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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%%
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%% Filename: spec.tex
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%%
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%% Project: A Wishbone Controlled Real-Time clock Core
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%%
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%% Purpose: This LaTeX file contains all of the documentation/description
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%% currently provided with this FPGA Real-time Clock Core.
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%% It's not nearly as interesting as the PDF file it creates,
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%% so I'd recommend reading that before diving into this file.
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%% You should be able to find the PDF file in the SVN distribution
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%% together with this PDF file and a copy of the GPL-3.0 license
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%% this file is distributed under. If not, just type 'make'
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%% in the doc directory and it (should) build without a problem.
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%%
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%%
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%% Creator: Dan Gisselquist
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%% Gisselquist Technology, LLC
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%%
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%%
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%% Copyright (C) 2015, Gisselquist Technology, LLC
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%%
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%% This program is free software (firmware): you can redistribute it and/or
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%% modify it under the terms of the GNU General Public License as published
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%% by the Free Software Foundation, either version 3 of the License, or (at
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%% your option) any later version.
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%%
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%% This program is distributed in the hope that it will be useful, but WITHOUT
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%% ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or
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%% FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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%% for more details.
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%%
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%% You should have received a copy of the GNU General Public License along
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%% with this program. (It's in the $(ROOT)/doc directory, run make with no
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%% target there if the PDF file isn't present.) If not, see
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%% <http://www.gnu.org/licenses/> for a copy.
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%%
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%% License: GPL, v3, as defined and found on www.gnu.org,
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%% http://www.gnu.org/licenses/gpl.html
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%%
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%%
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\documentclass{gqtekspec}
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\project{Real-Time Clock}
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\title{Specification}
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\author{Dan Gisselquist, Ph.D.}
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dgisselq |
\email{dgisselq (at) opencores.org}
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dgisselq |
\revision{Rev.~0.1}
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\begin{document}
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\pagestyle{gqtekspecplain}
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\titlepage
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\begin{license}
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Copyright (C) \theyear\today, Gisselquist Technology, LLC
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This project is free software (firmware): you can redistribute it and/or
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modify it under the terms of the GNU General Public License as published
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by the Free Software Foundation, either version 3 of the License, or (at
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your option) any later version.
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This program is distributed in the hope that it will be useful, but WITHOUT
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ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or
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FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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for more details.
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You should have received a copy of the GNU General Public License along
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with this program. If not, see \hbox{<http://www.gnu.org/licenses/>} for a
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copy.
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\end{license}
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\begin{revisionhistory}
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0.1 & 5/25/2015 & Gisselquist & First Draft \\\hline
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\end{revisionhistory}
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% Revision History
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% Table of Contents, named Contents
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\tableofcontents
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% \listoffigures
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\listoftables
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\begin{preface}
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Every FPGA project needs to start with a very simple core. Then, working
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from simplicity, more and more complex cores can be built until an eventual
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application comes from all the tiny details.
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This real time clock is one such simple core. All of the pieces to this
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clock are simple. Nothing is inherently complex. However, placing this
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clock into a larger FPGA structure requires a Wishbone bus, and being able
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to command and control an FPGA over a wishbone bus is an achievement in
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itself. Further, the clock produces seven segment display output values
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and LED output values. These are also simple outputs, but still take a lot
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of work to complete. Finally, this clock will strobe an interrupt line.
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Reading and processing that interrupt line requires a whole 'nuther bit of
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logic and the ability to capture, recognize, and respond to interrupts.
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Hence, once you get a simple clock working, you have a lot working.
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\end{preface}
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\chapter{Introduction}
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\pagenumbering{arabic}
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\setcounter{page}{1}
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This Real--Time Clock implements a twenty four hour clock, count-down timer,
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stopwatch
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and alarm. It is designed to be configurable to adjust to whatever clock
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speed the underlying architecture is running on, so with only minor changes
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should run on any fundamental clock rate from about 66~kHz on up to
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250~TeraHertz with varying levels of accuracy along the way.
