AM68A: SPI Pins without pinmux pullups/pulldowns odd signal

Amandio Neves

Part Number: AM68A

Tool/software:

Hello,

I've been observing something a little odd that I'd like some clarification on. We're using Linux SDK v 10.00.08 with the AM68A on a custom board and we're using a few of the MCSPI modules. I noticed that if I don't specify a pullup or pulldown in the pinctl pinmux in the device tree for the spi nodes, I'll see the associated signals on the spi module (D0, D1, CS, CLK) float high when idle. Whenever spi communication needs to take place, I'll see the lines switch to their proper modes.

Is this expected behavior? Couldn't this confuse some slave devices? We had used pullups and pulldowns on the spi pinctrl to workaround this, but I don't think this should be required right?

Thanks!

8 months ago

0 Takuma Fujiwara 8 months ago

TI__Guru 53543 points

Hi Amandio,

Yes, this is expected behavior. If there is no specification for pullup or pulldown in the devicetree, and there is no external pullup or pulldown resistors, then pins will float to some undefined value, which would be high in your case.

Regards,

Takuma

0 Amandio Neves 8 months ago in reply to Takuma Fujiwara

Prodigy 250 points

Hi Takuma,

Thanks for the response! Just to be clear, even on pins like the clock pin which is set to be low on idle per spi mode 0, it is normal to float high as an output? Is this a power management thing, i.e., is the module going to sleep when not in use and causing these lines to float high?

Thanks!

0 Takuma Fujiwara 8 months ago in reply to Amandio Neves

TI__Guru 53543 points

Hi Amandio,

Yes, that is the behavior. Setting the PU/PD bits in the GPIO register enables the internal pullup/pulldown resistors.

This behavior of pins floating looks to be something that was inherited from the previous generation of AMx processors:

https://e2e.ti.com/support/processors-group/processors/f/processors-forum/496527/how-to-enable-mcspi-clk-in-kernel

Regards,

Takuma

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AM68A: SPI Pins without pinmux pullups/pulldowns odd signal