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TM4C123GH6PM: MOSC crystal frequency selection for the TivaC

can altineller
Expert 1760 points
Part Number: TM4C123GH6PM


Tool/software:

Hello,

Page 1379 of the datasheet states the crystal configururations for tm4c123 circuits. Some frequencies are not suitable for usb. Apart from that, the crystal oscillator frequency is converted to a 80mhz clock internally using an internal PLL, so what would be difference say using a 10mhz crystal as opposed to 20mhz?

In my custom circuit, I have a 20mhz oscillator. What would the effects be if I use a 24mhz oscillator or even a 10mhz oscillator? Since the PLL scales it up to 80mhz, should not affect anything? Am I correct?

Could it have an effect on system peripherals such as the SPI? or ADC?

Best Regards,

C.

  • 0
    TI__Guru**** 191906 points
    can altineller said:
    In my custom circuit, I have a 20mhz oscillator. What would the effects be if I use a 24mhz oscillator or even a 10mhz oscillator? Since the PLL scales it up to 80mhz, should not affect anything? Am I correct?

    Your crystal as an input source for the PLL can be between 5Mhz to 25Mhz. If you look at the sysctl.h file, 16, 20 and 24Mhz are supported. 80Mhz System clock can be achieved using these crystal frequencies. 

    can altineller said:

    Could it have an effect on system peripherals such as the SPI? or ADC?

    No effect. The SPI uses divided system clock for the SPICLK. ADC module can take among the internal PIOSC or System Clock. 

  • 0 in reply to Charles Tsai
    Expert 1760 points

    Dear Charles.

    I have used 16mhz and 20mhz crystals with no problems. One thing I noticed was that I was not able to do 2.4mhz spi.

    Are you absolutely sure that, none of the components in the microcontroller is dependent on the oscillator frequency?

    I am planning to use a 10mhz crystal for the next revision, because I can not find a 16mhz,20mhz, or 24mhz crystal with same specs? I need a crystal that I can pull the frequency +-10ppm, and there are many more options for 10mhz.

    Best Regards,

    C.

  • 0 in reply to can altineller
    TI__Guru**** 191906 points
    can altineller said:
    I am planning to use a 10mhz crystal for the next revision, because I can not find a 16mhz,20mhz, or 24mhz crystal with same specs? I need a crystal that I can pull the frequency +-10ppm, and there are many more options for 10mhz.

    Hi,

      All I can say is that crystal between 5Mhz to 25Mhz will work for the PLL as the input clock. 10Mhz should work as well.