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CC2651R3SIPA: Pull-Up/Pull-down requirements for SPI with multiple slaves

Santosh Kumar
Prodigy 50 points
Part Number: CC2651R3SIPA

Hi,

I have two components to communicate via SPI. CC2651R3SIPA is master.

  1. IMU IC - slave
  2. FLASH - slave

My configuration is Multi-Subnode Configuration. Therefore, MOSI, MISO and SCLK lines are shared, and I have two different CS lines for IMU and FLASH.

I looked into schematic of LP-CC2651r3sipa and found that a 2.2K pullup resistor is used.

I know that something between1k to 10k is a good value for a pullup. A simple calculation shows that when CS is low; during communication 1.5mA will be sunk by the pullup. A 10k can bring it down to 0.33 mA.

I have following questions:

  1. SPI doesn't need pullups as it is push-pull, so why do we need to use the pullup here?
  2. Why 2.2k why not a larger value?
  3. Why internal pullup of CS was not used, and instead an external resistor used - in that case pin shall sink much lesser current?
  4. Shall we require a pullup for MISO too in case we have multiple slaves?

Please give some advice,

Thanks.

  • 0
    TI__Expert 6450 points

    Hi Santosh,

    1. The pull-up is there to give this nCS noise immunity and depending on your system requirement it can be remove.

    2. Using a 10k ohm resistor would trade noise immunity for lower current consumption. We prefer to guarantee performance hence the 2.2K ohm resistor.

    3. This ties back to #1 and #2. The internal pull up resistance is higher which would draw a lower current but at the cost of noise immunity. 

    4. In general, yes but it really depends on your system requirements.

    Best regards,

    Bun

  • 0 in reply to Bun Kuch
    Prodigy 50 points

    Hi Bun,

    Thank you for pointing out that it is not only about floating line, but also about filtering the noise.

    Kind regards,

    Santosh.