Electronic Circuits and Projects Forum

Without knowing the EXACT frequency of the oscillator, there is no point in debating which one is better.

Also, polling sounds like a bad idea.
How do you know if you are polling at the correct point in time?
I think that you need to use interupts.

JimB

I use interrupts within the code allready to do other stuff, the interrupts run at 500 hz, and the bit I'm polling toggles at 125 hz, so I know theres no chance of a missed poll, if I was missing polls then I'dve thought the clock would have been slow not fast.

I didnt know drift was less with 32.768 kc xtals, the low power bit makes sense as these are common in watches which obviously are battery powered.
I'll have to mess about shifting one bit of my 7 segment display bits to another port pin, but that looks like the way to improve accuracy.

Incidently my initial idea used interrupts for timekeeping rather than polling timer1, and that didnt work it ran slow, I think what was happening was that every time I reloaded the timer to get the correct interrupt duration the prescaler was reset (the pic resets the prescaler when you write to the timer register) causing an error of a few clock cycles every interrupt.

I have just reconfigured the circuit, the 4 mc xtal is now removed and the chip runs from its internal rc, I've shifted one of the bits off portb to porta (not very elegant but works), and timer1 now runs from a 32,768 kc xtal.
hSo I'll leave the clock to run overnight and we'll see how accurate it is.
The circuit is built on vero so I'm not expecting the best possible result, but hopefully better than 10 secs per day.

Originally Posted by dr pepper

Obviously I'm missing something, with a 32kc xtal accuracy is worse.

hi dr,
You can buy 32.768KHz xtals in different stability ranges, some 'bog' standard 32KHz xtals have very poor temperature stability.

The other point are you using high stability xtal caps.?

Originally Posted by dr pepper

That could be the issue, the caps are ceramics out of the scrap box, judging by the lack of the black dot at the top they dont look like npo's.
What do you mean by high stability, silver mica or polystyrene, or the likes?

I would use poly or silver mica, if you 'know' the temperature drift range of your xtal, you can choose caps which drift with temperature in the 'opposite' sense.

One method I have used in the past is a xtal small oven, running at say 5 or 10C above the worst case ambient.

If you could find an old watch xtal, thats also an option.

If you require really precise time keeping, there are improved versions of the DS1307 IC available, for about £5.

EDIT:
http://uk.farnell.com/jsp/search/bro...D2031%2B203436

I have a PIC32 (with RTCC module) and am using a 32.768KHz crystal for the clock.

It is accurate to about 2 seconds a day (compared to my computer). That's a minute a month without any compensation (software or oscillator tuning).

Accurate enough for me