Question about the PIC16F676 ?

1. If I need to calculate a time for a delay, is it simply
number_of_instructions_in_loop * T where T is 1/F
or 1/4,000,000 in the case of the internal 4MHz clock ?

2. The second part of the question is if the accuracy of the
delay will be sufficient to implement a normal 24h clock ?

3. Should I use the built in timer ?

If anyone can answer some/all of these points I would be
very happy.

Tia.

 

Not exactly. First off the instruction rate is 1/4 the processor clock.
Second jump/skip instructions take 2 instruction clock.

Nope. I've build a sunrise/sunset light controller and even my supposedly
accurate 32768 HZ watch crystal drifts by several minutes a month.

Absolutely! it will make you life easier because you then don't have to keep
such a watchful eye on instruction cycles. It also allows your program to
go do something else while time is ticking away. For example my controller
scans its pot/switch interface, and updates the PED display showing the time
in the intervals between ticks.

If you want an accurate clock and it is plugged into the wall you should do 2
things:

1) Count the number of cycles on the power line. While line frequency may vary
ever so slightly during the course of day, there will be exactly 86400*(50/60)
cycles over the course of a day.

2) Battery backup. When you lose power you don't want to lose either the set
time or the interval that elapses while the power is out. By having a battery
backup you can retain the time and use the internal clock to maintain the time
over the interval that you lose ticks.

Hope this helps,

BAJ

 

Counting instructions is a pain on the PIC, since it doesn't have 16 or 32
bit datawords. Thus, you are always dealing with carries, etc.

One easy way to do this is to use the following macro:

a set 0 ; used to generate labels for the PAUSE macro

PAUSE MACRO t
banksel GPIO
bcf T1CON,TMR1ON ; turn off timer1
movlw ((65536 - (t)) & 0xFF) ; set it up so there are t ticks left till
rollover
movwf TMR1L
movlw (((65536 - (t)) >> 8) & 0xFF)
movwf TMR1H
bsf T1CON,TMR1ON ; turn it on
bcf PIR1,TMR1IF ; clear the interrupt flag
M_again#v(a):
btfss PIR1,TMR1IF ; wait till timer rolls over
goto M_again#v(a)
a += 1
ENDM

Set up the T1CON to have a 0 prescaler, and it'll pause t uS (assuming the
4MHz internal oscillator). If you use another prescaler, it'll pause for
longer, obviously. If you are using the internal oscillator, make sure
you've calibrated OSCCAL with the factory value.

If you want to use interrupts, just get rid of the M_again loop, and wait
for the TIMER1 interrupt.

Regards
Bob Monsen

 

Good discussion, thanks to all !!!