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%% Filename:    spec.tex

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%% Project:     A Wishbone Controlled Real-Time clock Core

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%% Purpose:     This LaTeX file contains all of the documentation/description

%%              currently provided with this FPGA Real-time Clock Core.

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%% Creator:     Dan Gisselquist

%%              Gisselquist Technology, LLC

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\documentclass{gqtekspec}

\project{Real-Time Clock}

\title{Specification}

\author{Dan Gisselquist, Ph.D.}

\email{dgisselq (at) opencores.org}

\revision{Rev.~0.1}

\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 \hbox{<http://www.gnu.org/licenses/>} for a

copy.

\end{license}

\begin{revisionhistory}

0.2 & 7/11/2015 & Gisselquist & Date interface added\\\hline

0.1 & 5/25/2015 & Gisselquist & First Draft \\\hline

\end{revisionhistory}

% Revision History

% Table of Contents, named Contents

\tableofcontents

% \listoffigures

\listoftables

\begin{preface}

Every FPGA project needs to start with a very simple core.  Then, working

from simplicity, more and more complex cores can be built until an eventual

application comes from all the tiny details.

This real time clock began with one such simple core.  All of the pieces to

this clock are simple.  Nothing is inherently complex.  However, placing this

clock into a larger FPGA structure requires a Wishbone bus, and being able

to command and control an FPGA over a wishbone bus is an achievement in

itself.  Further, the clock produces seven segment display output values

and LED output values.  These are also simple outputs, but still take a lot

of work to complete.  Finally, this clock will strobe an interrupt line.

Reading and processing that interrupt line requires a whole 'nuther bit of

logic and the ability to capture, recognize, and respond to interrupts.

Hence, once you get a simple clock working, you have a lot working.

\end{preface}

\chapter{Introduction}

\pagenumbering{arabic}

\setcounter{page}{1}

This Real--Time Clock implements a twenty four hour clock, count-down timer,

stopwatch and alarm.  It is designed to be configurable to adjust to whatever

clock speed the underlying architecture is running on, so with only minor

changes should run on any fundamental clock rate from about 66~kHz on up to

250~TeraHertz with varying levels of accuracy along the way.

Distributed with this clock is a similar Real--Time Date module.  This

second module can track the day, month, and year while properly accounting

for varying days in each month and leap years, when they happen.

Together, the clock and date module offer a fairly full feature set of

capability: date, time, alarms, a countdown timer and a stopwatch, all

features which are available from the wishbone bus.

Other interfaces exist as well.

Should you wish to investigate your clock's stability or try to guarantee

its fine precision accuracy, it is possible to provide a time hack pulse to

the clock and subsequently read what all of the internal registers were set

to at that time.

When either the count--down timer reaches zero or the clock reaches the alarm

time (if set), the clock module will produce an impulse which can be used as

an interrupt trigger.

This clock will also provide outputs sufficient to drive an external seven

segment display driver and 16 LED's.

Future enhancements may allow for button control and fine precision clock

adjustment.

The layout of this specification follows the format set by OpenCores.

This introduction is the first chapter.  Following this introduction is

a short chapter describing how this clock is implemented,

Chapt.~\ref{chap:arch}.  Following this description, the Chapt.~\ref{chap:ops}

gives a brief overview of how to operate the clock.  Most of the details,

however, are in the registers and their definitions.  These you can find in

Chapt.~\ref{chap:regs}.  As for the wishbone, the wishbone spec requires a

wishbone datasheet which you can find in Chapt.~\ref{chap:wishbone}.

That leaves the final pertinent information necessary for implementing this

core in Chapt.~\ref{chap:ioports}, the definitions and meanings of the

various I/O ports.

As always, write me if you have any questions or problems.

\chapter{Architecture}\label{chap:arch}

Central to this real time clock architecture is a 48~bit sub--second register.

