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External write register: LED segments, writing 1 turns on segment
DO: Segment A
D1: Segment B
D2: Segment C
D3: Segment D
D4: Segment E
D5: Segment F
D6: Segment G
D7: Decimal point
LED segment definition
a
...._____-
f
el
d
LEDs are not decoded-software directly writes LED segments. Numeric to seven-segment
decoding must be performed in software.
Software must multiplex (scan) display digits.
Switch inputs are not debounced.
Watchdog timer has approximately a half-second timeout.
Example System Software Description
Requirements
Implement functionality as described in system definition.
Implement additional functionality as described in hardware definition.
CPU Resource Usage
Timer 1: 250Hz interrupt
Timers 0 and 2: Unused
Ports: As described in hardware definition.
Bit 3.4 is reserved as diagnostic output for oscilloscope.
Functional Software Description for Pool Pump Timer
This is a high-level logical description, one step above pseudocode.
290 Appendix A
