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).
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