This register is incremented every clock by a user defined 32~bit value,

{\tt CKSPEED}.  When the register turns over at the end of each second, a

second has taken place and all of the various clock (and date) registers are

adjusted.

Well, not quite but almost.  The 48~bit register is actually split into a

lower 40~bit register that is common to all clock components, as well as

separate eight bit upper registers for the clock, timer, and stopwatch.  In

this fashion, these separate components can have different definitions for

when seconds begin and end, and with sufficient precision to satisfy most

applications.

The next thing to note about this architecture is the format of the various

clock registers: Binary Coded Decimal, or BCD.  Hence an {\tt 8'h59} refers

to a value of 59, rather than 89.  In this fashion, setting the time to

{\tt 24'h231520} will set it to 23~hours, 15~minutes, and 20~seconds.  The

only exception to this BCD format are the subseconds fields found in the

stopwatch and time hack registers.  Seconds and above are all encoded as BCD.

\chapter{Operation}\label{chap:ops}

\section{Time}

To set the time, simply write to the clock register the current value of the

time.  If the seconds hand is written as zero, subsecond time will be cleared

as well.  The new clock value takes place one clock period after the value

is written to the bus.

To set only some parts of the time and not others, such as the minutes but

not seconds or hours, write all '1's to the seconds and hours.  In this way,

writing a {\tt 24'h3f17ff} will set the minutes to 17, but not affect the

rest of the clock.

This is also the way to adjust the display without adjusting time.  Suppose

you wish to switch to display option '1', just write a {\tt 32'h013fffff} to

the register and the display will switch without adjusting time.

\section{Count-down Timer}

To use the count down timer, set it to the amount of time you wish to count

down for.  When ready, or even in the same cycle, enable the count--down

timer by setting the RUN bit high.  At this point in time, the count--down

timer is running.  When it gets to zero, it will stop and trigger an interrupt.

You can tell if the alarm has been triggered by the TRIGGER bit being set.

Any write to the timer register will clear the alarm condition.

While the timer is running, writing a '0' to the timer register will stop it

without clearing the time remaining.  In this state, writing to the register

the RUN bit by itself will restart the timer, while anything else will set the

timer to a new value.  Further, if the timer is stopped at zero, then writing

zero to the timer will reset the timer to the last start time it had.

\section{Stopwatch}

The stop watch supports three operations: start, stop, and clear.  Writing a

'1' to the stop watch register will start the stopwatch, while writing a '0'

will stop it.  When it starts next, it will start where it left off unless the

register is cleared.  To clear the register and set it back to zero, write a

'2' to the register.  This will effectively stop the register and clear it in

one step.  If the register is already stopped, writing a '3' will clear and

start it in one step.  However, the register can only be cleared while stopped.

If the register is running, writing a '3' will have no effect.

\section{Alarm}

To set the alarm, just write the alarm time to the alarm register together

with alarm enable bit.  As with the time register, setting any field,

whether hours, minutes, or seconds, to {\tt 8'hff} has no effect on that

field.  Hence, the alarm may be activated by writing {\tt 25'h13fffff} to

the register and deactivated by writing {\tt 25'h03fffff}.

Once the alarm is tripped, the RTC core will generate an interrupt.  Further,

the tripped bit in the alarm register will be set.  To clear this bit and the

alarm tripped condition, either disable the alarm or write a '1' to this bit.

\section{Time Hacks}

For finer precision timing, the RTC module allows for setting a time

hack and reading the value from the device.  On the clock following the

time hack being high, the internal state, to include the time and the 48~bit

counter, will be recorded and may then be read out.  In this fashion,

it is possible to capture, with as much precision as the device offers,

the current time within the device.

It is the users responsibility to read the time hack registers before a

subsequent time hack pulse sets them to new values.

\section{Date}

The Real--Time Date module is really a separate module from the Real--Time

Clock module, but that doesn't prevent it from working just like the others.

To set the date, just write the new date value to the address of the date.

