Launch Pad EK-LM4F120XL, duty 0 not work of　PWM mode of GPTM

Hello Kechan,

I don;t think the 0% DC with the timer is possible for PWM. There was a thread in the forum few months back where the same issue we debugged to be a limitation from the GPT's PWM mode.

Regards

Amit

Agree - w/Amit - many ARM MCUs have difficulty in reaching PWM extremes - either 0 or 100% PWM duty.

As a work-around you may gate the MCU's PWM output - so that an additional GPIO provides both the 0 or 100% duty cycles - as/if/when needed.

Our group has found that for many applications - the necessity for 0 and 100% PWM is minimal - we drive Brushless Motors & various lighting schemes quite effectively via 3-97% PWM duty range.

I can confirm that 0% and 100% duty will not work. 

I did had a application that i needed 0% duty but  it's 1 rare application. You need to configure a pin to be a GPIO and use it as a GPIO to set 0% and 100% duty. To use it as a timer pwm output again you have to re-configure it back.

Luis Afonso said:

...to use it as a timer pwm output again you have to re-configure it back.

And while that may work for some applications - the earlier suggestion of using the added GPIO as a "gating" signal (or control) eliminates the time/effort penalty imposed by any such, "re-configuration" - thus proves a superior solution...

Is that possible without external hardware (yes i know it's just a few transistors)?

A properly chosen series R upon PWM output enables a simple GPIO to, "pull down" that PWM - yielding 0 duty cycle.  That GPIO reverts to open-drain (off) when 0 duty is not ordered.

To force the output to 100% duty cycle the GPIO is switched to push-pull and driven high.  No transistors are required.

To gain performance a small (SOT-23) gate (available from this vendor) may be imposed between the MCU's PWM output and the external device's PWM input.  GPIO then is employed @ PWM extremes...

In either case - the original PWM signal & set-up/configuration is untouched...

Hi,

Yes, there are some other brands which build micros with this feature. 

Petrei

Petrei said:

other brands which build micros with this feature. 

Quite true - we use ARM MCUs (currently) from 4 makers - and each/every one enjoys/displays this PWM, "feature!"

But again - series R and extra GPIO creates 0% duty-cycle - and for most all other Apps - 97-98% duty-cycle - achieved via these MCUs w/out modification - proves, "Good for G'ovt work."

I'd bet that poster experimented (which is good) and noted this PWM limitation.  Yet - in the "real world" such should not prove "game-breaker" (and if not acceptable) work-arounds have been described...

Hi,

Although I did not expressed, this is better than having that feature and then looking for some other circuitry to detect the missing pulse - I am in the area where this is a dangerous, so thanks to ARM/TI to avoid such. The user is free to choose whatever is more convenient. But your solution is of coarse much more simpler to realise than detecting a missing pulse and stopping the micro and the rest of hardware.

Regards,

Petrei

cb1- said:

A properly chosen series R upon PWM output enables a simple GPIO to, "pull down" that PWM - yielding 0 duty cycle.  That GPIO reverts to open-drain (off) when 0 duty is not ordered.

To force the output to 100% duty cycle the GPIO is switched to push-pull and driven high.  No transistors are required.

To gain performance a small (SOT-23) gate (available from this vendor) may be imposed between the MCU's PWM output and the external device's PWM input.  GPIO then is employed @ PWM extremes...

In either case - the original PWM signal & set-up/configuration is untouched...

alright, no argues there. gonna try it out latter (to see if i understood it also)

Helio TI Emplyee,

Question1
"Figure 11-7. CCP Output, GPTMTnMATCHR = GPTMTnILR
CCP not set if GPTMnMATCHR = GPTMnILR" of "11 General-Purpose Timers"in device datasheet.
Is Stellaris LM4F120H5QR and Tiva TM4C123GH6PM data sheet description incorrect?

When data sheet is incorrect,

Question2
To the duty to zero, what to zero by the GPIO as follows and next PWM set reconfig as follows?

