Capture Compare basic question

matthias23 · ‎2014-02-07

Posted on February 07, 2014 at 11:33

Hi Everybody,

I have a problem understanding the capture compare functionality.

I want to use channel1 to fire an interrupt if a match between CCR and counter is detected. In this case, CCR1 is set to 0xFFF+1. So the interrupt should never rise, because the counter restarts at 0xFFF this works fine for the TIM_OCMode_Toggle initialized on Channel3. Why is the TIM_IT_CC1 interrupt raised each counter overflow?

http://guyvo-cortex.blogspot.de/2009/04/pulse-timers.html this guy describes how it works, but why do the interrupt rises when the pulse is higher than the counter counts?

Can someone help me understanding this?

By the way, I'm using an STM32F4

void
TIM_test(
void
)
{
TIM_TimeBaseInitTypeDef TIM_TimeBaseStructure;
TIM_OCInitTypeDef TIM_OCInitStructure;
TIM_DeInit( TIM4 );
TIM_TimeBaseStructInit( &TIM_TimeBaseStructure );
TIM_TimeBaseStructure.TIM_Period =0xFFF;
TIM_TimeBaseStructure.TIM_Prescaler = 10000;
//((Prescaler/2)-1);
TIM_TimeBaseStructure.TIM_ClockDivision = 0;
TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;
TIM_TimeBaseInit(TIM4, &TIM_TimeBaseStructure);
TIM_OCStructInit( &TIM_OCInitStructure );
TIM_OCInitStructure.TIM_OCMode =TIM_OCMode_Toggle;
TIM_OCInitStructure.TIM_Pulse = 0xFFF+1; 
//When a match is found between the capture/compare register and the counter
TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Enable;
TIM_OCInitStructure.TIM_OCPolarity = TIM_OCPolarity_High;
TIM_OCInitStructure.TIM_OCIdleState = TIM_OCIdleState_Reset;
TIM_OC3Init(TIM4, &TIM_OCInitStructure);
TIM_OCStructInit( &TIM_OCInitStructure );
TIM_OCInitStructure.TIM_OCMode = TIM_OCMode_Toggle;
TIM_OCInitStructure.TIM_Pulse = 0xFFF+1;
TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Enable;
TIM_OCInitStructure.TIM_OCPolarity = TIM_OCPolarity_High;
TIM_OCInitStructure.TIM_OCIdleState = TIM_OCIdleState_Reset;
TIM_OC1Init(TIM4, &TIM_OCInitStructure);
TIM_ITConfig(TIM4,TIM_IT_CC1,ENABLE);
TIM_Cmd(TIM4, ENABLE);
}

#clive1 #capture-copmpare-stm32-tim

chen · ‎2014-02-07

Posted on February 07, 2014 at 13:21

Hi

''I have a problem understanding the capture compare functionality.'' Everybody does - its complicated. ''

I want to use channel1 to fire an interrupt if a match between CCR and counter is detected. In this case, CCR1 is set to 0xFFF+1.

'' ''

http://guyvo-cortex.blogspot.de/2009/04/pulse-timers.html

this guy describes how it work'' ''

TIM_TimeBaseStructure.TIM_Prescaler = 10000;
//((Prescaler/2)-1);
TIM_TimeBaseStructure.TIM_ClockDivision = 0;

'' Firstly - do you understand the basic timer operation (count, overflow, reset)? (Looking at the prescaler value - your timer is going to run at a slow rate - is that what you want?) ''I have a problem understanding the capture compare functionality.'' Capture (input capture) and compare (output compare) are actually 2 different modes of the timer!

matthias23 · ‎2014-02-07

Posted on February 07, 2014 at 13:34

Of course I know the prescaler is very high

I want to visualize the problem using an LED!

So again:

Actually I just want to raise an interrupt if there is a match between counter a CCR. This works. When I use half the value of the counter ARR-register, the interrupt is raised on this value.