Further, as with the clock time, setting any particular field of the date to

all ones, such as setting the month to {\tt 8'hff}, will cause that portion of

the date to retain it's current value.  In this way, one part of the date

may be set and not others.

\chapter{Registers}\label{chap:regs}

This RTC clock module supports eight registers, as listed in

Tbl.~\ref{tbl:reglist}.  Of these eight, the first four have been so placed

as to be the more routine or user used registers, while the latter four are

more lower level.

\begin{table}[htbp]

\begin{center}

\begin{reglist}

CLOCK   & 0 & 32 & R/W & Wall clock time register\\\hline

TIMER   & 1 & 32 & R/W & Count--down timer\\\hline

STPWTCH & 2 & 32 & R/W & Stopwatch control and value\\\hline

ALARM   & 3 & 32 & R/W & Alarm time, and setting\\\hline\hline

CKSPEED & 4 & 32 & R/W & Clock speed control.\\\hline

HACKTIME &5 & 32 & R & Wall clock time at last hack.\\\hline

HACKCNTHI&6 & 32 & R & Wall clock time.\\\hline

HACKCNTLO&7 & 32 & R & Wall clock time.\\\hline

\end{reglist}\caption{List of Registers}\label{tbl:reglist}

\end{center}\end{table}

Each register will be discussed in detail in this chapter.

The Date module supports an additional register, listed in

Tbl.~\ref{tbl:datereg}.

\begin{table}[htbp]

\begin{center}

\begin{reglist}

DATE    & 0 & 32 & R/W & Calendar date register\\\hline

\end{reglist}\caption{Date Register}\label{tbl:datereg}

\end{center}\end{table}

This register will be discussed after we discuss the time registers.

\section{Clock Time Register}

The various bit fields associated with the current time may be found in

the {\tt CLOCK} register, shown in Tbl.~\ref{tbl:clockreg}.

\begin{table}[htbp]\begin{center}

\begin{bitlist}

28--31 & R & Always return zero.\\\hline

24--27 & R/W & Seven Segment Display Mode.\\\hline

22--23 & R & Always return zero.\\\hline

16--21 & R/W & Current time, BCD hours\\\hline

8--15 & R/W & Current time, BCD minutes\\\hline

0--7 & R/W & Current time, BCD seconds\\\hline

\end{bitlist}

\caption{Clock Time Register Bit Definitions}\label{tbl:clockreg}

\end{center}\end{table}

This register contains six clock digits: two each for hours, minutes, and

seconds.  Each of these digits is encoded in Binary Coded Decimal (BCD).

Therefore, 23~hours would be encoded as 6'h23 and not 6'h17.  Writes to each

of the various subcomponent registers will set that register, unless the

write value is a 8'hff.  The behaviour of the clock when non--decimal

values are written, other than all F's, is undefined.

Separate from the time, however, is the seven segment display mode.  Four

values are currently supported: 4'h0 to display the hours and minutes,

4'h1 to display the timer in minutes and seconds, 4'h2 to display the

stopwatch in lower order minutes, seconds, and sixteenths of a second, and

4'h3 to display the minutes and seconds of the current time.  In all cases,

the decimal point will appear to the right of the lowest order digit

and will blink with the second hand.  That is, the decimal will be high for

the second half of any second, and low at the top of the second.

\section{Countdown Timer Register}

The countdown timer register, whose bit--wise values are shown in

Tbl.~\ref{tbl:timer},

\begin{table}[htbp]

\begin{center}

\begin{bitlist}

26--31 & R & Unused, always read as '0'.\\\hline

25 & R/W & Alarm condition.  Write a '1' to clear.\\\hline

24 & R/W & Running, stopped on '0'\\\hline

16--23 & R/W & BCD Hours\\\hline

8--15 & R/W & BCD Minutes\\\hline

0--7 & R/W & BCD Seconds\\\hline

\end{bitlist}

\caption{Count--down Timer register}\label{tbl:timer}

\end{center}\end{table}

controls the operation of the count--down timer.  To use this timer, write

some amount of time to the register, then write zeros with bit 24 set.  The

register will then reach an alarm condition after counting down that amount

of time.  (Alternatively, you could set bit 24 while writing the register,

to set and start it in one operation.)  To stop the register while it is

running, just write all zeros.  To restart the register, provided more than a

second remains, write a {\tt 26'h1000000} to set it running again.  Once

the timer alarms, the timer will stop and the alarm condition will be set.

