MSP430F5418A: I need to change the PWM frequency to control Heater levels

Point of order, it is not the frequency that affects the heater power but the duty cycle. As long as you keep the pulse width the same as you are changing the frequency, you are changing the duty cycle.

And just to check - I can at any time change the mode direction? Somehow I found that changing between modes gives me the 3 settings - though I will have to experiment with different duty cycles to realize the full potential of the heater.

Thanks,

Mechi

I put the following in my code:

  // Set up PWM for heater
  //P1.4 output and options select
  P1SEL |= HEATER_BIT;                      // P1.4 options select
  TA0CCR0 = 512-1;                          // PWM Period
  TA0CCTL3 = OUTMOD_7;                      // CCR3 reset/set
  TA0CCR3 = 384;                            // CCR3 PWM duty cycle
  TA0CTL = TASSEL_2 + MC_1 + TACLR;         // SMCLK, up mode, clear TAR

  //__bis_SR_register(LPM0_bits);             // Enter LPM0

if the line

__bis_SR_register(LPM0_bits);             // Enter LPM0

is not commented, the Heater works - but I don't get any other interrupts and the system stops.

Without this line, the heater doesn't work.

How do I start the PWM and TA0 in a way that won't bother the rest of the system?

Can I change the PWM duty cycle (TA0CCR3) during the program? Do I have to stop and start anything, or can I just set the register at will?

With this code I receive the output pictured below.

4kHz frequency and 75% duty cycle - which is what I expected.

But why is there a 15+ms break between activity? As if the PWM itself works on a 50% duty cycle.

I figured this out - the gap was because the code linked to SMCLK (TASSEL_2) - and when I linked to TACLK (TASSEL_0), there isn't a gap.

But if I'm linked to any other clock - I can only get 50% duty cycle.

I'm going back to SMCLK - so that I can change the kHz frequency and the duty cycle.

Again - why are there the gaps?

The program has RS232 communication and A2D readings as well as the heater.

When running in the debugger, the program stops at this line and doesn't continue.

What is meant by "until explicitly awakened"?

Why must it be awakened? Isn't there a way to start the PWM without stopping...so it should be continuous and doesn't need any intervention?

Thanks,

Mechi

"explicitly awakened" by an ISR issuing a "LPM0_EXIT;" [Ref User Guide (SLAU208Q) Sec 1.4].

The PWM does not require any intervention. It runs until you stop it. (One way to stop it is to stop its clock, but SMCLK is not stopped in LPM0.) That example I mentioned above goes into LPM0 (with interrupts disabled) at the end. Its main() will never proceed, but the PWM continues to run.

Since your code is doing other things, are you sure you want to use LPM here?

[Edit: In case it needs to be said: You don't need to use LPM to use PWM. That LPM0 at the end of the Example is not part of the PWM. It's just part of the demonstration -- they ran out of things to do, so they (deliberately) stopped the CPU (forever) there, rather than, say, spinning in a "while(1);". ]

I don't want to use LPM0.

But if I remove that line, then the PWM doesn't work at all.

That's my question.

Hello Mechi,

Have you tried replacing __bis_SR_register(LPM0_bits); with __bis_SR_register( GIE ); and then if you need to enter a low power mode you do that on a separate line. The __bis_SR_register( GIE ); will enable interrupts globally and shouldn't force you into a LPM.

Luke

Thanks Bruce.

You're right - I thought of this also after work yesterday - and there is something that's interfering with the clock.

I changed the main clock to TB0 and now the device is working with the heater.

Thanks,

Mechi