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This clock offers a fairly full feature set of capability: time, alarms,
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a countdown timer and a stopwatch, all features which are available from the
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wishbone bus.
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Other interfaces exist as well.
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Should you wish to investigate your clock's
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stability or try to guarantee it's fine precision accuracy, it is possible to
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provide a time hack pulse to the clock and subsequently read what all of the
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internal registers were set to at that time.
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When either the count--down timer reaches zero or the clock reaches the alarm
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time (if set), the clock module will produce an impulse which can be used
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as an interrupt trigger.
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This clock will also provide outputs sufficient to drive an external seven
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segment display driver and 16 LED's.
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Future enhancements may allow for button control and fine precision clock
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adjustment.
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The layout of this specification follows the format set by OpenCores.
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This introduction is the first chapter. Following this introduction is
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a short chapter describing how this clock is implemented,
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Chapt.~\ref{chap:arch}. Following this description, the Chapt.~\ref{chap:ops}
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gives a brief overview of how to operate the clock. Most of the details,
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however, are in the registers and their definitions. These you can find in
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Chapt.~\ref{chap:regs}. As for the wishbone, the wishbone spec requires a
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wishbone datasheet which you can find in Chapt.~\ref{chap:wishbone}.
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That leaves the final pertinent information necessary for implementing this
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core in Chapt.~\ref{chap:ioports}, the definitions and meanings of the
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various I/O ports.
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As always, write me if you have any questions or problems.
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\chapter{Architecture}\label{chap:arch}
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Central to this real time clock architecture is a 48~bit sub--second register.
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This register is incremented every clock by a user defined 32~bit value,
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{\tt CKSPEED}.
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When the register turns over at the end of each second, a second has taken
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place and all of the various clock registers are adjusted.
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Well, not quite but almost. The 48~bit register is actually split into a
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lower 40~bit register that is common to all clock components, as well as
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separate eight bit upper registers for the clock, timer, and stopwatch. In
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this fashion, these separate components can have different definitions for
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when seconds begin and end, and with sufficient precision to satisfy most
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applications.
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The next thing to note about this architecture is the format of the various
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clock registers: Binary Coded Decimal, or BCD. Hence an {\tt 8'h59} refers
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to a value of 59, rather than 89. In this fashion, setting the time to
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{\tt 24'h231520} will set it to 23~hours, 15~minutes, and 20~seconds. The
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only exception to this BCD format are the subseconds fields found in the
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stopwatch and time hack registers. Seconds and above are all encoded as BCD.
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\chapter{Operation}\label{chap:ops}
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\section{Time}
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To set the time, simply write to the clock register the current value of the
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time. If the seconds hand is written as zero, subsecond time will be cleared
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as well. The new clock value takes place one clock period after the value
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is written to the bus.
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To set only some parts of the time and not others, such as the minutes but
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not seconds or hours, write all '1's to the seconds and hours. In this way,
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writing a {\tt 24'h3f17ff} will set the minutes to 17, but not affect the
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rest of the clock.
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This is also the way to adjust the display without adjusting time. Suppose
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you wish to switch to display option '1', just write a {\tt 32'h013fffff} to
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the register and the display will switch without adjusting time.
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\section{Count-down Timer}
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To use the count down timer, set it to the amount of time you wish to count
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down for. When ready, or even in the same cycle, enable the count--down
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timer by setting the RUN bit high. At this point in time, the count--down
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timer is running. When it gets to zero, it will stop and trigger an interrupt.
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You can tell if the alarm has been triggered by the TRIGGER bit being set.
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Any write to the timer register will clear the alarm condition.
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While the timer is running, writing a '0' to the timer register will stop it
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without clearing the time remaining. In this state, writing to the register
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the RUN bit by itself will restart the timer, while anything else will set the
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timer to a new value. Further, if the timer is stopped at zero, then writing
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zero to the timer will reset the timer to the last start time it had.
