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TMS570LS3137: Maximum SPI baud rate inquiry

BrandonAzbell
Guru 64560 points
Part Number: TMS570LS3137


There is some confusion regarding the MibSPI max baud rate for the TMS570LS3137. The tech reference manual states a maximum baud rate of 20MHz.

See spnu499b section 25.1:

 

The data sheet specifies a minimum cycle time of the SPI clock pin as 40ns (which equates to a 25MHz baud rate).

See spns162 section 5.10.4:

 

Also we are currently configuring MibSPI 1 to run at 26.667 MHz and it appears to be working (see attached scope plot).


What is the actual maximum baud rate of the MibSPI on the TMS570LS3137? Can we expect any issues running above the 20MHz specified in the technical reference manual?

  • 0
    TI__Guru 59540 points
    Hello Brandon,

    The TRM value is a legacy limit. i.e., we improved the performance on this generation of MCUs to support up to 25MHz. As an FYI, if there is every a conflict between the datasheet and the TRM, the datasheet always takes precedence as the datasheet is a commitment from TI that we have characterized operation at the given spec and no issues were identified.

    In regard to operating above the specified limit. As noted, the limit is characterized across many devices produced at far corners of our manufacturing process and at the extreme conditions such as max and min temperatures and max an min voltages. What the specified limits indicate is that we found that all devices will perform to this specification when produced according to the limits in place for our manufacturing process, specified voltage range, and specific temperature range. In short, you may be able to find a significant number of parts from TI that will work fine at the faster SPI bit rates; however, there is also some probability that you will receive devices that won't function properly at that rate or that conditions on your board such as supply voltages or temperature may cause devices to fail at those same bit rates. This is also something that could change over time due to aging and degradation of the internal components of the MCU.

    Hope this helps for understanding where the spec limits come from. In regard to the discrepancy between the datasheet and TRM, I will submit a document feedback to identify the difference so it can be corrected in the next update.
  • 0 in reply to Chuck Davenport
    TI__Guru 64560 points

    There is one more discrepancy to be aware of. The data sheet also specifies a 20MHz baud rate in section 5.10.1, while the timing section 5.10.4 specifies 25MHz:

    There is also a discrepancy between the data sheet and the TRM with regard to the number of bits specified for the baud clock generator. The TRM specifies an 11-bit baud clock generator, while the data sheet specifies an 8-bit baud clock generator.

  • 0 in reply to BrandonAzbell
    TI__Guru 59540 points
    Hello Brandon,

    Thanks for the additional information. I will submit the document feedback to our technical publications department.