Any write to the timer register after the alarm condition has been set will

clear the alarm condition.

\section{Stopwatch Register}

The various bits of the stopwatch register are shown in

Tbl.~\ref{tbl:stopwatch}.

\begin{table}[htbp]

\begin{center}

\begin{bitlist}

24--31 & R & Hours\\\hline

16--23 & R & Minutes\\\hline

8--15 & R & Sub Seconds\\\hline

1--7 & R & Sub Seconds\\\hline

1 & W & Clear\\\hline

\end{bitlist}

\caption{Stopwatch Register}\label{tbl:stopwatch}

\end{center}\end{table}

Of note is the bottom bit that, when set, means the stop watch is running.

Set this bit to '1' to start the stopwatch, or to '0' to stop the stopwatch.

Further, while the stopwatch is stopped, a '1' can be written to the clear

bit.  This will zero out the stopwatch and set it back to zero.

\section{Alarm Register}

The various bits of the alarm register are shown in Tbl.~\ref{tbl:alarm}.

\begin{table}[htbp]

\begin{center}

\begin{bitlist}

26--31 & R & Always reads zeros. \\\hline

25 & R/W & Alarm tripped.  Write a '1' to this register to clear any alarm

        condition.  (A tripped alarm will not trip again.)\\\hline

24 & R/W & Alarm enabled\\\hline

16--23 & R & Alarm time, BCD hours\\\hline

8--15 & R & Alarm time, BCD minutes\\\hline

0--7 & R/W & Alarm time, BCD Seconds\\\hline

\end{bitlist}

\caption{Alarm Register}\label{tbl:alarm}

\end{center}\end{table}

Basically, the alarm register consists a time and two more bits.  The extra

two bits encode whether or not the alarm is enabled, and whether or not it has

been tripped.  The alarm will be {\em tripped} whenever it is enabled, and the

time changes to equal the alarm time.  Once tripped, the alarm will stay

in the alarmed or tripped condition until either a '1' is written to the

tripped bit, or the alarm is disabled.

As with the clock and timer registers, writing eight ones to any of the

BCD fields when writing to this register will leave those fields untouched.

\section{Clock Speed Register}

The actual speed of the clock is controlled by the {\tt CKSPEED} register,

shown in Tbl.~\ref{tbl:ckspeed}.

\begin{table}[htbp]

\begin{center}

\begin{bitlist}

0--31 & R/W & 48~bit counter time increment\\\hline

\end{bitlist}

\caption{Clock Speed Register}\label{tbl:ckspeed}

\end{center}\end{table}

This register contains a simple 32~bit unsigned value.  To step the clock,

this value is extended to 48~bits and added to the fractional seconds value.

This value should be set to $2^{48}$ divided by the clock frequency of the

controlling clock.  Hence, for a 100~MHz clock, this value would be set to

{\tt 32'd2814750}.  For clocks near 100~MHz, this allows adjusting speed

within about 40~clocks per second.  For clocks near 500~MHz, this allows

time adjustment to an accuracy of about about 800~clocks per second.  In

both cases, this is good enough to maintain a clock with less than a

microsecond loss over the course of a year.  Hence, this RTC module provides

more logical stability than most hardware clocks on the market today.

\section{Time--hack time}

To support finer precision clock control, the time--hack capability exists.