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\section{Stopwatch}
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The stop watch supports three operations: start, stop, and clear. Writing a
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'1' to the stop watch register will start the stopwatch, while writing a '0'
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will stop it. When it starts next, it will start where it left off unless the
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register is cleared. To clear the register and set it back to zero, write a
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'2' to the register. This will effectively stop the register and clear it in
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one step. If the register is already stopped, writing a '3' will clear and
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start it in one step. However, the register can only be cleared while stopped.
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If the register is running, writing a '3' will have no effect.
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\section{Alarm}
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To set the alarm, just write the alarm time to the alarm register together
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with alarm enable bit. As with the time register, setting any field,
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whether hours, minutes, or seconds, to {\tt 8'hff} has no effect on that
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field. Hence, the alarm may be activated by writing {\tt 25'h13fffff} to
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the register and deactivated by writing {\tt 25'h03fffff}.
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Once the alarm is tripped, the RTC core will generate an interrupt. Further,
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the tripped bit in the alarm register will be set. To clear this bit and the
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alarm tripped condition, either disable the alarm or write a '1' to this bit.
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\section{Time Hacks}
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For finer precision timing, the RTC module allows for setting a time
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hack and reading the value from the device. On the clock following the
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time hack being high, the internal state, to include the time and the 48~bit
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counter, will be recorded and may then be read out. In this fashion,
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it is possible to capture, with as much precision as the device offers,
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the current time within the device.
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It is the users responsibility to read the time hack registers before a
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subsequent time hack pulse sets them to new values.
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\chapter{Registers}\label{chap:regs}
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This RTC clock module supports eight registers, as listed in
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Tbl.~\ref{tbl:reglist}. Of these eight, the first four have been so placed
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as to be the more routine or user used registers, while the latter four are
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more lower level.
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\begin{table}[htbp]
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\begin{center}
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\begin{reglist}
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CLOCK & 0 & 32 & R/W & Wall clock time register\\\hline
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TIMER & 1 & 32 & R/W & Count--down timer\\\hline
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STPWTCH & 2 & 32 & R/W & Stopwatch control and value\\\hline
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ALARM & 3 & 32 & R/W & Alarm time, and setting\\\hline\hline
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CKSPEED & 4 & 32 & R/W & Clock speed control.\\\hline
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HACKTIME &5 & 32 & R & Wall clock time at last hack.\\\hline
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HACKCNTHI&6 & 32 & R & Wall clock time.\\\hline
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HACKCNTLO&7 & 32 & R & Wall clock time.\\\hline
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\end{reglist}\caption{List of Registers}\label{tbl:reglist}
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\end{center}\end{table}
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Each register will be discussed in detail in this chapter.
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\section{Clock Time Register}
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The various bit fields associated with the current time may be found in
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the {\tt CLOCK} register, shown in Tbl.~\ref{tbl:clockreg}.
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\begin{table}[htbp]\begin{center}
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\begin{bitlist}
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28--31 & R & Always return zero.\\\hline
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24--27 & R/W & Seven Segment Display Mode.\\\hline
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22--23 & R & Always return zero.\\\hline
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16--21 & R/W & Current time, BCD hours\\\hline
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8--15 & R/W & Current time, BCD minutes\\\hline
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0--7 & R/W & Current time, BCD seconds\\\hline
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\end{bitlist}
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\caption{Clock Time Register Bit Definitions}\label{tbl:clockreg}
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\end{center}\end{table}
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This register contains six clock digits: two each for hours, minutes, and
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seconds. Each of these digits is encoded in Binary Coded Decimal (BCD).
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Therefore, 23~hours would be encoded as 6'h23 and not 6'h17. Writes to each
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of the various subcomponent registers will set that register, unless the
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write value is a 8'hff. The behaviour of the clock when non--decimal
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values are written, other than all F's, is undefined.
