt readflash} again later.

\item At this point, it's time to build the programs for the Zip~CPU.  To do

        this, you will first need to download the Zip~CPU project.  When

        building that project, it will create a directory of programs

        (including its compiler) in

        {\tt zipcpu/trunk/sw/install/cross-tools/bin}.

        Include this directory into your path.

\item Change into {\tt trunk/sw/dev} to build some device testing files.

        {\tt make} by itself should build some of these.

        You should now be ready to run some basic tests on the S6~SoC.

\item Let's test the UART first: back out to the main directory {\tt trunk/},

        and run\break

        {\tt sw/host/zipload sw/dev/helloworld} and then {\tt make xload}.

        Now, examine your UART port.  (You do have the PModUSBUART installed

        and connected, right?)  You should see ``Hello, world!'' printed

        over and over again once each second.

\item You may try other test files in a similar manner, such as

        {\tt trunk/sw/dev/doorbell} and\break

        {\tt trunk/sw/dev/doorbell2}.  The first of these will just play the

        doorbell over and over again, whereas the second one will wait for a

        button press before playing the doorbell sound.

\item Now let's go and build the ZipOS together with it's user files.  To do

        this, enter into the {\tt trunk/sw/zipos} directory and type

        {\tt make}.  If all goes well, you should now have a program named

        {\tt trunk/sw/zipos/doorbell} which you can load into your S6~SoC as

        well.

\item A final load, and we'll be done.  To do this, make {\tt axload} again,

        and this time {\tt sw/host/zipload xilinx/toplevel.bit sw/zipos/doorbell}.

        When you power on your device the next time, or after you

        {\tt make xload}, you'll find the ZipOS running on the Zip~CPU.

\item To test the doorbell, press one of the buttons.  You should hear a

        doorbell coming out of the PModAMP2 audio port.

\item You should also be able to read the time on the LCD display.  It will be

        the wrong time (the number of seconds since power on \ldots).  To set

        the correct time, press `A' on the keypad and then type in the 6--digit

        time: HHMMSS.

        If you make a mistake, the `C' key can be used for a backspace.

\item You can also set the time of ``dawn'' by pressing a `B' on the keypad

        and then typing in the time ``dawn'' should be at.  The same is

        true for dusk, only you'll need to start that by pressing a `C' on the

        keypad.

\item Now, when the doorbell rings, the LCD will show the time the doorbell

        was pressed.  If the time is at night, the ourdoor light (oops, I

        mean LED\#3) will turn on for a half an hour (currently set to

        30~seconds, since I don't have the patience to wait a half hour while

        testing).

\end{enumerate}

Now that you've made it this far, you can go back and examine what was done

along the way, and perhaps even modify it for your own personal application.

\chapter{Registers}

There are several address regions on the S6~SoC, as shown in

Tbl.~\ref{tbl:memregions}.

\begin{table}[htbp]

\begin{center}\begin{tabular}{|p{0.75in}|p{0.75in}|p{0.5in}|p{3.0in}|}\hline

\rowcolor[gray]{0.85} Start & End & & Purpose \\\hline\hline

\scalebox{0.9}{\tt 0x000100} & \scalebox{0.9}{\tt 0x000107} & R/W & Peripheral I/O Control \\\hline

\scalebox{0.9}{\tt 0x000200} & \scalebox{0.9}{\tt 0x000201} & R/(W) & Debugging scope\\\hline

\scalebox{0.9}{\tt 0x000400} & \scalebox{0.9}{\tt 0x00043f} & R/W & Internal Configuration Access Port\\\hline

\scalebox{0.9}{\tt 0x000800} & \scalebox{0.9}{\tt 0x000803} & R/W & RTC Clock (if present)\\\hline

\scalebox{0.9}{\tt 0x002000} & \scalebox{0.9}{\tt 0x002fff} & R/W & 16kB On-Chip Block RAM \\\hline

\scalebox{0.9}{\tt 0x400000} & \scalebox{0.9}{\tt 0x7fffff} & R & 16~MB SPI Flash memory\\\hline

\end{tabular}

\caption{Address Regions}\label{tbl:memregions}

\end{center}\end{table}

In general, the address regions that are made up of RAM or flash act like

memory.  The RAM can be read and written, and the flash acts like read only

memory.\footnote{The Flash can be written, but only by an external command

while in the alternate configuration.}

This isn't quite true with the other address regions.  Accessing the I/O

region, while it will act like a memory, it may also have side-effects.  For

example, reading from the debugging scope device's data port will read a word

from the scope's buffer and advance the buffer pointer.  (More on that later.)

