UM0306 General purpose timer (TIMx)
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Slave mode: External Clock mode 2 + trigger mode
The external clock mode 2 can be used in addition to another slave mode (except external
clock mode 1 and encoder mode). In this case, the ETR signal is used as external clock
input, and another input can be selected as trigger input when operating in reset mode,
gated mode or trigger mode. It is recommended not to select ETR as TRGI through the TS
bits of TIMx_SMCR register.
In the following example, the up-counter is incremented at each rising edge of the ETR
signal as soon as a rising edge of TI1 occurs:
1. Configure the external trigger input circuit by programming the TIMx_SMCR register as
follows:
– ETF = 0000: no filter
– ETPS=00: prescaler disabled
– ETP=0: detection of rising edges on ETR and ECE=1 to enable the external clock
mode 2.
2. Configure the channel 1 as follows, to detect rising edges on TI:
– IC1F=0000: no filter.
– The capture prescaler is not used for triggering and does not need to be
configured.
– CC1S=01in TIMx_CCMR1 register to select only the input capture source
– CC1P=0 in TIMx_CCER register to validate the polarity (and detect rising edge
only).
3. Configure the timer in trigger mode by writing SMS=110 in TIMx_SMCR register. Select
TI1 as the input source by writing TS=101 in TIMx_SMCR register.
A rising edge on TI1 enables the counter and sets the TIF flag. The counter then counts on
ETR rising edges.
The delay between the rising edge of the ETR signal and the actual reset of the counter is
due to the resynchronization circuit on ETRP input.
Figure 111. Control circuit in external clock mode 2 + trigger mode
13.4.15 Timer synchronization
The TIMx timers are linked together internally for timer synchronization or chaining. When
one Timer is configured in Master Mode, it can reset, start, stop or clock the counter of
another Timer configured in Slave Mode.
The following figure presents an overview of the trigger selection and the master mode
COUNTER CLOCK = CK_CNT = CK_PSC
COUNTER REGISTER
35 3634
ETR
CEN/CNT_EN
TIF
TI1
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