Clock Selection; Figure 108. Control Circuit In Normal Mode, Internal Clock Divided By 1 - ST STM32F100 Series Reference Manual

RM0041
13.3.3

Clock selection

The counter clock can be provided by the following clock sources:
•
Internal clock (CK_INT)
•
External clock mode1: external input pin (TIx)
•
External clock mode2: external trigger input (ETR).
•
Internal trigger inputs (ITRx): using one timer as prescaler for another timer, for
example, Timer1 can be configured to act as a prescaler for Timer 2. Refer to
one timer as prescaler for another timer
Internal clock source (CK_INT)
If the slave mode controller is disabled (SMS=000 in the TIMx_SMCR register), then the
CEN, DIR (in the TIMx_CR1 register) and UG bits (in the TIMx_EGR register) are actual
control bits and can be changed only by software (except UG which remains cleared
automatically). As soon as the CEN bit is written to 1, the prescaler is clocked by the internal
clock CK_INT.
Figure 108
without prescaler.

Figure 108. Control circuit in normal mode, internal clock divided by 1

CEN=CNT_EN
Counter clock = CK_CNT = CK_PSC
Counter register
External clock source mode 1
This mode is selected when SMS=111 in the TIMx_SMCR register. The counter can count at
each rising or falling edge on a selected input.
shows the behavior of the control circuit and the upcounter in normal mode,
Internal clock
UG
CNT_INIT
31
General-purpose timers (TIM2 to TIM5)
for more details.
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