Finally, to keep the address decoder simple, many of these addresses are

multiply mapped.  Hence you may find the I/O peripherals mapped throughout the

{\tt 0x0100}--{\tt 0x01ff} address region.  Other memory addresses are similarly

overmapped.  This overmapping was a resource minimization feature, to get the

bus to fit within a minimum number of FPGA resources.

\section{Peripheral I/O Control}

Tbl.~\ref{tbl:ioregs}

\begin{table}[htbp]

\begin{center}\begin{reglist}

PIC    &\scalebox{0.8}{\tt 0x0100} & 32 & R/W & Interrupt Controller \\\hline

BUSERR &\scalebox{0.8}{\tt 0x0101} & 32 & R & Last Bus Error Address\\\hline

TIMA   &\scalebox{0.8}{\tt 0x0102} & 32 & R/W & ZipTimer A\\\hline

TIMB   &\scalebox{0.8}{\tt 0x0103} & 32 & R/W & ZipTimer B\\\hline

PWM    &\scalebox{0.8}{\tt 0x0104} & 32 & R/W & PWM Audio Controller\\\hline

SPIO   &\scalebox{0.8}{\tt 0x0105} & 32 & R/W & Special Purpose I/O, Keypad, LED Controller \\\hline

GPIO   &\scalebox{0.8}{\tt 0x0106} & 32 & R/W & GPIO Controller \\\hline

UART   &\scalebox{0.8}{\tt 0x0107} & 32 & R/W & UART data\\\hline

VERSION &\scalebox{0.8}{\tt 0x0108} & 32 & R & Build date\\\hline

\end{reglist}

\caption{I/O Peripheral Registers}\label{tbl:ioregs}

\end{center}\end{table}

shows the addresses of various I/O peripherals included as part of the SoC.

We'll walk through each of these peripherals in turn, describing how they work.

\subsection{Interrupt Controller}

The programmable interrupt controller (PIC) is identical to the one found with

the ZipSystem.  The layout of the PIC bits is shown in Fig.~\ref{fig:picreg}.

\begin{figure}\begin{center}

\begin{bytefield}[endianness=big]{32}

\bitheader{0-31} \\

        \bitbox{1}{E}

        \bitbox{15}{Enabled Ints}

        \bitbox{1}{A}

        \bitbox{15}{Currently Active Ints}

        \\

\end{bytefield}

\caption{Programmable Interrupt Control (PIC) Register}\label{fig:picreg}

\end{center}\end{figure}

This controller supports up to fifteen interrupts, however only twelve are

defined within the SoC.  These are listed in Tbl.~\ref{tbl:hw-ints}.

\begin{table}[htbp]

\begin{center}\begin{tabular}{|p{0.9in}|p{0.75in}|p{3.75in}|}\hline

\rowcolor[gray]{0.85} Name & Bit Mask & Description \\\hline\hline

INT\_BUTTON & 0x001 & A Button has been pressed. \\\hline

INT\_BUSERR & 0x002 & A Wishbone bus error has taken place\\\hline

INT\_SCOPE  & 0x004 & The Scope has completed its collection\\\hline

INT\_RTC    & 0x008 & An alarm or timer has taken place (assuming the RTC

                is installed, and includes both alarm or timer)\\\hline

INT\_TIMA   & 0x010 & Timer-A has reached zero\\\hline

INT\_TIMB   & 0x020 & Timer-B has reached zero\\\hline

INT\_UARTRX & 0x040 & A character has been received via the UART\\\hline

INT\_UARTTX & 0x080 & The transmit UART is idle, and ready for its next

                character.\\\hline

INT\_KEYPAD & 0x100 & One of the keypad wires has been pulled low. \\\hline

INT\_AUDIO  & 0x200 & The audio device is ready for its next sample\\\hline

INT\_GPIO   & 0x400 & The GPIO input lines have changed values.\\\hline

INT\_FLASH  & 0x800 & The flash device has finished either its erase or

                write cycle, and is ready for its next command. (Alternate

        config only.)\\\hline

\end{tabular}

\caption{Hardware Interrupts}\label{tbl:hw-ints}

\end{center}\end{table}

If any interrupt line is active, the PIC controller

will have that bit set among its active set.  Once set, the bit and hence the

interrupt can only be cleared by writing to the controller.  Interrupts can

also be enabled as well.  The enabled bit mask controls which interrupt lines

are permitted to interrupt the CPU. Hence, just because an interrupt is active

doesn't mean it will interrupt the CPU--the corresponding bit in the enable

mask must be set as well.

Finally, then {\tt A} or {\tt ANY} bit will be high if any interrupts are both

enabled and active, whereas the {\tt E} or global interrupt enable bit can be

set to allow the PIC to interrupt the CPU or cleared to disable all interrupts.

