Electronic – Is the datasheet of the AVR ATmega32 wrong

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I'm currently using an ATmega32 to keep time, so I carefully read the datasheet and configured the Timer 1 to generate an interrupt each second.

According to the following formula on the datasheet, using a 16 MHz clock, a prescaler of 256, and OCR1A = 31249, I should get a frequency of 1 Hz in CTC mode:

\$ f_{OCnA} = \frac{f_{clk\_I/O}}{2\: \cdot \: N \cdot \: (1 \: + \: OCRnA)} \$

This formula is available on page 99 of the ATmega32 datasheet.

I've configured Timer 1 as follows:

TCCR1A = 0;
TCCR1B = (1<<WGM12)|(1<<CS12);
OCR1A = 31249;
TIMSK = (1<<OCIE1A);

Here is the interrupt code

ISR(TIMER1_COMPA_vect){
    sec++;
}

Then, I use an LCD to continuously print the value of sec, and I get a timing of 0.5 seconds. In other words, for each second that passes, the timer generates two interrupts.

At first I thought maybe I got the fuses wrong, but that doesn't seem to be the case, as delays work just fine. Anyway, here's how I configured the fuses:

-U lfuse:w:0xff:m -U hfuse:w:0x99:m

I have absolutely no idea of what's going on, I've double checked everything, tried another ATmega32, tried another 16 MHz crystal oscillator, and I still get the same result.

Is the datasheet wrong, or am I missing something here?

Best Answer

If the system clock is 16 MHz and the prescaler is 256, you timer will run on \$ \large \frac{16\,MHz}{256} = \small 62500\,Hz \$ , so there will be an increment in every \$ \large \frac{1}{62500\,Hz} = \small 16\, \mu s \$. An interrupt will be generated when the counter's value is 31249. If we calculate this time: \$ \small 16\, \mu s \times 31250\, ticks = 0.5\, s\$, which is exactly the same result you have.

The equation in the datasheet is for a different purpose:

For generating a waveform output in \$ \small CTC\$ mode, the \$ \small OC1A\$ output can be set to toggle its logical level on each compare match by setting the compare output mode bits to toggle mode \$ \small (COM1A1:0 = 1)\$. The \$ \small OC1A\$ value will not be visible on the port pin unless the data direction for the pin is set to output \$ \small (DDR_{OC1A} = 1)\$. The waveform generated will have a maximum frequency of \$ \small fOC1A = fclk_{I/O}/2 \$ when \$ \small OCR1A\$ is set to zero \$(0x0000)\$. The waveform frequency is defined by the following equation: (...)

If you would measure the frequency of specific pin, I think it would be 1 Hz.

To achieve an 1 s, \$ \small 16\, \mu s \times 62500 = 1\,s\$ interrupt generation you need 62500 ticks and as the counter starts from 0, set OCR1A to \$ \small 31250 \times 2 - 1= 62499\$, and since Timer1 is a 16 bit timer this value is not too high (<65535, \$ 2^{16}-1 \$).

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