---------------------------------------------------------------------------------
test soft sorce

duty    full-1 -> 0 ->  full-1 change example

//*************************************
// pwm width full-1
//*************************************
    //Timer0 stop
    TimerDisable(TIMER0_BASE, TIMER_B);

    //PWM value SET  PWM cycle 4ms
    ulCycle = ulSysClock / 1000 * 4;
    ulCycleScale = ulCycle >> 16;
    ulCycle &= 0xFFFF;
    TimerPrescaleSet(TIMER0_BASE, TIMER_B, ulCycleScale);
    TimerLoadSet(TIMER0_BASE, TIMER_B, ulCycle);

    //Pulse width SET
    ulDutyScale = 0;
    ulDuty =  1;
    TimerPrescaleMatchSet(TIMER0_BASE, TIMER_B, ulDutyScale);
    TimerMatchSet(TIMER0_BASE, TIMER_B, ulDuty);

    TimerEnable(TIMER0_BASE, TIMER_B);  //Timer0 start

    //set change timing monitor
 GPIOPinWrite(GPIO_PORTF_BASE, GPIO_PIN_2,4);
    SysCtlDelay(ulSysClock / 1000 * 1);  //3ms
    GPIOPinWrite(GPIO_PORTF_BASE, GPIO_PIN_2,0);

    //delay
    SysCtlDelay(ulSysClock / 1000 * 4);  //12ms

//*************************************
// pwm width 0  GPIO:0
//*************************************

     //GPIO_PIN_1: PWM out   Timer0-A  port B6 red
     GPIOPinTypeGPIOOutput(GPIO_PORTF_BASE, GPIO_PIN_1);
     GPIOPinWrite(GPIO_PORTF_BASE, GPIO_PIN_1, 0x00);

    //set change timing monitor
    GPIOPinWrite(GPIO_PORTF_BASE, GPIO_PIN_2,4);
    SysCtlDelay(ulSysClock / 1000 * 1);  //3ms
    GPIOPinWrite(GPIO_PORTF_BASE, GPIO_PIN_2,0);

    //delay
    SysCtlDelay(ulSysClock / 1000 * 4);  //12ms

//next pwm

//*************************************
// reconfig
//*************************************
    //configure gptm
        SysCtlPeripheralEnable(SYSCTL_PERIPH_TIMER0);
        SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOF);
        TimerConfigure(TIMER0_BASE, TIMER_CFG_SPLIT_PAIR | TIMER_CFG_B_PWM);
        TimerControlLevel(TIMER0_BASE, TIMER_B, 0);      // Timer0-A High-Active

    // GPIO configure
        GPIOPinConfigure(GPIO_PF1_T0CCP1);
        GPIOPinTypeTimer(GPIO_PORTF_BASE, GPIO_PIN_1);

//*************************************
// full-1
//*************************************
        //Timer0 stop
        TimerDisable(TIMER0_BASE, TIMER_B);

        //PWM value SET  PWM cycle 4ms
        ulCycle = ulSysClock / 1000 * 4;
        ulCycleScale = ulCycle >> 16;
        ulCycle &= 0xFFFF;
        TimerPrescaleSet(TIMER0_BASE, TIMER_B, ulCycleScale);
        TimerLoadSet(TIMER0_BASE, TIMER_B, ulCycle);

        //Pulse width SET
        ulDutyScale = 0;
        ulDuty =  1;
        TimerPrescaleMatchSet(TIMER0_BASE, TIMER_B, ulDutyScale);
        TimerMatchSet(TIMER0_BASE, TIMER_B, ulDuty);

        TimerEnable(TIMER0_BASE, TIMER_B);  //Timer0 start

        //set change timing monitor
     GPIOPinWrite(GPIO_PORTF_BASE, GPIO_PIN_2,4);
        SysCtlDelay(ulSysClock / 1000 * 1);  //3ms
        GPIOPinWrite(GPIO_PORTF_BASE, GPIO_PIN_2,0);

Hello Kechan,

There are multiple such data sheet issues which we need to fix. When!!! I can't give a date right now, but not soon enough.

Regarding the second point of making it 0 or 1, a better way has been shown before by cb1. The way to implement it in TM4C would be different on the SW side. Do you want to use tat method or the custom method you have shown?

Regards

Amit

Hello TI Emplyee,

Excuse me, Which source I mistake is different from method that cb1 shown?

Hello Kechan,

cb1 mentioned using the Open Drain feature and Push Pull with driven '1' for 0 and 100% Duty Cycle. The above method is not inline with that. That is what I was stating.

Regards

Amit