How to configure the starting direction of center-aligned Timer

[Zheng Liang]

Posted on March 02, 2018 at 06:25

Device: STM32F103

I wanna set the direction(counting up or down) when starting a center-aligned Timer, but once I set the DIR and then set the CMS to center-aligned mode, the DIR bit will be cleared by hardware.

I found this description in datasheet:

When starting in center-aligned mode, the current up-down configuration is used. It

means that the counter counts up or down depending on the value written in the DIR bitin the TIMx_CR1 register. Moreover, the DIR and CMS bits must not be changed at thesame time by the software.

How could I start the center-aligned mode with down direction?

If someone knows, please tell me.

thanks.

#stm32

[Jan Waclawek]

Posted on March 02, 2018 at 10:41

Show us code.

Read out the timer registers immediately before enabling it and check/post.

What's the CNT startng position?

JW

[Zheng Liang]

Posted on March 03, 2018 at 09:00

Hi Jan, here are the codes：

--------------------------------------------------------------------

//Enbale Timer1 clock

RCC->APB2ENR |= RCC_APB2ENR_TIM1EN;

//Set single pulse mode

TIM1->CR1 |= TIM_CR1_OPM;

//Set Center-aligned mode 3

TIM1->CR1 |= TIM_CR1_CMS;

//Set prescaler

TIM1->PSC = 0;

//Set auto-reaload value

TIM1->ARR = 71 * 4;

//Set repetition

TIM1->RCR = 2;

//Channel 1 at Toggle Mode

TIM1->CCMR1 |= 3 << 4;TIM1->CCR1 = TIM1->ARR >> 1;

//Enable channel 1 output

TIM1->CCER |= TIM_CCER_CC1E;TIM1->BDTR |= TIM_BDTR_MOE;

//Set direction to downcounting

TIM1->CR1 |= TIM_CR1_DIR;

//Set counter register to max value

TIM1->CNT = TIM1->ARR;

//Enable Timer1

TIM1->EGR |= TIM_EGR_UG;TIM1->CR1 |= TIM_CR1_CEN;--------------------------------------------------------------------

The problem is when  Center-aligned mode 3 is set, the code to set direction bit in CR1 register to downcounting will not work at all.

Is the Center-aligned mode direction could only be set by hardware? means the counter could only start with upcounting mode from the first time?

Thanks,

Zheng

[waclawek.jan]

Posted on March 04, 2018 at 02:32

Zheng,

Your problem is in that you set up the bits in CR1 separately. Then, when you get to set DIR, the following note below DIR's description in RM applies:

Note: This bit is read only when the timer is configured in Center-aligned mode or Encoder

mode.

Set CR1 at once, as the last line:

TIM1->CR1 = TIM_CR1_DIR | TIM_CR1_OPM | TIM_CR1_CMS | TIM_CR1_CEN;

Note also that setting TiMx_EGR.UG has quite an aggressive effect: it resets TIMx_CNT to 0, and clears the DIR bit.

JW