In the datasheet I read:

When a match is found between the capture/compare register and the counter, the output

compare function: - Generates an interrupt if the corresponding interrupt mask is set (CCXIE bit in the

TIMx_DIER register).

So if the CCR is higher than the counter ARR there should be no interrupt. But there is on? Why? Can I disable it?

CLIVE1 where are you

J

chen · ‎2014-02-07

Posted on February 07, 2014 at 13:48

Hi

''

So if the CCR is higher than the counter ARR there should be no interrupt. But there is on? Why? Can I disable it?

''

Which event is causing the IRQ?

Which bit is set in TIMx_SR?

''

CLIVE1 where are you

J

''

If you do not want my help - OK

matthias23 · ‎2014-02-07

Posted on February 07, 2014 at 14:21

Of coure I also want your help!

Here my Problem in pictures:

void
TIM_test(
void
)
{
TIM_TimeBaseInitTypeDef TIM_TimeBaseStructure;
TIM_OCInitTypeDef TIM_OCInitStructure;
TIM_DeInit( TIM4 );
TIM_TimeBaseStructInit( &TIM_TimeBaseStructure );
TIM_TimeBaseStructure.TIM_Period =20000;
TIM_TimeBaseStructure.TIM_Prescaler = 10000;
TIM_TimeBaseStructure.TIM_ClockDivision = 0;
TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;
TIM_TimeBaseInit(TIM4, &TIM_TimeBaseStructure);
TIM_OCStructInit( &TIM_OCInitStructure );
TIM_OCInitStructure.TIM_OCMode =TIM_OCMode_Toggle;
TIM_OCInitStructure.TIM_Pulse = 20000; 
//When a match is found between the capture/compare register and the counter
TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Enable;
TIM_OCInitStructure.TIM_OCPolarity = TIM_OCPolarity_High;
TIM_OCInitStructure.TIM_OCIdleState = TIM_OCIdleState_Reset;
TIM_OC3Init(TIM4, &TIM_OCInitStructure);
TIM_OCStructInit( &TIM_OCInitStructure );
TIM_OCInitStructure.TIM_OCMode = TIM_OCMode_Inactive;
TIM_OCInitStructure.TIM_Pulse = 20000+1;
TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Enable;
TIM_OCInitStructure.TIM_OCPolarity = TIM_OCPolarity_High;
TIM_OCInitStructure.TIM_OCIdleState = TIM_OCIdleState_Reset;
TIM_OC1Init(TIM4, &TIM_OCInitStructure);
TIM_ITConfig(TIM4,TIM_IT_CC1,ENABLE);
TIM_Cmd(TIM4, ENABLE);
}

I use the toggle mode to output a waveform on TIM4_CH3 And TIM4_CH1 to generate an interrupt. so I have 4 scenarios: The question is why is there a pulse in scenario 4 on CH1??

CH1_Pulse = 20000

CH3_Pulse = 20000

CH1_Pulse = 10000

CH3_Pulse = 20000

CH1_Pulse = 20000

CH3_Pulse = 20000+1

CH1_Pulse = 20000+1

CH3_Pulse = 20000

waclawek.jan · ‎2014-02-07

Posted on February 07, 2014 at 14:24

This is a feature (*).

RM0090 rev.5 p.619, ch.18.4.5 - description of TIMx_SR.CC1IF:

When the contents of TIMx_CCR1 are greater than the contents of TIMx_ARR, the CC1IF bit

goes high on the counter overflow (in upcounting and up/down-counting modes) or underflow

(in downcounting mode)

JW

(*)

if (Surprising()) {

if (Documented()) {

IsAFeature();

}else {

IsABug();

}

matthias23 · ‎2014-02-07

Posted on February 07, 2014 at 14:31

Thanks

waclawek.jan

, this is the answer I was searching for, but not the one I was hoping for!

this do not make any sense! If I want to have this interrupt I can use TIM4_UPDATE!

Edit: This is new with cortex M4 in Cortex M3 reference manual the world was still in order!