This capability consists of three registers, the time--hack time register

shown in Tbl.~\ref{tbl:hacktime},

\begin{table}[htbp]

\begin{center}

\begin{bitlist}

24--31 & R & BCD Hours.\\\hline

16--23 & R & BCD Minutes.\\\hline

8--15 & R & BCD seconds.\\\hline

0--7 & R & Subseconds, encoded in 256ths of a second\\\hline

\end{bitlist}

\caption{Time Hack Time Register}\label{tbl:hacktime}

\end{center}\end{table}

and two registers (Tbls.~\ref{tbl:hackcnthi}

\begin{table}[htbp]

\begin{center}

\begin{bitlist}

0--31 & R & Upper 32 bits of the internal 40~bit counter.\\\hline

\end{bitlist}

\caption{Time Hack Counter, High}\label{tbl:hackcnthi}

\end{center}\end{table}

and~\ref{tbl:hackcntlo})

\begin{table}[htbp]

\begin{center}

\begin{bitlist}

24--31 & R & Bottom 8~bits of the internal 40~bit counter.\\\hline

0--23 & R & Always read as '0'.\\\hline

\end{bitlist}

\caption{Time Hack Counter, Low}\label{tbl:hackcntlo}

\end{center}\end{table}

capturing the contents of the 40~bit internal counter at the time of the hack.

The time--hack time register is perhaps the simplest to understand.  This

captures the time of the time--hack in hours, minutes, seconds, and 8~fractional

subsecond bits.  The top 24~bits of this register will match the bottom 24~bits

of the clock~time register at the time of the time hack.  The bottom eight

bits are the top eight bits of the 48~bit subsecond time counter.  The

rest of those 48~bits may then be returned in the other two time hack counter

registers.

At present, this functionality isn't yet truly fully featured.  Once fully

featured, there will (should) be a mechanism for adjusting this counter based

upon information gleaned from the hack time.  Implementation details have

to date prevented this portion of the design from being implemented.

\section{Date Register}

The year, month, and day of month fields may all be found within the

{\tt DATE} register of the Real--Time Date module, shown in

Tbl.~\ref{tbl:datebits}.

\begin{table}[htbp]\begin{center}

\begin{bitlist}

30--31 & R & Always return zero.\\\hline

16--29 & R/W & Four digit BCD year\\\hline

13--15 & R & Always return zero.\\\hline

8--12 & R/W & Two digit BCD month\\\hline

6--7 & R & Always return zero.\\\hline

0--5 & R/W & Two digit BCD day of month\\\hline

\end{bitlist}

\caption{Date Register Bit Definitions}\label{tbl:datebits}

\end{center}\end{table}

Further, according to the common calendar convention, the minimum day and month

are one and not zero.

\chapter{Wishbone Datasheet}\label{chap:wishbone}

Tbl.~\ref{tbl:wishbone}

\begin{table}[htbp]

