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TM4C1297NCZAD: Using MOSC as oscillator source

Silke
Prodigy 30 points
Part Number: TM4C1297NCZAD

We are currently running our system clock from MOSC, no PLL, with an external 25MHz oscillator (using SysCtlClockFreqSet() from TivaWare 2.1.0).

According to errata SYSCTL#23 (bus fault on reset) it is not recommended to use MOSC as oscillator source. The last line of the new SysCtlClockFreqSet() function forces the PIOSC as oscillator source. That seems to be a rather big constraint. Here are my questions:

 

1. Can I really only run this processor from the internal oscillator (PIOSC)?

 

2. Would using the MOSC in combination with the PLL also be a work-around for SYSCTL#23?

 

3. Does the issue (bus fault) apply to any kind of reset? Including a reset from an external watchdog?

I thought about switching from MOSC to PIOSC prior to an intentional software reset but if the external watchdog resets I would still be running from the MOSC.

 

Thanks for your help!

 

What is your recommendation to go forward?

  • 0
    Guru 13040 points
    SIlke,

    I am not the expert on this subject, but can share my reading:

    The errata says not to use MOSC directly as a source of OSCCLK, but rather to use PLL - and then on, PLL can be used then into SYSCLK.

    You are still perfectly able to run the processor from the crystal. The SYSCTL_USE_PLL parameter must be used, exactly what you are asking/suggesting on your question #2. I actually don't know why not to use PLL, and use OSCCLK instead? To save some energy perhaps?

    I'd say you proceed business as usual, use your external xtal with PLL. DO NOT use the ROM API to set your clock, but rather use Flash Tivaware, taken from a version 2.1.3 on (also, is there a particular reason why you use an older TivaWare?)

    If the processor had been good to be used ONLY from PIOSC, it would probably not even pass pre-production stage... We've never experience bus fault on any kind of reset coming from MOSC->PLL->SYSCLK.

    Bruno
  • 0 in reply to Bruno Saraiva
    Guru 117855 points

    Good job (we feel) Bruno - nicely detailed & the "necessary limitations" enforced by such, "PIOSC-only" usage - very much - "make your case!"