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.