Electronic – Estimating the frequency at which your microcontroller is running

atmelavrcusbasp

I have recently started programming an ATtiny85 microcontroller, which as per the datasheet runs at 10Mhz given 3v3 supply. So, to experiment, I have created a small program, as shown below.

#include <avr/io.h>

int main(void) {
    DDRB |= 8;
    while(1) {
        for(int i = 0; i < 1000; i++)
            for(int j = 0; j < 1000; j++)
                asm volatile("nop");
        PORTB ^= 8;
    }
}

As per documentation, each nop will take exactly one clock cycle to complete. Now, I have compiled this program using Atmel-Studio-7, and programmed the tiny using a usbasp clone and avrdude. As a side-note, the power supply jumper on usbasp is set to 3v3.

The programming worked fine, and I started looking at the blinking LED at pin-2 and it looked quite slow to me. When I measured the delay between LED toggles using a stop-watch, I could measure it to be exactly 5 seconds. As per my code, each time, 1000000 nops are executed before each toggle. By a simple calculation, it looks like my tiny could execute only 200000 nops per second, meaning it is running at 200Khz, which does not make any sense.

What could be the contributing factors to this error? How to estimate any microcontroller's clock speed by its output?

Best Answer

Take a look at the compiler output here https://godbolt.org/g/TxZSgt. Notice that the inner for loop and the asm("nop") lines correspond to 3 assembly instructions, two of which (sbiw and brne) take two cycles (for brne when taken). So instead of taking one cycle per inner loop like you predicted, it takes 5.

And actually, if you multiply your measured clock frequency by 5, it comes out to 1MHz, which is the ATTiny default clock speed as per the datasheet.

The device is shipped with CKSEL = “0010”, SUT = “10”, and CKDIV8 programmed. The default clock source setting is therefore the Internal RC Oscillator running at 8 MHz with longest start-up time and an initial system clock prescaling of 8, resulting in 1.0 MHz system clock

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