Sleep in 16F877A

[Brian S]

I'm having trouble "Sleeping". What should be a timer that wakes every 2304ms to produce a 10ms pulse is acting like a free-running mutivibrator. What I see at power-on is an immediate 10ms high followed by a 22.5ms low....repeating endlessly. See code attached; I've cut out a lot of irrelevant operations. But I need to use 19.2kHz PWM, and I need a 65ms tick timer for other operations.

I'm obviously getting truncated sleep mode, but I can't figure out why. Is my setup for the other Timers upsetting the WDT timeout? Is there a workaround?


What am I missing?

[#include "16F877A.h"
#device ADC=10
#fuses HS, WDT, NOLVP, NOPROTECT
#use delay (clock=20000000)

void main()
{
setup_adc_ports(AN0_AN1_AN2_AN3_AN4);
setup_adc(ADC_CLOCK_INTERNAL);
setup_wdt(WDT_2304MS);
setup_psp(PSP_DISABLED);
setup_spi(FALSE);
setup_timer_0(RTCC_INTERNAL);
setup_timer_1(T1_INTERNAL|T1_DIV_BY_1);
setup_timer_2(T2_DIV_BY_1,255,1);
enable_interrupts(GLOBAL);

while (TRUE)
{
disable_interrupts(GLOBAL);
output_high (PIN_C1);
delay_ms(10);
output_low(PIN_C1);
restart_wdt();
enable_interrupts(GLOBAL);
sleep();

} /*end WHILE TRUE*/

} /*end MAIN*/
][/code]

[Ttelmah]

The other timers should not interfere with the sleep, since their oscillators will stop when the chip is asleep. However there is a problem if an interrupt has occurred in the period between disabling and enabling the interrupt, when the 'sleep' instruction will be prefetched, and the interrupt immediately serviced (I am assuming you have got interrupt service routines present - you do not show these, otherwise enabling an interrupt without the routine will result in chaos!). I'd try pausing for a moment between enabling the interrupts and trying to enable sleep. I have always used the 'setup_counters' control, rather than the setup_wdt, so:
setup_counters(RTCC_INTERNAL,WDT_2304MS);

The three functions 'setup_wdt', 'setup-countrs', and 'setup_timer_0', all sort of 'overlap' in functionality, with 'setup_counters', doing in one statement, what the other two functions do together. I wonder if the call to 'setup_timer_0', is overriding the earlier wachdog setup. Try using the 'combined' version and see if it helps.

Best Wishes