Allready some time ago I tried to build a clock with a AVR Mega 8 and a clock crystal (32,768kHz). But by some reason this clock didn't get precise. The error was about 10min a day, so really huge - and I had no explaination for it. Why was the clock crystal not precise enough?
Now, I found again some time for this small side project - and I tried the following circuit with the clock crystal.
Of course, there was a display too, but I only wanted to show the important part.
But still, even with the changeable capacitor the clock was crazy wrong. There was no chance at all to pull it to the correct frequency. So, I read again the manual of the Mega 8 and this time I found the reason.
The fuse CKOPT was programmed by default, which means that internal capacitors of 36pF are activated. And because of this, the specification of the clock crystal was totally exceeded. Thats why my clock was allways so wrong. Now, its less than a second a day, which is good enough for me.
Important Assemblerparts:
ldi r16, 0b00000101 ; Prescaler for Timer2 = 128 -> 128*256 = 32768 1 overflow per second
out tccr2, r16
ldi r16, 0b01000000; Overflow Interrupt of Timer 2 activated
out timsk, r16
TIM2_OVF:
push r16 ; Working register to stack
in r16, sreg ; Sreg to stack as CPI instructions will change ist
push r16 ;
inc r21 ; increase second
cpi r21, 60
brlo nothingtodo
clr r21 ; second is 60, clear it and increase minute
inc r22
cpi r22, 60
brlo nothingtodo
clr r22
inc r23 ; increase hour
cpi r23, 24
brlo nothingtodo
clr r23
nothingtodo:
pop r16 ;
out sreg, r16 ; restore sreg
pop r16 ; restore working register
reti
Typical questions to this article:
Why is the clock crystal 32.768kHz not running correct?
How can I make a clock with a Atmel Mega 8 and a clock crystal?
My Clock is running to fast