Arduino millis timer12/3/2023 To be certain of catching that overflow, the ISR therefore has to be called within 1.024 ms (probably slightly less because of the time taken to enter the ISR, so say: 1 ms). Since there is only one overflow flag, if the ISR misses an overflow then both millis and micros will be out by 1.024 ms (or more, if it misses multiple overflows). The timer is configured to tick every 4 µs on a 16 MHz system (because of the prescaler of 64: 64 * 62.5 ns = 4000 ns) and overflow every 1.024 ms (1024 µs) - because it overflows after 256 ticks ( 4 µs * 256 = 1024 µs ). To get accurate results this ISR must not miss an overflow. Its purpose is to count Timer 0 overflows. The code called by the (default) Timer 0 overflow interrupt vector (TIM0_OVF_vect) is used by millis and micros to help return their results. Would this affect code inside the main loop that relies on the millis() function since a significant amount of time is spent processing the ISRs?įrequency of the Timer 0 overflow ISR being called A RTC is a good idea for long term time keeping. Note: if you require extreme accuracy, the built in function isn't the best choice. The general rule of thumb of ISRs still apply: keep it short!!! In this case, you would have more serious issues than this, but it is something to keep in mind when you're writing code. If a lot of the other ISRs have a higher interrupt priorities, it is possible that the code for the millis() may never be ran. This may be a result of using noInterrupts() for an extended period of time or when too many other interrupts are being ran and there is not enough CPU for all of the tasks. However, there is a problem when the ISR is ran less often than it should be triggered. So, even if some non-interruptable code is being ran for a fraction of a millisecond, it won't matter since the bit for that interrupt vector will still remain set. When that ISR is ran, it increments a variable that is used by the millis() function. In simpler terms, it has it set up so a certain piece of code (the ISR) is run approximately once per millisecond on a 16MHz system (and proportionately less frequently on slower clocked systems). After some digging around in the core, it seems like Arduino updates millis() with a 8 bit timer: it uses overflow with a prescaler value of 64.
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