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Separate from the time, however, is the seven segment display mode. Four
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values are currently supported: 4'h0 to display the hours and minutes,
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4'h1 to display the timer in minutes and seconds, 4'h2 to display the
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stopwatch in lower order minutes, seconds, and sixteenths of a second, and
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4'h3 to display the minutes and seconds of the current time. In all cases,
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the decimal point will appear to the right of the lowest order digit
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and will blink with the second hand. That is, the decimal will be high for
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the second half of any second, and low at the top of the second.
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\section{Countdown Timer Register}
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The countdown timer register, whose bit--wise values are shown in
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Tbl.~\ref{tbl:timer},
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\begin{table}[htbp]
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\begin{center}
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\begin{bitlist}
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26--31 & R & Unused, always read as '0'.\\\hline
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25 & R/W & Alarm condition. Write a '1' to clear.\\\hline
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24 & R/W & Running, stopped on '0'\\\hline
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16--23 & R/W & BCD Hours\\\hline
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8--15 & R/W & BCD Minutes\\\hline
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0--7 & R/W & BCD Seconds\\\hline
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\end{bitlist}
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\caption{Count--down Timer register}\label{tbl:timer}
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\end{center}\end{table}
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controls the operation of the count--down timer. To use this timer, write
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some amount of time to the register, then write zeros with bit 24 set. The
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register will then reach an alarm condition after counting down that amount
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of time. (Alternatively, you could set bit 24 while writing the register,
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to set and start it in one operation.) To stop the register while it is
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running, just write all zeros. To restart the register, provided more than a
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second remains, write a {\tt 26'h1000000} to set it running again. Once
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the timer alarms, the timer will stop and the alarm condition will be set.
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Any write to the timer register after the alarm condition has been set will
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clear the alarm condition.
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\section{Stopwatch Register}
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The various bits of the stopwatch register are shown in
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Tbl.~\ref{tbl:stopwatch}.
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\begin{table}[htbp]
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\begin{center}
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\begin{bitlist}
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24--31 & R & Hours\\\hline
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16--23 & R & Minutes\\\hline
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8--15 & R & Sub Seconds\\\hline
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1--7 & R & Sub Seconds\\\hline
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1 & W & Clear\\\hline
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\end{bitlist}
|
316 |
|
|
\caption{Stopwatch Register}\label{tbl:stopwatch}
|
317 |
|
|
\end{center}\end{table}
|
318 |
|
|
Of note is the bottom bit that, when set, means the stop watch is running.
|
319 |
|
|
Set this bit to '1' to start the stopwatch, or to '0' to stop the stopwatch.
|
320 |
|
|
Further, while the stopwatch is stopped, a '1' can be written to the clear
|
321 |
|
|
bit. This will zero out the stopwatch and set it back to zero.
|
322 |
|
|
|
323 |
|
|
\section{Alarm Register}
|
324 |
|
|
The various bits of the alarm register are shown in Tbl.~\ref{tbl:alarm}.
|
325 |
|
|
\begin{table}[htbp]
|
326 |
|
|
\begin{center}
|
327 |
|
|
\begin{bitlist}
|
328 |
|
|
26--31 & R & Always reads zeros. \\\hline
|
329 |
|
|
25 & R/W & Alarm tripped. Write a '1' to this register to clear any alarm
|
330 |
|
|
condition. (A tripped alarm will not trip again.)\\\hline
|
331 |
|
|
24 & R/W & Alarm enabled\\\hline
|
332 |
|
|
16--23 & R & Alarm time, BCD hours\\\hline
|
333 |
|
|
8--15 & R & Alarm time, BCD minutes\\\hline
|
334 |
|
|
0--7 & R/W & Alarm time, BCD Seconds\\\hline
|
335 |
|
|
\end{bitlist}
|
336 |
|
|
\caption{Alarm Register}\label{tbl:alarm}
|
337 |
|
|
\end{center}\end{table}
|
338 |
|
|
Basically, the alarm register consists a time and two more bits. The extra
|
339 |
|
|
two bits encode whether or not the alarm is enabled, and whether or not it has
|
340 |
|
|
been tripped. The alarm will be {\em tripped} whenever it is enabled, and the
|
341 |
|
|
time changes to equal the alarm time. Once tripped, the alarm will stay
|
342 |
|
|
in the alarmed or tripped condition until either a '1' is written to the
|
343 |
|
|
tripped bit, or the alarm is disabled.
