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TMS570LC4357: SPI in slave mode, configure PRESCALE or not?

Robert Finger
Genius 17077 points
Part Number: TMS570LC4357

Team,

When the SPI is operating in slave mode, is PRESCALE (SPIFMTn Register) configuration needed or not? TRM and datasheet are not clear. 

The TRM says in Table 28-30 that configuration is not needed:

But the datasheet lists conditions for the prescaler in table 7-39 (SPI slave mode timing):

Which one is true? 

Thanks,
  Robert

  • 0
    TI__Guru**** 193106 points

    Hi Robert,

    The SPICLK is driven by the SPI master. The prescale is not used by the SPI slave. The description in TRM is correct. For SPI slave, whatever the PRESCALE is configured, the input SPI clock should meet this requirement: period >= 40ns or the SPICLK frequency <= 25MHz 

  • 0 in reply to QJ Wang
    TI__Genius 17077 points

    Thanks QJ!

    I guess if SPICLK is 25 MHz, then VCLK needs to run at 50 MHz or more?! 

    The TRM says (page 1527): The maximum input frequency on the SPICLK pin when in slave mode is VCLK frequency /2.

     

  • 0 in reply to Robert Finger
    TI__Guru**** 193106 points

    Hello,

    SPI Baudrate = VCLK / (PRESCALEx + 1)

    When PRESCALEx is cleared to 0, the SPI clock rate defaults to VCLK/2.

    So the SPI baudrate should not be > VCLK/2 in master side.

    Yes, you are right. SPI baudrate should be <= slave VCLK/2