ACU Real Time Clock (DIY)

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

• The RTC 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).

Uses a Z80 PIO, MM58174 RTC, 6116 (2Kx8bit SRAM), 4040 (12bit counter for sequential SRAM addressing), and 32.768kHz Crystal (for the RTC). Both RTC and RAM are backed by rechargeable 3.6V NiCad battery. Used I/O Ports are (with only A10,A4,A3 decoded):

 FBE7h  ACU Real Time Clock Z80 PIO Data In/Out Port A
             Bit7..4  SRAM Data7..4
             Bit3..0  SRAM Data3..0 and 4bit RTC Data
 FBEFh  ACU Real Time Clock Z80 PIO Data In/Out Port B
             Bit7     SRAM /CS and /OE (0=Select)
             Bit6     RTC /CS (0=Select)
             Bit5     RTC /RD (0=Read)
             Bit4     RTC /WR (0=Write)
             Bit3     RTC Address A3 and SRAM /WE (0=Write)
             Bit2     RTC Address A2 and SRAM Address Counter RST (1=Reset, 0=Active)
             Bit1     RTC Address A1
             Bit0     RTC Address A0 and SRAM Address Counter /CLK (1-to-0=Increment)
 FBF7h  ACU Real Time Clock Z80 PIO Configure Port A
 FBFFh  ACU Real Time Clock Z80 PIO Configure Port B

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 (0=Stop and set Seconds to 00.0, 1=Start) (W)
 F  Interrupt (R/W)

Registers 1..C are BCD (range 0..9), a value of 0Fh in these registers indicates Failure (this may happen if the time changed while reading - if that happens, restart reading ALL time registers).

Observe that the part number, MM58147, in the headline is wrong (it's meant to be MM58174).