\begin{center}

\begin{wishboneds}

Revision level of wishbone & WB B4 spec \\\hline

Type of interface & Slave, Read/Write \\\hline

Port size & 32--bit \\\hline

Port granularity & 32--bit \\\hline

Maximum Operand Size & 32--bit \\\hline

Data transfer ordering & (Irrelevant) \\\hline

Clock constraints & Faster than 66~kHz \\\hline

Signal Names & \begin{tabular}{ll}

                Signal Name & Wishbone Equivalent \\\hline

                {\tt i\_clk} & {\tt CLK\_I} \\

                {\tt i\_wb\_cyc} & {\tt CYC\_I} \\

                {\tt i\_wb\_stb} & {\tt STB\_I} \\

                {\tt i\_wb\_we} & {\tt WE\_I} \\

                {\tt i\_wb\_addr} & {\tt ADR\_I} \\

                {\tt i\_wb\_data} & {\tt DAT\_I} \\

                {\tt o\_wb\_ack} & {\tt ACK\_O} \\

                {\tt o\_wb\_stall} & {\tt STALL\_O} \\

                {\tt o\_wb\_data} & {\tt DAT\_O}

                \end{tabular}\\\hline

\end{wishboneds}

\caption{Wishbone Datasheet}\label{tbl:wishbone}

\end{center}\end{table}

is required by the wishbone specification, and so

it is included here.  The big thing to notice is that both the real time clock

and the real time date modules act as wishbone slaves, and that all accesses

to the registers of either module are 32--bit reads and writes.  The address

bus does not offer

byte level, but rather 32--bit word level resolution.  Select lines are not

implemented.  Bit ordering is the normal ordering where bit~31 is the most

significant bit and so forth.  Although the stall line is implemented, it is

always zero.  Access delays are a single clock, so the clock after a read or

write is placed on the bus the {\tt i\_wb\_ack} line will be high.

\iffalse

\chapter{Clocks}\label{chap:clocks}

This core is based upon the Basys--3 design.  The Basys--3 development board

contains one external 100~MHz clock, which is sufficient to run this

core.  The logic within the core can also be run faster, or slower, as is

necessary to meet the timing constraints associated with the internal

operations of the core and it's surrounding environment.  See

Table.~\ref{tbl:clocks}.

\begin{table}[htbp]

\begin{center}

\begin{clocklist}

i\_clk & External & 250~THz & 66~kHz & System clock.\\\hline

\end{clocklist}

\caption{List of Clocks}\label{tbl:clocks}

\end{center}\end{table}

\fi

\chapter{I/O Ports}\label{chap:ioports}

The I/O ports for this clock are shown in Tbls.~\ref{tbl:iowishbone}

\begin{table}[htbp]

\begin{center}

\begin{portlist}

i\_clk & 1 & Input & System clock, used for time and wishbone interfaces.\\\hline

i\_wb\_cyc & 1 & Input & Wishbone bus cycle wire.\\\hline

i\_wb\_stb & 1 & Input & Wishbone strobe.\\\hline

i\_wb\_we & 1 & Input & Wishbone write enable.\\\hline

i\_wb\_addr & 5 & Input & Wishbone address.\\\hline

i\_wb\_data & 32 & Input & Wishbone bus data register for use when writing

        (configuring) the core from the bus.\\\hline

o\_wb\_ack & 1 & Output & Return value acknowledging a wishbone write, or

                signifying valid data in the case of a wishbone read request.

                \\\hline

o\_wb\_stall & 1 & Output & Indicates the device is not yet ready for another

                wishbone access, effectively stalling the bus.\\\hline

o\_wb\_data & 32 & Output & Wishbone data bus, returning data values read

                from the interface.\\\hline

\end{portlist}

\caption{Wishbone I/O Ports}\label{tbl:iowishbone}

\end{center}\end{table}

and~Tbl.~\ref{tbl:ioother}.

\begin{table}[htbp]

\begin{center}

\begin{portlist}

o\_sseg & 32 & Output & Lines to control a seven segment display, to be

                sent to that display's driver.  Each eight bit byte controls

                one digit in the display, with the bottom bit in the byte

                controlling the decimal point.\\\hline

o\_led & 16 & Output & Output LED's, consisting of a 16--bit counter counting

                from zero to all ones each minute, and synchronized with each

                minute so as to create an indicator of when the next minute

                will take place when only the hours and minutes can be

                displayed.\\\hline

o\_interrupt & 1 & Output & A pulsed/strobed interrupt line.  When the

                clock needs to generate an interrupt, it will set this line

                high for one clock cycle.  \\\hline

o\_ppd & 1 & Output & A `pulse per day' signal which can be fed into the

        real--time date module.  This line will be high on the clock before

        the stroke of midnight, allowing the date module to turn over to the

        next day at exactly the same time the clock module turns over to the

        next day.\\\hline

i\_hack & 1 & Input & When this line is raised, copies are made of the

        internal state registers on the next clock.  These registers can then

        be used for an accurate time hack regarding the state of the clock

        at the time this line was strobed.\\\hline

\end{portlist}

\caption{Other I/O Ports}\label{tbl:ioother}

\end{center}\end{table}

Tbl.~\ref{tbl:iowishbone} reiterates the wishbone I/O values just discussed in

Chapt.~\ref{chap:wishbone}, and so need no further discussion here.

