ACU Real Time Clock (DIY)

ACU Real Time Clock, DIY from magazine "Amstrad User April 85".

Uses a Z80 PIO, MM58174 RTC, 6116 (2K RAM), 32.768kHz Crystal. Both RTC and RAM are backed by rechargeable 3.6V NiCad battery. Used I/O Ports are (with only A10,A4,A3 decoded):

 FBE7 Z80 PIO Data In/Out Port A
 FBEF Z80 PIO Data In/Out Port B
 FBF7 Z80 PIO Configure Port A
 FBFF Z80 PIO Configure Port A

The 4bit RTC Registers are:

 0  Not used / Test (W)
 1  Seconds 1/10s Fraction (0..9) (R)
 2  Seconds LSB (0..9) (R)
 3  Seconds MSB (0..5) (R)
 4  Minutes LSB (0..9) (R/W)
 5  Minutes MSB (0..5) (R/W)
 6  Hours LSB (0..9) (R/W)
 7  Hours MSB (0..2) (R/W)
 8  Day LSB (0..9) (R/W)
 9  Day MSB (0..3) (R/W)
 A  Day of Week (1..7) (R/W)
 B  Month LSB (0..9) (R/W)
 C  Month MSB (0..1) (R/W)
 D  Leap Year Shift Register (W)
 E  Stop/Start (W)
 F  Interrupt (R/W)

Registers 1..C are BCD (range 0..9), a value of 0Fh in these registers indicates Failure. The chip doesn't include a Year register (aside from the leap-year shift register, but it can span only 4 years, and it is write-only).