Using the PIC16F690′s (and others’) Internal Timers to Keep Time

This one is so easy a kid could do it. Actually, once you see how easy it is  to keep a somewhat accurate count with an internal oscillator, you’ll probably wonder why I ever spent $7 on a DS1305. If you just want a simple, somewhat inaccurate timepiece, neglecting to use an RTC can make things really simple. The $7 for a real RTC is worth it :).

The first experiment is to check if our interrupts work. We are using timer 2 to generate an interrupt every millisecond. Inside the interrupt we have a 16-bit variable counting to 1000, and then it increments the seconds variable. Then, when seconds hit ten, it starts the loop back at 0. I then output the seconds byte to port C on the Pickit 2. The LEDs show a binary count of the seconds. Tune in for the next installment. I’ll have a clock with four blue 7-segment LEDs that I’m stealing from my first clock project that kept time right, but nothing else.

#include <16f690.h>
#fuses intrc_io,nowdt,nomclr,noborownout,put
int seconds=0; int16 milliseconds=0;

void incrementSeconds()
if (milliseconds==1000)
{milliseconds==0; seconds++}
if (seconds==10) seconds=0;

void main(void)
setup_timer_2(timer_2_div_by_1,0×64,10); //1ms interrupt
while (1)
{ output_c(seconds); } //binary count of seconds

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