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TM4C123GH6PM: 20 mhz oscillator for TM4C123GH6PM

can altineller
Expert 1500 points
Part Number: TM4C123GH6PM

Hello,

I have a custom circuit where I used a 20 mhz oscillator fed into osc0 and with osc1 left floating.

I have been initializing the programs with:

    ROM_FPUEnable();
    ROM_FPULazyStackingEnable();
    ROM_SysCtlClockSet(SYSCTL_SYSDIV_2_5 | SYSCTL_USE_PLL | SYSCTL_XTAL_20MHZ | SYSCTL_OSC_MAIN);

Before I was using 16mhz crystals for my custom tiva circuits, but now I am using 20mhz source, what changes do I have to make, if I want to run the tm4c123gh6pm 'n the fastest mode?

I only changed the SYSCTL_XTAL_16MHZ to SYSCTL_XTAL_20MHZ, leaving SYSCTL_SYSDIV_2_5 unchanged.

I also have tested the USB, and it also does work. Is there anything I need to change apart from these?

Best Regards,

C.A.

  • +1
    TI__Guru**** 169336 points

    Hi Can,

      What you are doing in ROM_SysCtlClockSet(SYSCTL_SYSDIV_2_5 | SYSCTL_USE_PLL | SYSCTL_XTAL_20MHZ | SYSCTL_OSC_MAIN) is correct. When using the PLL, the VCO frequency of 400 MHz is predivided by 2 before the divisor is applied. In another word, you will be dividing 200Mhz by 2.5 which gives you 80Mhz. This is the maximum frequency TM4C123 is spec'ed for.