EM357-RTR datasheet(130/241 Pages) SILABS

130 / 241 page

Rev 1.3

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Figure 9.21. Output Compare Mode, Toggle on OC1

9.3.9. PWM Mode

Pulse width modulation mode allows you to generate a signal with a frequency determined by the value of the

TIMx_ARR register, and a duty cycle determined by the value of the TIMx_CCRy register.

PWM mode can be selected independently on each channel (one PWM per OCy output) by writing 110 (PWM

mode 1) or 111 (PWM mode 2) in the TIM_OCyM bits in the TIMx_CCMR1 register. The corresponding buffer

center-aligned mode the auto-reload buffer register must be enabled by setting the TIM_ARBE bit in the TIMx_CR1

Because the buffer registers are only transferred to the shadow registers when a UEV occurs, before starting the

counter initialize all the registers by setting the TIM_UG bit in the TIMx_EGR register.

OCy polarity is software programmable using the TIM_CCyP bit in the TIMx_CCER register. It can be programmed

as active high or active low. OCy output is enabled by the TIM_CCyE bit in the TIMx_CCER register. Refer to the

TIMx_CCER register description in "9.5. Registers" on page 145 for more details.

In PWM mode (1 or 2), TIMx_CNT and TIMx_CCRy are always compared to determine whether TIMx_CCRy ?

TIMx_CNT or TIMx_CNT ? TIMx_CCRy, depending on the direction of the counter. The OCyREF signal is asserted

only:



When the result of the comparison changes, or



When the output compare mode (TIM_OCyM bits in the TIMx_CCMR1 register) switches from the "frozen"

configuration (no comparison, TIM_OCyM = 000) to one of the PWM modes (TIM_OCyM = 110 or 111).

This allows software to force a PWM output to a particular state while the timer is running.

The timer is able to generate PWM in edge-aligned mode or center-aligned mode depending on the TIM_CMS bits

in the TIMx_CR1 register.

9.3.9.1. PWM Edge-Aligned Mode: Up-Counting Configuration

Up-counting is active when the TIM_DIR bit in the TIMx_CR1 register is low. Refer to "9.3.2.1. Up-Counting Mode"

on page 117.

The following example uses PWM mode 1. The reference PWM signal OCyREF is high as long as TIMx_CNT <

TIMx_CCRy, otherwise it becomes low. If the compare value in TIMx_CCRy is greater than the auto-reload value in

TIMx_ARR, then OCyREF is held at 1. If the compare value is 0, then OCyREF is held at 0. Figure 9.22 shows

some edge-aligned PWM waveforms in an example, where TIMx_ARR = 8.

EM357-RTR 数据表(PDF) 130 Page - Silicon Laboratories