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AN4013

General-purpose timer modes

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2

General-purpose timer modes

General-purpose timers can be programmed to work in one of the following configurations.

2.1

Clock input sources

The timer 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 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

a. For the STM32F100/101/102/103/105/107 lines, polarity selection for both edges can be achieved by using

TI1F_ED, but only for TI1 input.