This clock is designed for command and control via the wishbone.  No other

registers, beyond the wishbone bus, are required.  However, several other

may be valuable.  These other registers are listed in Tbl.~\ref{tbl:ioother}.

We'll discuss each of these in turn.

First of the other I/O registers is the {\tt o\_sseg} register.  This register

encodes which outputs of a seven segment display need to be turned on to

represent the value of the clock requested.  This register consists of four

eight bit bytes, with the highest order byte referencing the highest order

display segment value.  In each byte, the low order bit references a decimal

point.  The other bits are ordered around the zero, with the top bit being

the top bar of a '0', the next highest order bit and so on following the

zero clockwise.  The final bit of each byte, the bit in the two's place,

encodes whether or not the middle line is to be displayed.  When either timer

or alarm is triggered, this display will blink until the triggering conditions

are cleared.

This output is expected to be the input to a seven segment display driver,

rather than being the output to the display itself.

The next output lines are the 16~lines of the {\tt o\_led} bus.  When connected

with 16~LED's, these lines will create a counting display that will count up

to each minute, synchronized to the minute.  When either timer or alarm has

triggered, all of the LED's will flash together until the triggered condition

is reset.

The third other line is the {\tt o\_interrupt} line.  This line will be

strobed by the RTC module any time the alarm is triggered or the timer runs

out.  The line will not remain high, but neither will it trigger a second

time until the underlying interrupt is cleared.  That is, the timer will only

trigger once until cleared as will the alarm, but the alarm may trigger after

the timer has triggered and before the timer clears.

As a fourth additional line, the clock module produces a one pulse per day

signal, {\tt o\_ppd}.  This signal is designed to be the only necessary

coordinated input between the clock and date module.  Feeding it straight

into the date module will keep the two synchronized.

The final other I/O line is a simple input line.  This line is expected to be

strobed for one clock cycle any time a time hack is required.  For example,

should you wish to read and synchronize to a GPS PPS signal, strobe the device

with the PPS (after dealing with any metastability issues), and read the time

hacks that are produced.

The real--time date module has a similar set of I/O ports to the clock.  These

are listed in Tbl.~\ref{tbl:iodate}.

\begin{table}[htbp]

\begin{center}

\begin{portlist}

i\_clk & 1 & Input & The system clock.\\\hline

i\_ppd & 1 & Input & The one pulse per day strobe from the clock module.\\\hline

i\_wb\_cyc & 1 & Input & Wishbone bus cycle.\\\hline

i\_wb\_stb & 1 & Input & Wishbone strobe.\\\hline

i\_wb\_we & 1 & Input & Wishbone write enable.\\\hline

i\_wb\_data & 32 & Input & Wishbone bus data register for use when writing

        (configuring) the core from the bus.\\\hline

o\_wb\_ack & 1 & Output & Equal to the bus cycle line anded with the strobe

                line, and delayed by one clock---essentially acknowledging any

                wishbone access.\\\hline

o\_wb\_stall & 1 & Output & Fixed to zer.\\\hline

o\_wb\_data & 32 & Output & Wishbone data bus, returning data values read

                from the interface.\\\hline

\end{portlist}

\caption{Wishbone I/O Ports}\label{tbl:iodate}

\end{center}\end{table}

There are two big things to notice.  The first is the {\tt i\_ppd} signal.

This should be connected straight from the clock module's {\tt o\_ppd} signal

into this module.  The second difference is the lack of any address lines.

This is appropriate since the date module provides a single register only.

% Appendices

% Index

\end{document}