|
344 |
|
|
|
345 |
|
|
As with the clock and timer registers, writing eight ones to any of the
|
346 |
|
|
BCD fields when writing to this register will leave those fields untouched.
|
347 |
|
|
|
348 |
|
|
\section{Clock Speed Register}
|
349 |
|
|
The actual speed of the clock is controlled by the {\tt CKSPEED} register,
|
350 |
|
|
shown in Tbl.~\ref{tbl:ckspeed}.
|
351 |
|
|
\begin{table}[htbp]
|
352 |
|
|
\begin{center}
|
353 |
|
|
\begin{bitlist}
|
354 |
|
|
0--31 & R/W & 48~bit counter time increment\\\hline
|
355 |
|
|
\end{bitlist}
|
356 |
|
|
\caption{Clock Speed Register}\label{tbl:ckspeed}
|
357 |
|
|
\end{center}\end{table}
|
358 |
|
|
This register contains a simple 32~bit unsigned value. To step the clock,
|
359 |
|
|
this value is extended to 48~bits and added to the fractional seconds value.
|
360 |
|
|
|
361 |
|
|
This value should be set to $2^{48}$ divided by the clock frequency of the
|
362 |
|
|
controlling clock. Hence, for a 100~MHz clock, this value would be set to
|
363 |
|
|
{\tt 32'd2814750}. For clocks near 100~MHz, this allows adjusting speed
|
364 |
|
|
within about 40~clocks per second. For clocks near 500~MHz, this allows
|
365 |
|
|
time adjustment to an accuracy of about about 800~clocks per second. In
|
366 |
|
|
both cases, this is good enough to maintain a clock with less than a
|
367 |
|
|
microsecond loss over the course of a year. Hence, this RTC module provides
|
368 |
|
|
more logical stability than most hardware clocks on the market today.
|
369 |
|
|
|
370 |
|
|
\section{Time--hack time}
|
371 |
|
|
To support finer precision clock control, the time--hack capability exists.
|
372 |
|
|
This capability consists of three registers, the time--hack time register
|
373 |
|
|
shown in Tbl.~\ref{tbl:hacktime},
|
374 |
|
|
\begin{table}[htbp]
|
375 |
|
|
\begin{center}
|
376 |
|
|
\begin{bitlist}
|
377 |
|
|
24--31 & R & BCD Hours.\\\hline
|
378 |
|
|
16--23 & R & BCD Minutes.\\\hline
|
379 |
|
|
8--15 & R & BCD seconds.\\\hline
|
380 |
|
|
0--7 & R & Subseconds, encoded in 256ths of a second\\\hline
|
381 |
|
|
\end{bitlist}
|
382 |
|
|
\caption{Time Hack Time Register}\label{tbl:hacktime}
|
383 |
|
|
\end{center}\end{table}
|
384 |
|
|
and two registers (Tbls.~\ref{tbl:hackcnthi}
|
385 |
|
|
\begin{table}[htbp]
|
386 |
|
|
\begin{center}
|
387 |
|
|
\begin{bitlist}
|
388 |
|
|
0--31 & R & Upper 32 bits of the internal 40~bit counter.\\\hline
|
389 |
|
|
\end{bitlist}
|
390 |
|
|
\caption{Time Hack Counter, High}\label{tbl:hackcnthi}
|
391 |
|
|
\end{center}\end{table}
|
392 |
|
|
and~\ref{tbl:hackcntlo})
|
393 |
|
|
\begin{table}[htbp]
|
394 |
|
|
\begin{center}
|
395 |
|
|
\begin{bitlist}
|
396 |
|
|
24--31 & R & Bottom 8~bits of the internal 40~bit counter.\\\hline
|
397 |
|
|
0--23 & R & Always read as '0'.\\\hline
|
398 |
|
|
\end{bitlist}
|
399 |
|
|
\caption{Time Hack Counter, Low}\label{tbl:hackcntlo}
|
400 |
|
|
\end{center}\end{table}
|
401 |
|
|
capturing the contents of the 40~bit internal counter at the time of the hack.
