getADC() returns 0.0 with TMR5 active

pjc30943 · Joined: 02 Dec 2005

I looked in the resource usage section, but I'm still uncertain why I'm seeing this specific effect:

When TMR5 is untouched, the getADC() function works all the time.
After setting up TMR5, getADC() often returns 0.0.

What shared resources do they have?

At first though it seemed to be an interrupt-related issue since some are enabled, but interrupts are disabled around the getADC() command with no change.

dkinzer · Site Admin Joined: 03 Sep 2005 Location: Portland, OR

dkinzer · Site Admin Joined: 03 Sep 2005 Location: Portland, OR

It could also be caused by a resource conflict if multiple tasks attempt to use the ADC. There is only one ADC in the system and if a conversion is underway when another call to GetADC() is made the latter call will silently return 0.

The locking method described in my previous response will guard against this problem unless an ISR uses the ADC. The alternate method to protect against this problem is to create a semaphore variable representing the ADC. Then, in each place that you call GetADC() you'll want to first acquire the semaphore, call GetADC() and then clear the semaphore. This technique can be used on any resource that must be used serially. Note, however, that you probably do not want to use this technique in an ISR because of the delay that it will introduce.