ADAU1761 GPIO clarification

I've been struggling with the ambiguously-defined GPIO settings on the ADAU1761 and hope that sharing my findings here will save others some time.

On the ADAU1761, one can drive general-purpose outputs from either the DSP core or the I2C/SPI ports. In each case, we are offered the choice of with and without an internal pull-up, which is specified as "nominally 250K ohms" (p82 of datasheet).

It is not made clear whether the outputs are totem-pole, open-collector or switchable. There is no explicit option offered to select one or the other - although the GPIO section of the datasheet (p50) uses the phrase "When the GPIO pins are configured as open-collector outputs...", there is no obvious way of doing so.

In reality, if you select the "no pull-up" option then the pins do indeed act as open-drain outputs. The polarity is such that the outputs are high-impedance when driven high and low-impedance (about 70 ohms to ground) when driven low.

However, if you select the "with pull-up" option you do not get an open-drain output with a 250K pull-up - you get a full totem-pole output driver. There is negligible voltage drop (< 10mV) driving a high level into a 100K load. If it was a 250K passive pull-up then we would expect the high-level output to be only about 1V.

So, IMHO at least, p82 of the datasheet is not quite correct. The real GPIO output settings in Table 79 should be:

1001 = output set by I2C/SPI port, totem-pole output
1010 = output set by I2C/SPI port, open-drain output
1001 = output set by DSP core, totem-pole output
1100 = output set by DSP core, open-drain output

...and the reference to an optional internal pull-up should be deleted.

I hope this helps somebody!

Cheers,

Steve L.

Audio Signal Processors

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adau1761