|
402 |
|
|
|
403 |
|
|
The time--hack time register is perhaps the simplest to understand. This
|
404 |
|
|
captures the time of the time--hack in hours, minutes, seconds, and 8~fractional
|
405 |
|
|
subsecond bits. The top 24~bits of this register will match the bottom 24~bits
|
406 |
|
|
of the clock~time register at the time of the time hack. The bottom eight
|
407 |
|
|
bits are the top eight bits of the 48~bit subsecond time counter. The
|
408 |
|
|
rest of those 48~bits may then be returned in the other two time hack counter
|
409 |
|
|
registers.
|
410 |
|
|
|
411 |
|
|
At present, this functionality isn't yet truly fully featured. Once fully
|
412 |
|
|
featured, there will (should) be a mechanism for adjusting this counter based
|
413 |
|
|
upon information gleaned from the hack time. Implementation details have
|
414 |
|
|
to date prevented this portion of the design from being implemented.
|
415 |
|
|
|
416 |
|
|
\chapter{Wishbone Datasheet}\label{chap:wishbone}
|
417 |
|
|
Tbl.~\ref{tbl:wishbone}
|
418 |
|
|
\begin{table}[htbp]
|
419 |
|
|
\begin{center}
|
420 |
|
|
\begin{wishboneds}
|
421 |
|
|
Revision level of wishbone & WB B4 spec \\\hline
|
422 |
|
|
Type of interface & Slave, Read/Write \\\hline
|
423 |
|
|
Port size & 32--bit \\\hline
|
424 |
|
|
Port granularity & 32--bit \\\hline
|
425 |
|
|
Maximum Operand Size & 32--bit \\\hline
|
426 |
|
|
Data transfer ordering & (Irrelevant) \\\hline
|
427 |
|
|
Clock constraints & Faster than 66~kHz \\\hline
|
428 |
|
|
Signal Names & \begin{tabular}{ll}
|
429 |
|
|
Signal Name & Wishbone Equivalent \\\hline
|
430 |
|
|
{\tt i\_clk} & {\tt CLK\_I} \\
|
431 |
|
|
{\tt i\_wb\_cyc} & {\tt CYC\_I} \\
|
432 |
|
|
{\tt i\_wb\_stb} & {\tt STB\_I} \\
|
433 |
|
|
{\tt i\_wb\_we} & {\tt WE\_I} \\
|
434 |
|
|
{\tt i\_wb\_addr} & {\tt ADR\_I} \\
|
435 |
|
|
{\tt i\_wb\_data} & {\tt DAT\_I} \\
|
436 |
|
|
{\tt o\_wb\_ack} & {\tt ACK\_O} \\
|
437 |
|
|
{\tt o\_wb\_stall} & {\tt STALL\_O} \\
|
438 |
|
|
{\tt o\_wb\_data} & {\tt DAT\_O}
|
439 |
|
|
\end{tabular}\\\hline
|
440 |
|
|
\end{wishboneds}
|
441 |
|
|
\caption{Wishbone Datasheet}\label{tbl:wishbone}
|
442 |
|
|
\end{center}\end{table}
|
443 |
|
|
is required by the wishbone specification, and so
|
444 |
|
|
it is included here. The big thing to notice is that this real time clock
|
445 |
|
|
acts as a wishbone slave, and that all accesses to the
|
446 |
|
|
clock registers are 32--bit reads and writes. The address bus does not offer
|
447 |
|
|
byte level, but rather 32--bit word level resolution. Select lines are not
|
448 |
|
|
implemented. Bit ordering is the normal ordering where bit~31 is the most
|
449 |
|
|
significant bit and so forth. Although the stall line is implemented, it is
|
450 |
|
|
always zero. Access delays are a single clock, so the clock after a read or
|
451 |
|
|
write is placed on the bus the {\tt i\_wb\_ack} line will be high.
