99 Minute Timer Replacement Datasheet

99 Minute Timer - Electromechanical Timer Replacement

This 99 minute timer replaces the electro-mechanical timer. The hardware is made simple and economical. The timer is very simple to operate. Two 7-segment displays are provided to display minutes from 0 to 99. To simplify operation, only two push-on switches are provided to set minutes. Each switch is dedicated to each display.

“TENS OF MIN SW” increments tens of minutes display from 0 through 9 at each push and “UNITS OF MIN SW” increments units of minutes displays from 0 through 9 at each push. Debouncing of switches is taken care by software.

APPLICATION OPERATION
• The TMR0 prescaler is set to x16. Software checks TMR0 for 250 rollovers, which happens every 4 msec. 4 msec is counted 25 times to generate 100 msec, from which seconds and minutes are generated.
• Every 100 msec the keyboard is read and the required minutes display (TENS or UNITS) is incremented at the rate of 2 Hz.
• If the displayed minutes is equal to 0 then relay is put off and UNITS DISPLAY's decimal point is not flashing to indicate the timer is off.
• If displayed minutes is not equal to 0, then a relay is turned on and UNITS DISPLAY's decimal point is flashed to indicate the timer is active. During this mode the display decrements every minute till it reaches 0, where the relay is put off.

Notes:
1 Total instruction time [one loop] Approximately 375 µsec, maximum.
2 Set Processor configuration word as 0000 0000 1010b.
a) MCLR tied to VDD (internally).
b) Code protection off.
c) WDT disabled.
d) Internal RC oscillator.

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