Real Time Clock
This Real Time Clock addon was published as a DIY project in a special edition of the the German magazine: CPC Schneider International(Sonderheft, #3-1986)
Like most other homecomputers of the era, the Amstrad CPC did not include a Real Time Clock.
This project overcomes this limitation.
Technical specs
Uses a RTC-58321 chip (with 4bit databus and built-in 32.786kHz oscillator), four 74LSxx chips, and a rechargeable battery. Port addresses are jumper-selectable (the defaults, used in the included example program, are FBE2h/FBE3h):
FBE2h FBE6h FBEAh FBEEh FBF2h FBF6h FBFAh FBFEh - RTC-58321 Index (W) FBE3h FBE7h FBEBh FBEFh FBF3h FBF7h FBFBh FBFFh - RTC-58321 Data (R/W)
All registers are 4bit wide (Bit0-3 used, Bit4-7=don't care/garbage):
0 S1 Seconds units (0..9) 1 S10 Seconds tens (0..5) (bit3=unused) 2 MI1 Minutes units (0..9) 3 MI10 Minutes tens (0..5) (bit3=unused) 4 H1 Hours units (0..9) 5 H10 Hours tens (0..1 or 0..2) (bit2=PM, bit3=24-hour) 6 W Day-of-week (0..6) (bit3=unused) (Epson datasheet: 0=Sunday) 7 D1 Day units (0..9) 8 D10 Day tens (0..3) (bit2-3=Leap year offset, usually zero) 9 MO1 Month units (0..9) A MO10 Month tens (0..1) (bit1-3=unused) B Y1 Year units (0..9) C Y10 Year tens (0..9) D RES Reset register (strobe) (bit0-3=unused) E REF Reference signal (bit0=1024Hz, bit1=1sec, bit2=1min, bit3=1hour) F REF Reference signal (bit0=1024Hz, bit1=1sec, bit2=1min, bit3=1hour)
In 24hour mode: range is 00:00 to 23:59 and the PM flag is always zero. In 12hour mode: range is 12:00 AM to 11:59 PM. Day and month start counting with one (ie. 01..31 and 01..12). Day-of-Week is a general purpose 7-day counter (the Epson datasheet suggests 0=Sunday, the CPC magazine suggests 0=Monday). Leap years are generated when "((Year + LeapYearOffset) MOD 4)=0", which can be matched to different calendars:
1926..1989 Showa 1..64 (reign of japanese emperor Hirohito) 1989..today Heisei 1..today (reign of japanese emperor Akihito)
Ie. for Gregorian and Heisei calendars set LeapYearOffset=0, for Showa calendar set LeapYearOffset=1 (as described in the CPC magazine). Anyways, the article recommends to use Gregorian calendar, ie. using year=86 for 1986, so, if the battery survived, it wrapped to year=00 in year 2000.
Commercial versions
There are or have been other projects with an own Real Time Clock solution:
How to build a RTC interface (German)
- RTC
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