ST STM32F301 6 Series Reference Manual page 354

Advanced-control timer (TIM1)
1. As no filter is needed in this example, write ETF[3:0]=0000 in the TIMx_SMCR register.
2. Set the prescaler by writing ETPS[1:0]=01 in the TIMx_SMCR register
3. Select rising edge detection on the ETR pin by writing ETP=0 in the TIMx_SMCR
register
4. Enable external clock mode 2 by writing ECE=1 in the TIMx_SMCR register.
5. Enable the counter by writing CEN=1 in the TIMx_CR1 register.
The counter counts once each 2 ETR rising edges.
The delay between the rising edge on ETR and the actual clock of the counter is due to the
resynchronization circuit on the ETRP signal. As a consequence, the maximum frequency
which can be correctly captured by the counter is at most ¼ of TIMxCLK frequency. When
the ETRP signal is faster, the user should apply a division of the external signal by proper
ETPS prescaler setting.
f CK_INT
CNT_EN
ETR
ETRP
ETRF
Counter clock =
CK_CNT =CK_PSC
Counter register
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Figure 118. Control circuit in external clock mode 2
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