The STM32's timer input channel has a filter unit located on each input path (the yellow box in the figure below) and the external trigger input path (the blue box in the figure below). Their function is to filter the input. High frequency interference on the signal.
The specific operation principle is as follows:
The CKD[1:0] in TIMx_CR1 allows the user to set the sampling frequency reference for the input signal. There are three options:
1) Sampling frequency reference fDTS = timer input frequency fCK_INT
2) Sampling frequency reference fDTS=timer input frequency fCK_INT/2
3) Sampling frequency reference fDTS=Timer input frequency fCK_INT/4
The input signal is then sampled using the above frequency as a reference, and is considered a valid input level when successively sampled to N active levels.
The actual sampling frequency and sampling times can be selected by the user program as needed; the filtering parameters of the external trigger input channel are set in the ETF[3:0] of the slave mode control register (TIMx_SMCR); the filtering parameters of each input channel are captured/ Compare mode register 1 (TIMx_CCMR1) or capture/compare mode register 2 (TIMx_CCMR2) in IC1F[3:0], IC2F[3:0], IC3F[3:0], and IC4F[3:0].
For example, when fCK_INT=72MHz, select fDTS=fCK_INT/2=36MHz, sampling frequency fSAMPLING=fDTS/2=18MHz and N=6, then the signal with frequency higher than 3MHz will be filtered by this filter, effectively shielding Interference above 3MHz.
For example, combined with the input capture interrupt, the debounce function of the button can be easily implemented without software intervention; this is a hardware-implemented debounce function, which greatly saves the software overhead and the length of the program code.
Each timer can input up to 4 buttons. This method can also be used for keyboard matrix scanning, and because it is implemented by interrupts, the software does not need to perform frequent scanning operations.
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