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LMX2820: Serial clock performing far below that specified in datasheet.

Luke Dyson
Prodigy 20 points
Part Number: LMX2820
Other Parts Discussed in Thread: SN74LV125A,

Trying to run this device with a standard SPI bus interface (beagle bone black) and am unable to approach the rated speed of 40MHz. This is not surprising, since I am going through a harness, but I cannot even reach 4MHz of clock speed before the connection becomes intermittent. I am not the only customer who has this experience, and even so it was with a different SPI bus interface on a different system.

Anecdotally, it was noted that clock edges appeared to be "blunted" by some aspect of the input circuitry, perhaps a parasitic capacitance of a non-linear nature.

If there are errata for this part, with respect to timing or otherwise, then please provide it on the website.

Regards,

Luke

  • 0
    TI__Mastermind 34270 points

    Luke,

    Final test for our devices is actually conducted with 55MHz SPI on packaged devices, so we know all production devices should work at speeds up to 40MHz.

    There may be some series resistance for damping, or parasitic capacitance accumulated along the run of the SPI bus lines, which could be impacting the SPI timings. Alternately, the drive strength on the BBB or the other system SPI controller may not be good enough for the rate you're trying to drive - I see most pins are 6mA but a handful are limited to 4mA. You may consider testing with a stronger line driver on the output, maybe something with ±16mA drive strength like SN74LV125A.

    When you say you're not the only customer with this experience, do you have another E2E thread or equivalent correspondence we can reference?

    Are you using the LMX2820EVM? It doesn't sound like it from your description of "a different SPI bus interface on a different system", but just in case: there are 33Ω damping resistors in series with the SPI lines on the EVM which could be impacting your rise/fall times.

    Regards,

    Derek Payne