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AM335x - Unused GPMC_CLK

Daisuke Maeda
Guru 20205 points

For GPMC_CLK, TRM describes as follows.

"It is also recommended to place a 33-ohm resistor in series (close to the processor) to avoid signal reflections."

I understand that the recommendation means the series termination.

If GPMC_CLK is not selected by the mux mode, can the recommendation be ignored?

Best regards,

Daisuke

 

  • 0
    TI__Guru**** 393215 points
    Hi Daisuke-san,
     
    Yes, this recommendation is only valid if GPMC_CLK is selected by the mux mode. For other muxmodes series termination is not necessary.
     
    In general this series termination rule applies for most clock signals that are sourced from the processor - like SPI clock, MMC clock, etc.
  • 0 in reply to Biser Gatchev-XID
    Guru 20205 points

    Hi Biser,

    Thank you for your reply.

    I have other questions. I would like to make sure how to use the GPMC correctly.

    TRM describes:

    "GPMC_CLK is also used as a re-timing input. The associated CONF_<module>_<pin>_RXACTIVE bit for the output clock must be set to 1 to enable the clock input back to the module. "

    If GPMC is used, must the RXACTIVE bit be set to 1?

    To enable the output clock, must GPMC_CLK be selected by the mux mode?

    Best regards,

    Daisuke

     

  • 0 in reply to Daisuke Maeda
    TI__Guru**** 393215 points
    Output clock will only be enabled if GPMC_CLK is selected by the mux mode. In this case RXACTIVE must also be set. Please note that the GPMC will output GPMC_CLK for synchronous accesses only.
  • 0 in reply to Biser Gatchev-XID
    Guru 20205 points

    Hi Biser,

    Thank you for your reply.

    Our customer uses the GPMC as the asynchronous mode.
    GPMC_WAIT pins are used. RXACTIVE bit is set to 1 because GPMC_WAIT timing is expressed as a number of GPMC_CLK clock cycles.
    GPMC_CLK is not output as it is not selected by the mux mode.

    Is their configuration correct?

    Best regards,

    Daisuke

     

  • 0 in reply to Daisuke Maeda
    TI__Guru**** 393215 points
    I suppose you are talking about the WAITPINMONITORING parameter. RXACTIVE is not relevant in this case, since GPMC_CLK is not output to the pin. Do they have anything else connected to this pin?
  • 0 in reply to Biser Gatchev-XID
    Guru 20205 points

    Hi Biser,

    Thank you for your reply.

    They do not have anything connected to GPMC_CLK pin. GPMC is only used as the asynchronous mode with the wait pins. In their case should RXACTIVE be not set to 1?

    If GPMC_CLK is used as the re-timing input, RXACTIVE bit must be set to 1. I do not know what the re-timing input is used for.

    Best regards,

    Daisuke

     

  • 0 in reply to Daisuke Maeda
    TI__Guru**** 393215 points
    If the pin muxmode is not set to GPMC_CLK it doesn't matter what RXACTIVE is set to, as the clock signal does not reach the pin.
     
    Retiming means that the clock signal, which is output from the pin is also fed back to the processor, and input data is clocked by this signal, rather than an internal clock signal. In your case this is not necessary.
  • 0 in reply to Biser Gatchev-XID
    Guru 20205 points

    Hi Biser,

    Thank you for your reply.

    I understand that RXACTIVE bit must be set to 1 if GPMC is used as the synchronous mode.

    Best regards,

    Daisuke