As an exclusive to readers of this launch edition of this brand new column, Wilf has
given permission for the following article on his latest hardware project to be published
here in its entirety.
He's literally just finished writing it and posted it over to me whereupon, I have
edited it for HTML.
By the way, I was going to put up a MAJOR new article that Wilf had sent over to me in
BETA form and given permission for me to post here - but due to the subjects significance
and importance (I don't think there has ever been a work like this before), I have
contacted Wilf - and decided not to post it until it is all finished.
It will be worth the wait! ...
Anyway, I know you will all find this articles contents fascinating and remember, you
can't get it anywhere else - so without further delay - over to Wilf...
ZX81 REAL TIME CLOCK PROJECT
Written and developed by Wilf Rigter
Last revised: October, 1996
Here is a simple Real Time Clock (RTC) project I have recently built and tested on a
ZX81 but should be compatible with the Spectrum and other Z80 machines. Cost and
simplicity are the main reasons for choosing the DALLAS DS1287. These RTC modules were
used on some 286 AT motherboards which can often be had for free and contain other useful
parts for ZX81 projects. The DS1287 is functionally equivalent to the more common MC146818
but integrates all external components including crystal and battery in a single 24 pin
DIP module. Check the date code on the unit to determine the remaining life of the
internal lithium battery, which is normally good for 10 years or more. An internal flag
can also be used to verify a good battery. The DS1287 is designed to work with multiplexed
address/data bus MPU's like the 6805 or the 8088. The databook shows an example of a 68000
application but there are no Z80 application examples given. This may be the reason it is
seldom used in Z80 designs although the interface is straight forward. Rather than
multiplexing the data and address, I used two separate IO addresses: one for the address
port and one for the data port.
The DS1287/Z80 interface programmer model is a block of 80 bytes, each of which can be
selected by writing a byte address (0 to 79 decimal) to the address port and reading or
writing data for that byte through the data port. The 74HC138 decodes IO addresses
1F,3F,.,FF any two of which (except FF) can be assigned to the RTC. In this example I have
used 9F for the address register and BF for the data register. The AS address strobe and
the DS data strobe are active high and CE is active low. The R/W pin is connected to Z80
inverted RD line for timing purposes. CS is active only when selecting data port. The
reset pin may also be connected to the VCC line.
In this application I have connected an LED to the SQW output pin to provide a 2 Hz
blinking indication light. The SQW frequency is controlled with REGISTER A bits 0-3.
The IRQ pin is connected to a piezo buzzer which turns on when the RTC bytes are equal
to the ALARM bytes. The AIE alarm interrupt must be enabled in REGISTER B and when the IRQ
output is active it can be reset by reading the data of REGISTER C. The unused 74HC14
Schmitt trigger inverters can be used as oscillators, latches or push-button conditioning
for other applications.
I have included a brief introduction to the DS1287 specifications and some programming
examples to get you started, however you should consult the MC146818 or the DS1287 data
manual for more complex applications.
DS1287 IC pins:
Pin 1 Mode=1 for Motorola control bus timing used in this application.
Pin 4-11 AD0-7 used to load the internal address and data.
Pin 12 Vss connected to powersupply common.
Pin 13 CS active low chip select enables data read and write cycles.
Pin 14 AS active high loads internal address from AD0-7.
Pin 15 R/W select read or write cycle inverted RD is used to enable.
write cycle : inverter required for minimum hold time.
Pin 17 DS active high read or write data to internal address.
Pin 18 RESET active low to initialize flags etc. but not RTC data.
Pin 19 IRQ active low programmable interrupt request (i.e. piezo alarm).
Pin 23 SQW user programmable frequency output (i.e. flashing LED).
Pin 24 Vcc connected to +5V powersupply.
Note: M146818 uses additional pins for external crystal etc.
Consult data book for complete details.
DS1287 and MC146818 programmer model:
The byte addresses and their functions are as follows:
00 seconds 01 sec alarm
02 minutes 03 min alarm
04 hours 05 hr alarm
06 day of week 07 day of month
08 month 09 year
10 Register A 11 Register B
12 Register C 13 Register D
14-79 user bytes
The RTC bytes (byte 00,02,04,06-09) are updated with current data once
a second at which time the alarm bytes (01,03,05) are compared to the RTC. The alarm bytes
can be loaded with a don't care code (192 decimal) to generate alarms more than once a day
(i.e. once a minute or once an hour).
