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Tiva C Series Launchpad - PWM signals have low voltage

Other Parts Discussed in Thread: EK-TM4C123GXL

I am using the tiva c series launchpad ( EK-TM4C123GXL ). The issue I am having is when I initialize M0PWM0-7, but do not initialize M1PWM0-7, M0PWM0-1 show PWM waves with a peak voltage of 2.5v rather than 3.3v. This reduced voltage is too lower to control my servos. M0PWM2-7 still show 3.3v. Enabling all 16 pwm output (M0PWM0-7 and M1PWM0-7) results in all pwm pins outputting a 3.3v square wave. Disabling M1PWM0 and M1PWM1 results in M0PWM0 and M0PWM1 then again having a 2.5v peak wave. Any support in solving this issue would be much appreciated.

  • I've diagnosed this (correctly) before - likely issue is the (always hallowed) 0-ohm resistors - which short 2 pairs of MCU pins - most assuredly 2 of those your desired PWM outputs.

    Check your launchpad schematic for those 0R resistors - and remove/banish them.  Imho this was a NRND decision from the get-go - such Rs should have been clear-packed for use by those (extremely few) who sought compatibility w/M0 killer.  (aka MSP family)  Useless and destructive - for vast numbers here...

    Note - I'm assuming that you're not over-loading those PWM outputs via too low - ground impedance - circuitry.

  •   , thank you very much for the quick and knowledgable response.

     

    After looking at the schematic available here - http://www.ti.com/lit/ug/spmu296/spmu296.pdf - page 20 shows R9 and R10 0 ohm resistors linking the PWM pins I am having issues with, so I am fairly confident you are correct! I am a senior ECE student, and am questioning what the different is between a 0hm resistor and a wire, any chance you could aid me in understanding this? More importantly, how do you recommend I "Banish" these resistors? Is this a physical task? Thank you very much for your assistance, it is very appreciated.

  • Rare (for this reporter) to receive so polished & appreciative a response - thank you, Taylor.

    Again - like in dayze of olde - cars get repaired via "past experience/history" far faster/efficiently than detailed troubleshooting.

    Banishment (quite proper) of those offending 0-ohm resistors requires a solder iron and careful desoldering techniques.  (a desoldering tweezer - designed for smt (surface mount) proves best)  We have such - but we're "in the biz."

    0-ohm R and conductive wire perform the same task - yet 0-ohm R may be "auto-inserted."  We have "Pick n Place" automatic component inserter - easily capable of inserting 0-ohm R in resistor format.  Plain wire - not so much - that explains "why" a 0-ohm R exists.  (good question by you - actually)  Should you be Ebola free - and reasonably near Chicago - we'll invite you to our plant - you can watch Pick n Place and 20' reflow oven "do their thing."

    Surely one in your school has some soldering capability - go slowly/carefully - and insure that no solder continues to bridge the resistor connection once your board liberates both resistors...

    Bon chance, mon ami.