PWM Problem

[websterling 12]

I'm trying to use Energia for a project and I'm having problems with PWM. My understanding is that the following code should continually toggle the green led between full on and pwm with a value of 14. This is on a 2553 on a Rev. 1.4 Launchpad with the crystal installed.

 

void setup() {
 pinMode(GREEN_LED, OUTPUT);
}

void loop(){
digitalWrite(GREEN_LED, HIGH);
delay(2000);  
analogWrite(GREEN_LED,14);
delay(2000);
}

When the code runs the led is full on. After 2 seconds it goes dim with the pwm. After 2 more seconds I expected the led to go to full brightness as the code loops, but it remains dim. The problem also exists using analogWrite(GREEN_LED, 255) instead of digitalWrite(GREEN_LED, HIGH).

 

Am I missing something here, or is there a bug in Energia?

[energia 485]

Which release are you on?

 

Release 0101E0006 has a bug. TA0CTL is setup incorrectly for T0A1 which is P1.6 (GREEN_LED). The fix has already been checked into master. Diff is here: https://github.com/energia/Energia/comm ... 9b3ed05479

 

analogWrite(GREEN_LED, 255) should work though. Will look into it.

 

Robert

[websterling 12]

Hey, Robert,

 

I'm on 0101E0006, and I saw the fix and applied it a few days ago. Are there any other fixes that might need to be applied in this situation?

 

Should the code I posted work the way I described it?

 

Thanks,

George

[energia 485]

No other fixes have been done in this area.

Code should work but you might have hit a new bug. I will have a look at it with an o-scope this afternoon and see if I can reproduce it.

[energia 485]

This is confirmed to be a bug. digitalWrite leaves the pin in PWM mode. A digitalWrite drives the line high and then PWM takes over again immediately after that. Thank you for the report. WIll file a bug and post the patch in this thread.

 

A way around this is:

 

void setup() {
 pinMode(GREEN_LED, OUTPUT);
}

void loop(){
analogWrite(GREEN_LED,254); // write with 255 does a digitalWrite(, HIGH) so will not work
delay(2000);  
analogWrite(GREEN_LED,14);
delay(2000);
}

[websterling 12]

Thanks for filing the bug report. I already tried the work around while I was trying to get a handle on the problem.

 

I'll be glad when there's a patch.

 

Thanks,

George

[energia 485]

Bug is fixed. Pull request with fix has been posted for review:

https://github.com/energia/Energia/pull/64/files

 

--- a/hardware/msp430/cores/msp430/wiring_digital.c
+++ b/hardware/msp430/cores/msp430/wiring_digital.c
@@ -79,9 +79,17 @@ void digitalWrite(uint8_t pin, uint8_t val)
       uint8_t bit = digitalPinToBitMask(pin);
       uint8_t port = digitalPinToPort(pin);
       volatile uint16_t *out;
+       volatile uint16_t *sel;

       if (port == NOT_A_PORT) return;

+       /*
+        * Clear bit in PxSEL register to select GPIO function. Other functions like analogWrite(...) 
+        * will set this bit so need to clear it.
+        */
+       sel = portSelRegister(port);    /* get the port function select register address */
+       *sel &= ~bit;                   /* clear bit in pin function select register */
+
       out = portOutputRegister(port);

       if (val == LOW) {

[websterling 12]

Thanks, Robert,

 

It now works as expected.