In my project, I am using a low power timer (LPTIM1) to handle input from a quadrature encoder. I have set the timer up in CubeMX as follows:
- IN1 and IN2 are used for the hardware inputs (this is always true; I cannot set this up).
- Channel 1 and 2 are active in output compare mode with no external pins, only interrupts.
- Clock polarity is set to both edges.
- Sample time (glitch filter) is set to 8 transitions.
- The LSI (32 kHz) is selected as its input clock.
At boot, I set the value of both channels to be the current timer values plus and minus 2 respectively. This should mean I get an interrupt when the encoder is rotated to the next mechanically stable position.
On any interrupt, I reset the values of the channel registers to be the new value of the CNT register plus and minus 2.
What I expect to see, is a single interrupt for any mechanical click on the encoder, where I can use the CNT register to figure out if the rotation was up or down.
What I see instead, is that this works properly for down rotations (where the counter decreases in value), but for up rotations it needs to be turned 2 mechanical clicks. At that point, the value of the timer has correctly increased by 4.
In a debugger, I have paused the system after one click and inspected the hardware registers. This can be seen in this screen shot:
As can be seen, CCR1 is 1 and CCR2 is 5. This was set up when CNT was 3. I have rotated the encoder and CNT is correctly increased to 5. However, no interrupt triggered and ISR.CC2IF is zero, meaning it does not detect a match.
From this state, after another click on the encoder, CNT increased to 7, CCR1 and CCR2 were unchanged, but then an interrupt is triggered.
I want to know why this happens, and more importantly, how I can detect every click on the encoder in both directions.
On the LPTIM, when CCRx=5, interrupt happens when CNT changes from 5 to 6. So if CNT=3 and you want an interrupt when CNT changes to 5, set CCRx=4.
Per the reference manual:
At least that's how I interpret it, and it matches the observation.
