Let me first talk about frequency capture in general. According to my personal understanding, frequency capture uses a counter. When a timer interrupt occurs (it should be when the Autoreload register in the figure below is full), two values are recorded in two adjacent timer interrupts. The difference between the captured signals is calculated. The final frequency is your timer clock divided by your captured value.
Of course, if you directly call the function in the PWM_Input routine in the official 3.5 library to capture directly, you will find that the measured value will be very inaccurate below about 980Hz (under STM32F103RBT6). At this time, you need to use TIM_TimeBaseInit() to configure the pre-scaling parameters in PSC and ARR. The following figure is attached
Two pictures after the Prescaler changes
ARR determines the value of the Autoreload register. The two pictures above show that after changing PSC, the timer clock line will slow down, which means that more signals can be detected after one cycle, and the frequency that can be measured will be lower.
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