To keep operations on this register atomic, most of the bits of this register

have special meanings upon write.  The one exception to this is the global

interrupt enable bit.  On any write, interrupts will be globally enabled or

disabled based upon the value of this bit.  Further, the {\tt ANY} bit is a

read only bit, so writes to it have no effect.

Enabling specific interrupts, via writes to the enable lines, are different.

To enable a specific interrupt, enable all interrupts and

set the mask bit associated with the specific interrupt you wish to enable.

Hence writing a {\tt 0x80010000} will enable interrupt line zero

({\tt INT\_BUTTON}), while also enabling all previously enabled interrupts.

To disable a specific interrupt, disable all interrupts and write a one to the

enable line of the interrupt you wish to disable.  In this fashion, writing a

{\tt 0x00010000} will disable all interrupts and leave interrupt line zero

disabled when the interrupts are re--enabled later, whereas {\tt 0x07fff0000}

will disable all specific interrupts.

Interrupts are acknowledged in a fashion similar to enabling interrupts.  By

writing a `1' to the active bit mask, the interrupt will be acknowledged and

reset, whereas writing a `0' leaves the interrupt untouched.  In this fashion,

as individual interrupts are handled, a `1' may be written to this bottom mask

to clear the interrupt.  Be aware, however, that any interrupt acknowledgement

may also globally enable or disable interrupts.

\subsection{Last Bus Error Address}

The Bus Error peripheral simply records the address of the last bus error,

and sets an interrupt upon receiving a bus error.  (The interrupt itself

is kind of useless ...) The address can be useful when debugging.  While the

peripheral may only be read,

setting it is really as easy as creating a bus error and trapping the result.

Another use for this is upon any reboot, it is possible to read the address

of the last bus error and perhaps learn something of what caused the CPU to

restart.

\subsection{ZipTimer}

The S6~SoC contains two ZipTimers, available for the CPU to use.  These are

countdown timers.  Writing any non--zero value to them will cause them to

immediately start counting down from that value towards zero, and to interrupt

the CPU upon the transition to zero.  Writing a new value while the timer is

running will cause that new value to automatically load into the timer and

start counting from there.  Writing a zero to the timer disables the timer, and

causes it to stop.

ZipTimer A can be set to auto reload by setting the top bit as well as the

interval.  When so set, the timer will automatically

load it's last set value upon reaching zero and interrupting the CPU.  This

effectively turns it into an interrupt timer if desired.  To set this feature,

write to the timer the number of clock ticks before an interrupt, but also set

the high order bit.  In this fashion, writing a {\tt 0x8001387f} will interrupt

the CPU every millisecond, starting one millisecond after the write takes place

(assuming an 80~MHz system clock).\footnote{Note that, since the timer spends

a cycle at zero, setting it for a 80,000 cycle period requires setting the

timer value to one less than 80,000.}

ZipTimer B has been wired for a different purpose.  ZipTimer B does not support

auto reload, nor will it interrupt the CPU.  Instead, ZipTimer B has been wired

as a watchdog timer.  When this timer transitions to zero, the CPU will be

rebooted.  One way to use this timer would be in conjunction with the ZipTimer

A, and to write a number to it upon any entry to the interrupt service routine.

If given enough time, this would cause the CPU to reboot if for any reason it

locked up.

The ZipOS uses ZipTimer~A for task swapping.  By setting the timer for

1~ms, the ZipOS examines every task for a potential task swap every millisecond.

Of course, if the various tasks are running from Flash at 52~clocks per

instruction, this means that as few as 1,538~instructions may be executed

between timer interrupts, but this can be tuned if necessary for better

performance.

\subsection{PWM Audio Controller}

The bit fields of the PWM Audio controller are shown in Fig.~\ref{fig:pwmreg}.

\begin{figure}\begin{center}

\begin{bytefield}[endianness=big]{32}

\bitheader{0-31} \\

        \bitbox{10}{Unused}

        \bitbox{1}{S}

        \bitbox{1}{G}

        \bitbox{3}{}

        \bitbox{1}{E}

        \bitbox{16}{Sample}

        \\

\end{bytefield}

\caption{PWM Audio Controller Bitfields}\label{fig:pwmreg}

\end{center}\end{figure}

This controller has been designed for easy writing.  To send a sample to the

PWM audio controller, simply write the sample to the controller and clear the

PWM audio interrupt---{\em in that order}.  When the audio interrupts the CPU

again, it is ready for the next sample.  Do note, however, that the audio

interrupt can only be cleared once a new sample has been written to it.

Attempts to clear it prior to that will have no effect.  (This is why the

order matters.)

The audio sample rate has been fixed at 8~kHz.  While changing this rate is

easy to do within {\tt busmaster.v}, the rate itself takes some work to keep

up with, so I wouldn't recommend going faster.