|
452 |
|
|
|
453 |
|
|
\iffalse
|
454 |
|
|
\chapter{Clocks}\label{chap:clocks}
|
455 |
|
|
|
456 |
|
|
This core is based upon the Basys--3 design. The Basys--3 development board
|
457 |
|
|
contains one external 100~MHz clock, which is sufficient to run this
|
458 |
|
|
core. The logic within the core can also be run faster, or slower, as is
|
459 |
|
|
necessary to meet the timing constraints associated with the internal
|
460 |
|
|
operations of the core and it's surrounding environment. See
|
461 |
|
|
Table.~\ref{tbl:clocks}.
|
462 |
|
|
\begin{table}[htbp]
|
463 |
|
|
\begin{center}
|
464 |
|
|
\begin{clocklist}
|
465 |
|
|
i\_clk & External & 250~THz & 66~kHz & System clock.\\\hline
|
466 |
|
|
\end{clocklist}
|
467 |
|
|
\caption{List of Clocks}\label{tbl:clocks}
|
468 |
|
|
\end{center}\end{table}
|
469 |
|
|
|
470 |
|
|
\fi
|
471 |
|
|
|
472 |
|
|
\chapter{I/O Ports}\label{chap:ioports}
|
473 |
|
|
The I/O ports for this core are shown in Tbls.~\ref{tbl:iowishbone}
|
474 |
|
|
\begin{table}[htbp]
|
475 |
|
|
\begin{center}
|
476 |
|
|
\begin{portlist}
|
477 |
|
|
i\_wb\_cyc & 1 & Input & Wishbone bus cycle wire.\\\hline
|
478 |
|
|
i\_wb\_stb & 1 & Input & Wishbone strobe.\\\hline
|
479 |
|
|
i\_wb\_we & 1 & Input & Wishbone write enable.\\\hline
|
480 |
|
|
i\_wb\_addr & 5 & Input & Wishbone address.\\\hline
|
481 |
|
|
i\_wb\_data & 32 & Input & Wishbone bus data register for use when writing
|
482 |
|
|
(configuring) the core from the bus.\\\hline
|
483 |
|
|
o\_wb\_ack & 1 & Output & Return value acknowledging a wishbone write, or
|
484 |
|
|
signifying valid data in the case of a wishbone read request.
|
485 |
|
|
\\\hline
|
486 |
|
|
o\_wb\_stall & 1 & Output & Indicates the device is not yet ready for another
|
487 |
|
|
wishbone access, effectively stalling the bus.\\\hline
|
488 |
|
|
o\_wb\_data & 32 & Output & Wishbone data bus, returning data values read
|
489 |
|
|
from the interface.\\\hline
|
490 |
|
|
\end{portlist}
|
491 |
|
|
\caption{Wishbone I/O Ports}\label{tbl:iowishbone}
|
492 |
|
|
\end{center}\end{table}
|
493 |
|
|
and~Tbl.~\ref{tbl:ioother}.
