Timer interrupt flag behavior in encoder mode

[Lee3]

I have an STM32G474, but this encoder/timer behavior is likely similar for other MCUs. I have the timer set up in quadrature encoder mode on channels 1 and 2 and I'm using input capture on channel 3. This is tied to the encoder index so that it will latch the encoder value when the index occurs. This works but I also want to know when the first index has occurred. I tried reading CC3IF in the SR directly, but I've found it is temperamental. It seems to flag but only for a brief period. I'm not resetting any flags or writing to SR.

Can anyone tell me about this use case and what behavior I should expect from CC3IF?

Below is some code:

class QEPEncoder final : public EncoderBase {
 public:
 QEPEncoder(TIM_TypeDef &regs) : EncoderBase(), regs_(regs) { 
 regs_.SMCR = 3; // quadrature mode
 regs_.CCMR1 = (1 << TIM_CCMR1_CC1S_Pos) | (1 << TIM_CCMR1_CC2S_Pos) | // input selection
 (1 << TIM_CCMR1_IC1F_Pos) | (1 << TIM_CCMR1_IC2F_Pos); // filters at 1
 regs_.CCMR2 = (1 << TIM_CCMR2_CC3S_Pos) | (2 << TIM_CCMR2_IC3F_Pos); // index filter at 2
 regs_.CCER = TIM_CCER_CC1E | TIM_CCER_CC2E | TIM_CCER_CC3E; // enable
 regs_.CR1 |= TIM_CR1_CEN;
 }
 int32_t read() { value_ = regs_.CNT; return value_; }
 int32_t get_value() const { return value_; }
 int32_t get_index_pos() { return regs_.CCR3; }
 bool index_received() { return regs_.SR & TIM_SR_CC3IF; }
 private:
 TIM_TypeDef &regs_;
 int32_t value_ = 0;
};

[waclawek.jan]

JW

[Lee3]

Thank you much, that was my issue. My debugging was taking me in the wrong direction otherwise, thinking it was something with the configuration or specific to the encoder mode.