The audio controller supports two additional functionalities, however.  The

first is that the {\tt E} bit will be set upon any read when or if the audio

controller is ready for another sample and the Audio interrupt has been

asserted.  By polling this bit, for example, the audio driver can be run without

using the interrupt functionality.

The second functionality has to do with the two auxiliary control bits present

in the PModAMP2 audio device.  These are the gain and shutdown bits.  To set

these bits, write a sample to the controller while also setting the {\tt E}

bit.  When the {\tt E} bit is set upon any write, the shutdown and gain bits

will also be set.  (Be aware, the shutdown bit is negative logic.)  Hence, one

may start this interface by writing a {\tt 0x0310000} to the device, and later

shut it back off by writing a {\tt 0x010000}.

\subsection{Special Purpose I/O}

Register {\tt SPIO}, as shown in Fig.~\ref{fig:spioreg},

\begin{figure}\begin{center}

\begin{bytefield}[endianness=big]{32}

\bitheader{0-31} \\

\begin{leftwordgroup}{Read}\bitbox[lrt]{16}{Zeros}

        \bitbox[lrt]{4}{Kpad}

        \bitbox[lrt]{4}{Kpad}

        \bitbox[lrt]{2}{00}

        \bitbox[lrt]{2}{Btn}

        \bitbox[lrt]{4}{LED} \\

\bitbox[lrb]{16}{}

        \bitbox[lrb]{4}{Col Out}

        \bitbox[lrb]{4}{Row In}

        \bitbox[lrb]{2}{}

        \bitbox[lrb]{2}{}

        \bitbox[lrb]{4}{}\end{leftwordgroup} \\

\begin{leftwordgroup}{Write}\bitbox[lrt]{16}{Ignored}

        \bitbox[lrt]{4}{Col}

        \bitbox[lrt]{4}{Col}

        \bitbox[lrt]{4}{LED}

        \bitbox[lrt]{4}{LED} \\

\bitbox[lrb]{16}{}

        \bitbox[lrb]{4}{Out}

        \bitbox[lrb]{4}{Enable}

        \bitbox[lrb]{4}{Enable}

        \bitbox[lrb]{4}{}\end{leftwordgroup} \\

\end{bytefield}

\caption{SPIO Control Register}\label{fig:spioreg}

\end{center}\end{figure}

is a Special Purpose Input/Output (SPIO) register. It is

designed to control the on-board LED's, buttons, and keypad.  Upon any read,

the register reads the current state of the keypad column output, the keypad

row input, the buttons and the LED's.  Writing is more difficult, in order to

make certain that parts of these registers can be modified atomically.

Specifically, to change an LED, write the new value as well as a `1' to the

corresponding LED change enable bit.  The same goes for the keypad column

output, a `1' needs to be written to the corresponding change enable bit in

order for a new value to be accepted.

As examples, writing a {\tt 0x0ff} to the {\tt SPIO} register will turn all

LED's on, {\tt 0x0f0} will turn all LED's off, and {\tt 0x011} and {\tt 0x010}

will turn LED0 on and then off again respectively.

The keypad is a little bit tricker.  To wait for a keypad interrupt, one needs

to set the column outputs to zero.  To do this, write a {\tt 0x0f00} to the

{\tt SPIO} register.  When a user then presses a key, one of the row inputs

will go low and an interrupt will be asserted.  The key must then be debounced,

and the ZipOS debounces keys for 5~ms.  Once debounced, the key may be read.

To do this, set half of the columns to zero, such as by writing a {\tt 0x0cf00}

to the {\tt SPIO} register.  If one of the row values is still zero, then

one of the two columns tested corresponded with the key.  This can then be

repeated until the correct column has been determined, at which point the

row can be read and the key known.

The controller will generate a keypad interrupt whenever any row input is

zero, and a button interrupt whenever any button value is a one.  This is a

level triggered interrupt, not edge triggered.  What that means is that,

once generated, the interrupt will need to be disabled until the key or button

is released---there will be no interrupt for the release, that part will need

to be done in software.

\subsection{General Purpose I/O}

The General Purpose Input and Output (GPIO) control register, shown in

Fig.~\ref{fig:gpioreg},

\begin{figure}\begin{center}

\begin{bytefield}[endianness=big]{32}

\bitheader{0-31} \\

\bitbox[lrtb]{16}{Current Input Vals (x16)}\bitbox[lrt]{16}{Current Output} \\

\bitbox[lrtb]{16}{Output Change Enable}\bitbox[lrb]{16}{Values (16-outs)}

\end{bytefield}

\caption{GPIO Control Register}\label{fig:gpioreg}