Clock on Atmega8

Question

I am trying to do a clock on Atmega8. I have 8Mhz quartz.

I use timer0 interrupt for clock timeticks:

/* Settings */
#define TMR_RELOAD      80 - 5 /* 8 kHz / 80 = 100 Hz */

#define TMR_CNT_MAX     100 /* 1Hz internal counter */

/* internal variables */
static uint8_t tmr_cnt;

inline void TMR0_Init()
{
  /* set clock source f_t0 = 8МHz/1024 = 8 kHz */
  TCCR0 = _BV(CS00) | _BV(CS02);

  TIMSK |= _BV(TOIE0); /* Enable TMR0 interrupt on overflow*/
}

ISR (TIMER0_OVF_vect)
{
  if (tmr_cnt == 0)
    Clock_Tick1s();


  tmr_cnt++;
  if (tmr_cnt >= TMR_CNT_MAX)
    tmr_cnt = 0;


  TCNT0 -= TMR_RELOAD;
}

The problem is that my clock a running too fast or too slow.

Calculated value that I put in TCNT0 register is 80, but in this case the clock is running too slow. When I use 80-4 the clock is also running too slow. When I use 80-5, it's too fast.

I don't know, how it could be???

UPDATE: Now settings are the following, but problem still exists.

/* Settings */
#define TMR_RELOAD      125 /* 31.25 kHz / 125 = 250 Hz */
#define TMR_CNT_MAX     250 /* 1Hz internal counter */

inline void TMR0_Init()
{
  /* set clock source f_t0 = 8МHz/256 = 31.25 kHz */
  TCCR0 = _BV(CS02);

  TIMSK |= _BV(TOIE0); /* Enable TMR0 interrupt on overflow*/
}

ISR (TIMER0_OVF_vect)
{
  TCNT0 -= TMR_RELOAD;

  if (tmr_cnt == 0)
    Clock_Tick1s();

  tmr_cnt++;
  if (tmr_cnt >= TMR_CNT_MAX)
    tmr_cnt = 0;
 }

How much is "too fast"? How many seconds per hour for example? Do you have an oscilloscope available? — Rev, Oct 13 '14 at 06:32

score 2 · Answer 1 · answered Oct 14 '14 at 08:57

2

Note that when you call

TCNT0 -= TMR_RELOAD;

the TCNT0 register already is 0. So you get TCNT0 = 131 which requires to 124 ticks to overflow. This means that your clock is

1/31250 * 250 = 8ms

faster per second, which results in

8ms * 60 x 60 x3 = 86.4s

per 3 hours. This is not the 5 minutes you mentioned in the comments but the error is there. There is potentially some additional error coming from your 8MHz crystal. This depends on the crystal quality and how well your resonant circuit is designed.

I would advice you to use the output compare interrupt of Timer2 instead of using an overflow interrupt and reloading the TCNT value.

"The Output Compare Register contains an 8-bit value that is continuously compared with the counter value (TCNT2). A match can be used to generate an Output Compare interrupt..."

Its easier to handle and you don't have to deal with "reloading" the timer.

You could also consider using a 32.768kHz clock crystal.

answered Oct 14 '14 at 08:57

Rev

5,827
4
27
51

1) Interrupt occurs with frequency 250Hz. 1/250 = 4ms. It count 125 ticks of TMR0 => 1 extra count of TMR0 = 4ms / 125 = 32 uS. 32 uS * 60 * 60 = 115.2 ms per hour. Or 0.35 s per 3 hours. ------- 2) "The Output Compare Register contains an 8-bit value that is continuously compared with the counter value (TCNT2). A match can be used to generate an Output Compare interrupt..." In order to have **custom intervals of time** before interrupts, you anyway need to load new value into TCNT2. – vitperov Oct 14 '14 at 19:24
3) Today I've noticed a large delay when I was out of my flat (4 hours per 10 hours). Maybe is is also problems with power-supply. 4) **Thank you for your help!** – vitperov Oct 14 '14 at 19:28
@vitperov: Hmm. The register TCNT0 is 1 "tick" off (1/31250 = 32us) if it only counts 124 instead of 125. Since you count to 250 before a second is complete, you are 250 * 32us per second off -> 8ms. Or am I missing something? – Rev Oct 14 '14 at 19:38
You are right! 30 seconds per hour, 86.4s per 3 hours. – vitperov Oct 14 '14 at 20:46

score 1 · Answer 2 · answered Oct 11 '14 at 19:11

1

The first issue is obvious, frequency is 8MHz/1024=7812.5 Hz, i.e. using 80 makes no sense. Manipulating TCNT0 is also problem because there is some delay since the counter has reached zero and interrupt is executed and code flow reaches last instruction. The third is frequency of crystal, are you sure that is really 8MHz? Because unproper capacitive load or just frequency shift may bring timing error.

answered Oct 11 '14 at 19:11

TMa

723
1
5
14

Thank you! I fix frequecy devider and move `TCNT0 -= TMR_RELOAD` to the beginning of the interrupt function. We will see after several hours whether it helps. Thank you! – vitperov Oct 11 '14 at 19:56

Clock on Atmega8

2 Answers2