ssp mosi & open drain mode

[hendriksachse]

Why is it not possible to configure SSP MOSI as open drain?

I know this is not a normal operating mode for MOSI, but I wanted to use the SSP controller for the bittimings of a onewire bus. It would have been nice and easy without a need for external components if I could have set the MOSI as open drain.

But as soon as I set it to open drain, either via Chip_IOCON_PinMuxSet or via the debugger, it reverts to standard_gpio_output mode.

Regards

Hendrik

[kerryzhou]

Hi Hendrik Sachse,

   Please tell me what the chip you are using, the chip part number.

   Then I will help you to check it.

   Or, you also can check the according chip user manual, whether the opendrain is true or just pseudo open drain.

Have a great day,
Kerry

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[hendriksachse]

The chip is an LPC11C24 and the pin PIO0_9 is pseudo open drain, but I would have expected that the OD mode is independent of GPIO/SSP mode, just like I can set hyteresis for the SSP input pin.

This, combined with a "SPI loop back" that reads the actual pin state, would have spared me 1 IO pin and an external transistor for implementing onewire.

Regards

Hendrik

[kerryzhou]

Hi Hendrik Sachse,

  PIO0_9 just have pseudo open drain, and it won't work independent of GPIO/SSP mode, this is the chip character.

 The true open drain just in some I2C pin, so the SPI just can be limit to 3.3V, if you want to use other voltage, you need to use the external transistor or converter to change it.

Wish it helps you!

Have a great day,
Kerry

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