|
494 |
|
|
\begin{table}[htbp]
|
495 |
|
|
\begin{center}
|
496 |
|
|
\begin{portlist}
|
497 |
|
|
o\_sseg & 32 & Output & Lines to control a seven segment display, to be
|
498 |
|
|
sent to that display's driver. Each eight bit byte controls
|
499 |
|
|
one digit in the display, with the bottom bit in the byte
|
500 |
|
|
controlling the decimal point.\\\hline
|
501 |
|
|
o\_led & 16 & Output & Output LED's, consisting of a 16--bit counter counting
|
502 |
|
|
from zero to all ones each minute, and synchronized with each
|
503 |
|
|
minute so as to create an indicator of when the next minute
|
504 |
|
|
will take place when only the hours and minutes can be
|
505 |
|
|
displayed.\\\hline
|
506 |
|
|
o\_interrupt & 1 & Output & A pulsed/strobed interrupt line. When the
|
507 |
|
|
clock needs to generate an interrupt, it will set this line
|
508 |
|
|
high for one clock cycle. \\\hline
|
509 |
|
|
i\_hack & 1 & Input & When this line is raised, copies are made of the
|
510 |
|
|
internal state registers on the next clock. These registers can then
|
511 |
|
|
be used for an accurate time hack regarding the state of the clock
|
512 |
|
|
at the time this line was strobed.\\\hline
|
513 |
|
|
\end{portlist}
|
514 |
|
|
\caption{Other I/O Ports}\label{tbl:ioother}
|
515 |
|
|
\end{center}\end{table}
|
516 |
|
|
Tbl.~\ref{tbl:iowishbone} reiterates the wishbone I/O values just discussed in
|
517 |
|
|
Chapt.~\ref{chap:wishbone}, and so need no further discussion here.
|
518 |
|
|
|
519 |
|
|
This clock is designed for command and control via the wishbone. No other
|
520 |
|
|
registers, beyond the wishbone bus, are required. However, several other
|
521 |
|
|
may be valuable. These other registers are listed in Tbl.~\ref{tbl:ioother}.
|
522 |
|
|
We'll discuss each of these in turn.
|
523 |
|
|
|
524 |
|
|
First of the other I/O registers is the {\tt o\_sseg} register. This register
|
525 |
|
|
encodes which outputs of a seven segment display need to be turned on to
|
526 |
|
|
represent the value of the clock requested. This register consists of four
|
527 |
|
|
eight bit bytes, with the highest order byte referencing the highest order
|
528 |
|
|
display segment value. In each byte, the low order bit references a decimal
|
529 |
|
|
point. The other bits are ordered around the zero, with the top bit being
|
530 |
|
|
the top bar of a '0', the next highest order bit and so on following the
|
531 |
|
|
zero clockwise. The final bit of each byte, the bit in the two's place,
|
532 |
|
|
encodes whether or not the middle line is to be displayed. When either timer
|
533 |
|
|
or alarm is triggered, this display will blink until the triggering conditions
|
534 |
|
|
are cleared.
|
535 |
|
|
|
536 |
|
|
This output is expected to be the input to a seven segment display driver,
|
537 |
|
|
rather than being the output to the display itself.
|
538 |
|
|
|
539 |
|
|
The next output lines are the 16~lines of the {\tt o\_led} bus. When connected
|
540 |
|
|
with 16~LED's, these lines will create a counting display that will count up
|
541 |
|
|
to each minute, synchronized to the minute. When either timer or alarm has
|
542 |
|
|
triggered, all of the LED's will flash together until the triggered condition
|
543 |
|
|
is reset.
|
544 |
|
|
|
545 |
|
|
The third other line is the {\tt o\_interrupt} line. This line will be
|
546 |
|
|
strobed by the RTC module any time the alarm is triggered or the timer runs
|
547 |
|
|
out. The line will not remain high, but neither will it trigger a second
|
548 |
|
|
time until the underlying interrupt is cleared. That is, the timer will only
|
549 |
|
|
trigger once until cleared as will the alarm, but the alarm may trigger after
|
550 |
|
|
the timer has triggered and before the timer clears.
|
551 |
|
|
|
552 |
|
|
The final other I/O line is a simple input line. This line is expected to be
|
553 |
|
|
strobed for one clock cycle any time a time hack is required. For example,
|
554 |
|
|
should you wish to read and synchronize to a GPS PPS signal, strobe the device
|
555 |
|
|
with the PPS (after dealing with any metastability issues), and read the time
|
556 |
|
|
hacks that are produced.
|
557 |
|
|
|
558 |
|
|
% Appendices
|
559 |
|
|
% Index
|
560 |
|
|
\end{document}
|
561 |
|
|
|
562 |
|
|
|