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I2C Pull-up resistor influence on (Shared) SPI bus ?

matianfu
Intellectual 500 points
Other Parts Discussed in Thread: CC2420

In our design, we use USCI0 Channel B on MSP430F24x connect to both CC2420 (SPI mode) and I2C/1-Wire bridge (DS2482-101, I2C mode), though at one time, only one mode is used (both cc2420 and ds2482 could be shutdown by GPIO).

In DS2482 reference design, two pull-up resistors connects SDA/SCL to DVCC. Since I2C shared PINs with SPI, I'm not sure if the pull-up resistors have some (bad) influence on SPI mode data transmition, since they pull up SOMI and SIMO in SPI mode. Could anyone give me some advice? Thanks.

 

UGlee

  • 0
    TI__Guru 69887 points

    Assuming reasonable resistor values, pull-ups shouldn't affect any SPI communication. In SPI mode, all signals are driven through push-pull CMOS output stages on both SPI master and SPI slave, so the pull-up will be overdriven by the SPI signals. This can easily be verified by looking at the signals with a scope.

    In the same context, another thing to keep an eye on. You said these devices will be shutdown? It needs to be verified what happens to their respective pins in shut-down mode to prevent any side effects. Ideally they would become high-Z. But for example, if all the "shut-down" in your system is doing is removing power from the chip (e.g., in case of powering an external device through a GPIO pin), the unpowered chip might load the SPI/I2C signals which are still connected...

    Regards,
    Andreas

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