The data sheet for the ATmega328 states under the heading Interrupt Response Time: So what about the prologue and the epilogue? Adding those cycles up leads to another 43 cycles. Remember for later, 2 of these cycles can be subtracted since they are caused by the measurement. Since we have 16 cycles per µs, this leads to 55/16=3.4375 µs, which is close enough to what I have measured. One has to add 2 cycles from the CBI instruction (clearing the port bit), which makes it 55. In the worst case (when the if-branch is executed), one ends up with 53 execution cycles. Before we go into that, let us count the cycles between the two port manipulation instructions that I inserted.
Arduino millis timer interrupt series#
It is mainly a series of push instructions before (in order to save registers temporarily) and a series of pop instructions afterwards.
Arduino millis timer interrupt code#
So this is it? Definitely not! When looking into the generated assembler code, one notices that there is some code before the first port manipulation command is executed and some after. This looks completely plausible because sometimes, the if-branch will be executed, which needs a bit more time. Then, using the triggering mechanism, I looked for longer intervals and found that sometimes the routine needs 3.44 µs. The result was that one could see blips of length 3.13 µs. I did both.įirst, I inserted two port manipulation statements (such as PORTC |= 0x01 and PORTC &= ~0x01) in the beginning and the end of the routine and measured the time using a logic analyzer. Now, one can try to determine the runtime of this ISR either by looking at the generated code and count instruction cycles or one measures the timing empirically. (volatile variables must be read from memory on every access) copy these to local variables so they can be stored in registers Looking into the source code of the Arduino core, one finds in the file wiring.c the following code: #if defined(TIM0_OVF_vect) Since no other code can be executed while an interrupt is serviced, time critical functions (such as bit-banging I/O functions) are very sensitive to it. We want to find out how much time the interrupt handler that counts the milliseconds needs. The function calls cost some time and, moreover, the timekeeping in the background that is performed by the TIM0_OVF interrupt service routine (ISR) incurs some cost. In an ideal world, these timekeeping functions would not impose any computational costs. Fortunately, it is nevertheless possible to determine the length of time intervals correctly if they are not too long. Since the time counters use unsigned 32-bit numbers, micros() will go from UINT32_MAX to zero after roughly 70 minutes, millis() does that after 49 days. But beware, the counters roll over after some time. With these functions, you can measure time intervals between two events. Millis() and micros() are the two functions returning the number of milliseconds and microseconds since the Arduino program was started.
What is the overhead imposed by the millis() interrupt? And can we get rid of it?
The featured mage of this blog post is by Gerd Altmann from Pixabay
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