How are timer interrupts implemented in the AVR?
The AVR can be configured to execute interrupts if a timer event has occurred (the corresponding interrupt flag in the TIFRn is set). Normal program execution will be interrupted (almost) immediately and the processor will execute the code of the Interrupt Service Routine.
Is timer A interrupt?
The CPU then executes code for a key interrupt, which typically displays a character on a screen or performs a task. Perhaps the most important interrupt for operating system design is the “timer interrupt”, which is emitted at regular intervals by a timer chip.
When does an overflow interrupt occur in timer 1?
An overflow interrupt should have occurred at 65536, which is about half way through that time. But you are also reading timer1OverflowCount about half way through that time, so depending the exact timing of the interrupt it may or may not have occurred when you read the overflow count.
How to check overflow count in Arduino timer?
You can watch overflow count for a fixed time to see if has any delay. The code is fast but also executed without any break between loops. In the loop you have more or less five instructions, two transfers in the atomic block, and three outside. Can be added a jump and enable disable interrupts.
What happens if the timer overflows after executing tcnt1?
Think of what would happen if the timer overflows after executing timer1CounterValue = TCNT1; but before reading the TOV1 flag. Then we would unduly increment overflowCopy . This is why the code tests for (TIFR1 & bit (TOV1)) && timer1CounterValue < 256.
Why is there an overflow interrupt on my Arduino?
The problem is that you are not eliminating the actual issue with the ATOMIC statement- the counter is incrementing in hardware and there will be times when an interrupt is pending but not completed. It’s worse because the C int operations take a lot of cycles on an 8-bit processor, but it would show up even in tight asm code.