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TMS320F28377D: Input Clock Frequency using external crystal

Johnny Chen
Expert 4990 points
Part Number: TMS320F28377D


Hi Champ,

I am asking for my customer.

The input clock frequency is planned to gen. by external crystal.

As the DS mentioned, it is recommended to select the crystal freq. between 10M~20MHz. 

While the customer doesn't have the ones in stock, and ask what if they use the crystal with 25MHz  freq. ? Is there a big impact on the clocking, or we would strongly suggest not to do that ?

Second question is the EVM board crystal is designed to be connected with independent OSC_GND. Would the expert kindly give some advice would it be better to share the ground of Crystal (OSC_GND) with MCU S_GND ? Or it is better to separate independently?

Thanks and regards,

Johnny

  • +1
    TI__Guru* 95315 points
    Johnny Chen said:
    While the customer doesn't have the ones in stock, and ask what if they use the crystal with 25MHz  freq. ? I

    As shown in 8.9.3.2.1.1 Input Clock Frequency of the datasheet, maximum input frequency of an external crystal is capped at 20 MHz. I wouldn't advise a 25 MHz crystal.

    Johnny Chen said:
    would it be better to share the ground of Crystal (OSC_GND) with MCU S_GND ? Or it is better to separate independently?

    As advised in the datasheet, it must be separate.

  • 0 in reply to Hareesh Janakiraman
    TI__Expert 4990 points

    Hi Hareesh,

    Hareesh Janakiraman said:
    As advised in the datasheet, it must be separate.

    I found out the schematic of our F2837x controlCARD doesn't match with the word advised in the DS. It seems that they are having a sharing ground. Not sure if it is my misunderstanding ?

     (Figure 1)

    (Figure 2)

    (Figure 3)

    The source of VDD_3V3 and VDDOSC_MCU_3V3 in figure 1  actually do separate with connecting a bead between it, while it seems they are having the common GND.
    In figure 2 and figure 3, there is actually having another ground for crystal named "OSC_GND" connected with VSSOSC and the crystal. 

    I am confused whether the advise in DS is correct for must be separate, and there is wrong in the schematic of F2837x controlCARD ? Or actually the ground of Crystal (OSC_GND) could be share with MCU S_GND ? Would you pls clarify that for me ?

    Thanks and regards,

    Johnny.

  • 0 in reply to Johnny Chen
    TI__Guru* 95315 points

    Advice in the datasheet is correct. OSC_GND should not be shared with the common ground and that is how the schematic is. Perhaps C75, C76 & C91 should have been referenced to OSC_GND. I think that is what confused you.