REGISTER A is a read/write control byte with the following functions:
bit0-3 timebase divider selection i.e. 1111 = 2 Hz square wave
bit4-6 oscillator start code = 010 and any other code to stop
bit7 UIP update in progress bit : clock data is valid when low
Register A can be initialized by with a value of 47 to start the
oscilator and to select a 2 Hz square wave output. The RTC is updated once per second and
the UIP bit goes high 244 us before the RTC data is changed. To avoid errors of reading
changing data during the update, read data only if the UIP bit is low.
REGISTER B is a read/write control byte with the following functions:
bit0 DSE =1 daylight saving time in effect
bit1 24/12=1 enable 24 hour format
bit2 DM =1 RTC data in binary / DM =0 RTC data in BCD
bit3 SQWE =1 enable square wave output
bit4 UIE =1 enable IRQ generated at the end of the update cycle
bit5 AIE =1 enable IRQ generated by RTC alarm byte comparator
bit6 PIE =1 enable IRQ generated from timebase. (ie 2 Hz)
bit7 SET =1 used to freeze RTC updates. Normally SET =0
REGISTER B is initialized to 14 decimal in our simple example
REGISTER C is a read only byte with the following functions :
bit0-3 N/C =0 bits not used
bit4 UF =1 update flag indicates update cycle is finished
bit5 AF =1 alarm flag indicates alarm comparator match
bit6 PF =1 periodic flag set every new square wave period
bit7 IRQF =1 IRQ generated by any of the other status flags
The ZX81 does not provide user defined interrupts but the DS1287 IRQ
pin may be used to drive a small piezo beeper for alarms etc. Reading register C causes
the IRQ pin to be reset.
Register D is read only status byte with the following function:
Bit0-6 N/C =0 bits not used
bit7 VRT =1 indicates the internal lithium battery is good
DS1287 REAL TIME CLOCK CIRCUIT FOR THE ZX81 AND SPECTRUM
_______ |/ |
|DS1287 | 74HC14 |
+5V____________1|MODE | _____
D0____________4|AD0 IRQ|19_____|ALARM|____ ____+5V
D1____________5|AD1 | |_____| |
D2____________6|AD2 | 
D4____________8|AD4 | |\| LED
D6___________10|AD5 | |
D7___________11|AD7 CS|13______________ |
RST___________18|RESET | | |
0V___________12|Vss AS|14_________ | |
|_______| | | |
_______ | | |
|74HC138| HC14 | | |
+5V___________16|Vcc Y4|11___3|\4__| | |
A5____________1|A0 | |/ | HC14 |
A7____________3|A3 | |/
M1____________6|E | Y4 = 9F (ADDRESS PORT)
0V____________5|E | Y5 = BF (DATA PORT)
0V___9|\8___NC 0V___11|\10___NC 0V___13|\12___NC
|/ |/ |/
UNUSED 74HC14 INVERTERS
Note: in this simple example, RTC data is read regardless of update status
;STARTS AT +16516
ADRS LD A,XX ;POKE +16517,INTERNAL BYTE ADDRESS
OUT 9F ;WRITE TO ADDRESS PORT
WR LD A,XX ;POKE +16522, BYTE DATA
OUT BF ;WRITE TO DATA PORT
RD IN A,BF ;READ DATA PORT
LD C,A ;PASS DATA VIA REGISTER BC
LD B,00 ;BACK TO BASIC
DECIMAL CODE STARTING IN 1 REM LINE STARTING AT 16514:
BASIC CODE: RUN 1000 TO INITIALIZE: RUN TO ENTER DATA ETC.
10 PRINT "ENTER W OR R FOLLOWED BY ADDRESS 0 TO 79 (IE W2=MINUTES)"
20 INPUT A$
40 POKE 16516,VAL (A$(2 TO))
50 RAND USR 16516
60 IF A$(1) = "R" THEN PRINT "BYTE " ; A$ (2 TO) ; " = " ; USR 16526
70 IF A$(1) <> "W" THEN GOTO 10
80 PRINT "ENTER DATA FOR BYTE" ; A$ (2 TO)
90 INPUT A
100 POKE 16522,A
110 RAND USR 16526
120 GOTO 10
REM REGISTER A INITIALIZED FOR 2 Hz ON SQW PIN
1000 POKE 16517,10
1010 RAND USR 16516
1020 POKE 16522,47
1030 RAND USR 16521
REM REGISTER B UPDATE/BINARY DATA/24HR FORMAT/ALARM ENABLED
1040 POKE 16517,11
1050 RAND USR 16516
1060 POKE 16522,10
1070 RAND USR 16521
1080 GOTO 10
END OF ARTICLE
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