PICAXE outputting PWM at the wrong frequency??

[dmaxben]

After making do with a cheap chinese multimeter for years, I finally decided to treat myself to a "real" multimeter. So I picked up a nice shiny Fluke 87-V.

I was eager to try out the frequency and duty-cycle readout capabilities of the 87-V so first thing I did when I opened up the box was try it out on a little PICAXE PWM circuit that I have been using/running successfully for a long time.

The simple circuit I have here drives a small 12v PWM solenoid with an ST-Micro P20NF06L MOSFET via a PWM output on a Picaxe 14m2. The circuit works perfectly fine and the solenoid operates normally... The circuit is just running on the bench, off a proper/regulated DC power supply. I have the DC bench power supply set to 12.0 volts (to run the solenoid), and then stepped down with an LM2940t with the proper input/output caps to insure stable 5v operation for the PICAXE itself.

But after putting my new Fluke across the terminals of the solenoid, the duty cycle % reads normally (ramps up from 0% to 98% steady-state)...but when I set the multimeter to read frequency, it shows that the frequency of my PWM circuit is running at ~400hz. My code/program, as written, says it should be running the PWM at 100hz...

Using the PWM Wizard, this is what I came up with.

main:
pause 25
pwmout pwmdiv64, B.2, 155, 612
goto main

The above code 'should' result in 98% duty cycle at 100hz...but my multimeter shows the thing running at 400hz.

I am doing some "multi-tasking" on this program...I have the 14m2 doing the solenoid PWM while simualtaniously monitoring some button presses and 2 ADC inputs. This might be a stupid question that I completely missed when reading the details about how exactly the "multiple tasks" feature works on the M2 chips...but perhaps does the chip run faster when "multi-tasking", thus altering the PWM-out frequency? Or am I being a bonehead and simply not using my fancy new multimeter correctly?

I tried searching on the forum, but couldnt really find anything...

thanks
Ben

 

[papaof2]

You need to look at the voltage across the solenoid with a scope - there could very well be spikes produced when the PWM pulse turns off and the frequency meter is counting the spikes as well as the PWM pulses.

 

[inglewoodpete]

The above code 'should' result in 98% duty cycle at 100hz...but my multimeter shows the thing running at 400hz.

I am doing some "multi-tasking" on this program...I have the 14m2 doing the solenoid PWM while simualtaniously monitoring some button presses and 2 ADC inputs. This might be a stupid question that I completely missed when reading the details about how exactly the "multiple tasks" feature works on the M2 chips...but perhaps does the chip run faster when "multi-tasking", thus altering the PWM-out frequency? Or am I being a bonehead and simply not using my fancy new multimeter correctly?

I think you're spot on the money. When the M2s do their multitasking, their default speed is automatically increased to 16MHz. Um, that's 4 times the default speed! Did someone say 4 times?

 

[Technical]

Please take a read of the 'parallel tasks' section in part 1 of the PICAXE manual, this explains about this 16MHz speed and effect on pwm