TM4C123GH6PZ: Crystal Oscillator selection

Hello Ryan,

As long as the crystal oscillator meets the required specs for the device which are outlined in Section 24.9.5 Main Oscillator Specifications that should be fine. We also have a list of crystal oscillators provided in the device datasheet. For this device you can find the full list on Table 24-19 Crystal Parameters. This also provides the values of any external components that should pair with those oscillators. Most customers use this to find a suitable oscillator for their design.

Regarding the internal clock running at 80MHz, this is done by using the PLL inside of the device. The PLL can operate as long as the external oscillator ranges from 5 MHz up to 25 MHz. When running, the PLL operates at 200 MHz. From there, a clock divisor of 2.5 is applied in order to get the 80 MHz system clock frequency.

This is done in TivaWare with the following API:

    //
    // Set the system clock to run at 80MHz off PLL with external crystal as
    // reference.
    //
    SysCtlClockSet(SYSCTL_SYSDIV_2_5 | SYSCTL_USE_PLL | SYSCTL_XTAL_16MHZ |
                   SYSCTL_OSC_MAIN);

For more details about the PLL and Clocking schema for the TM4C123x, I would recommend reviewing Section 5.2.5 Clock Control of the device datasheet.

Best Regards,
Ralph Jacobi