Hello,

        I think that you are talking about the WDT timeout period, which is a
maximum of 2.3 seconds. HOWEVER, there are other ways to make a PIC wake up
from sleep.

        You can use the PB0 interrupt on pin change. You could attach a capacito
r
and a resistor in parallel from PB0 to ground. Then, you could also have a
wire from PB0 to some other pin on the pic(PB1, for example), then you
could do this:

bsf STATUS,RP0 ;Set PB1 to output
bcf TRISB,PB1
bcf STATUS,RP0

bsf PORTB,PB1  ;Charge capacitor thru PB1
                  ;Do other stuff for about 1 mS while waiting for large cap to
                  ;charge. You could speed this up with an external transistor.


bsf STATUS,RP0 ;Tri-state PB1
bsf TRISB,PB1
bcf STATUS,RP0

SLEEP           ;Sleep until cap discharges thru resistor and
                ;triggers interrupt on falling edge

You would of course need interrupt code at the interrupt vector (location
0x0004 if I remember correctly) to return from the interrupt. You would
also need to enable the interrupt in you initialization code.

C should be large (but not TOO large, about 10 uF) and
for R you should have about:

R=0.546*T/C

Where R is in ohms
C is in Farads
and T is sleep time in seconds.

Please note that this will be approximate, since it will depend on actual
pin input low threshold, temperature, capacitor and resistor tolerance,
input pin leakage current,etc.
This is only good for delays of up to about 30 seconds maximum.

        For greater or more accurate delays you can use an RTC (real time clock)
chip which will be able to, say, give very accurate pulses once a second.
These pulses could then be sent through a divider and into pin PB0,
triggering the interrupt.


Good luck,

Sean




At 08:47 PM 11/4/98 +0700, you wrote:
>I am asking advice how to increase sleep time to 3 min or more.
>In the datasheet the max sleep time only 2.3 sec
>
>TIA
>Hanafi Tan
>
+-------------------------------+
| Sean Breheny                  |
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