2.1 Clock input sources
The timer always needs a clock source. It also can be synchronized by several clocks
simultaneously:
• Internal clock
• External clock
– External mode1 (TI1 or TI2 pins)
– External clock mode2 (ETR pin)
– Internal trigger clock (ITRx).
2.1.1 Internal clock
The timer is clocked by default by the internal clock provided from the RCC. To select this
clock source, the TIMx_SMCR->SMS (if present) bits should be reset.
2.1.2 External clock
The external clock timer is divided in two categories:
• External clock connected to TI1 or TI2 pins
• External clock connected to ETR pin
In these cases, the clock is provided by an external signal connected to TIx pins or ETR pin.
The maximum external clock frequency should be verified.
Note: 1 In addition to all these clock sources, the timer should be clocked with the APBx clock.
2 The external clocks are not directly feeding the prescaler, but they are first synchronized
with the APBx clock through dedicated logical blocks.
External clock mode1 (TI1 or TI2 pins)
In this mode the external clock will be applied on timer input TI1 pin or TI2 pin. To do this:
1. Configure the timers to use the TIx pin as input:
a) Select the pin to be used by writing CCxS bits in the TIMx_CCMR1 register.
b) Select the polarity of the input:
For the STM32F100/101/102/103/105/107 lines: by writing CCxP in the
TIMx_CCER register to select the rising or the falling edge;
For the other series & lines: by writing CCxP and CCxNP in the TIMx_CCER
register to select the rising/falling edge, or both edges(a).
c) Enable corresponding channel by setting the CCEx bit in the TIMx_CCER register.
2. Select the timer TIx as the trigger input source by writing TS bits in the TIMx_SMCR
register.
3. Select the external clock mode1 by writing SMS=111 in the TIMx_SMCR register
External clock mode2 (ETR pin)
The external clock mode2 uses the ETR pin as timer input clock. To use this feature:
1. Select the external clock mode2 by writing ECE = 1 in the TIMx_SMCR register.
2. Configure, if needed, the prescaler, the filter and the polarity by writing ETPS [1:0], ETF
[3:0] and ETP in the TIMx_SMCR register.
Internal trigger clock (ITRx)
This is a particular mode of timer synchronization. When using one timer as a prescaler for
another timer, the first timer update event or output compare signal is used as a clock for the
second one.