• State TI Thinks Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 1 answer
  • Subscribers 31 subscribers
  • Views 752 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TM4C129EKCPDT: Current consumption with 25 MHz main clock

Alex Naylor
Prodigy 90 points
Part Number: TM4C129EKCPDT

Is there any current consumption information, like that listed in section 30.23 of the datasheet for a 25 MHz crystal? I was using one of the reference designs for a project and noticed that a 25 MHz crystal was used, but the closest current consumption information is for a 16 MHz crystal.

Thank you,

Alex

  • 0
    TI__Guru 72500 points
    The current consumption in section 30.23 is independent of the external crystal. The 120MHz and 60MHz values are based on using the main oscillator and PLL. The 16MHz version is for using the "PIOSC" (precision internal oscillator). The internal oscillator is always 16MHz. Are you trying to calculate current consumption when running at 25MHz from the external crystal without using the PLL? You can get a good estimate by taking the 60MHz and 120MHz values at the appropriate temperature and doing a straight line extrapolation down to 25MHz.
  • 0 in reply to Bob Crosby
    Prodigy 90 points
    Hi Bob,

    Thank you for the explanation, everything is a lot clearer now. I did not realize that the MCU had an internal clock source as well as an option for an external one that can be used with the internal PLL.

    Thank you,
    Alex
  • 0 in reply to Alex Naylor
    Guru 117855 points

    I've another "take" on this subject - may or may not prove of interest/value.

    Long ago (circa 2010 iirc) we experimented with several "LM3S" MCUs (first ones introduced here) and we discovered that, "The use of the highest "legal" external crystal frequency - yielded the lowest power consumption!"     In all such test cases - we employed the MCU's PLL - and operated at the highest legal, MCU System Clock.     Always and only - the crystal alone - was changed - then tested...

    In the attempt to be "scientifically sound" (at least upon the surface) we chose crystals from several "known/trusted" crystal makers - which provided, "Low, Medium, and Max" frequencies - all w/in the MCU's specifications.     (and employed the maker spec'ed, bypass caps, as well)

    Almost always - the use of the "highest legal frequency, external crystal" yielded the lowest MCU operating current.     (we insured that a "reasonable" use of our (usual) MCU peripherals occurred - that our test software was "locked down" - and that our, "Temperature and Voltage conditions"  were as close as possible.)

    More recently (perhaps 2015) we repeated such a test w/another's Cortex M4 (running 180MHz) and the identical results revealed.

    The difference between current consumption proved at/around 4-8% - with the greatest difference noted between (legal) "Min & Max" - external crystal frequencies.    In addition - current increased in accord w/the "number of MCU Peripherals active" (that's expected) and different peripherals altered the "quantitative" findings - never those "qualitative!"    (i.e. higher frequency xtals reduced power consumption!)

    Our (unproven) explanation is that the MCU's PLL required more energy to "spin up" - when lower crystal frequencies were employed...

  • 0 in reply to cb1_mobile
    Prodigy 90 points

    Interesting find, might be worth further investigation on my end!

  • 0 in reply to Alex Naylor
    Guru 117855 points
    One would hope - especially if "current consumption" proves significant to your design.

    To raise the test's accuracy - crystals chosen should be from the same crystal series w/in that crystal vendor's inventory.

    Take care to alter the bypass caps - if & when - vendor notes/directs. Temperature, Test Software, & MCU Voltage should be as consistent as possible - test to test...