Page 308 - Embedded Microprocessor Systems Real World Design

Other inputs:
Water low switch closure
Power: 9 to 12V DC input, using coaxial connector-onboard 5V regulator
Polarity protected
outputs:

Relay on/off/Relay powered by unregulated DC input
Other outputs:
Watchdog timer required

Circuit Description (Postdesign)

CPU: 8031, 6MHz input clock
EPROM: 8K x 8, external (2764); no internal ROM
8031 port usage:
Ports 0,Z: Address/data bus for external memory access
Port 1: LED/display control
Bit 0 Zero enables minutes, ones display digit
Bit 1: Zero enables minutes, tens display digit
Bit 2: Zero enables hours, ones display digit
Bit 3: Zero enables hours, tens display digit
Bit 4: One turns on override LED
Bit 5: Unused
Bit 6: One turns on set LED
Bit 7: One turns on ON LED
Port 3:
Bit 0: Unused
Bit 1: One turns on motor relay
Bit 2: Toggle to trigger watchdog timer
Bits 3 to 5: Unused
Bits 6 and 7: External register access (RD/WR)
External registers: One read buffer, one write register. No address decoding-read from
any external data memory address will enable the read buffer and any external write will
clock the write register.
External read buffer:
DO, D1: Unused, read as 0
D2: 0 = FCN key pressed
D3: 0 = OFF key pressed
D4: 0 = ON key pressed
D5: 0 = SET key pressed
D6: Unused
D7: 0 =Water Low switch closed

Appendix A 289