Open drain behaviour for alternative pin functions?

[lpcware]

Content originally posted in LPCWare by pwuertz on Fri Oct 11 09:03:37 MST 2013
Using the PINMODE and PINMODE_OD registers on the LPC1769 I can set an output pin to open-drain mode, which works fine with a pin's default GPIO function. In one of my applications I tried to use the pin P1.22 as an output of the timer match register MAT1.0 in open drain mode. However, the open drain mode seems to be ignored once I switch from GPIO to the MAT1.0 function.

Is this the intended behavior? The user manual (UM10360 Page 105) kind of gave me the impression that the pin mode is independent from the selected function

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Quote:
The on-chip pull-up/pull-down resistor can be selected for every port pin regardless of the
function on this pin...

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Does open drain only work for the GPIO function?

[mikaelstolt]

Hi,

I just had similar open drain-issues for the I2C interfaces.

According to the latest Errata for LPC176x the OD-registers only works for pins in GPIO-mode.

If the pin is used for any other function the OD-register is overridden and is only set when the interface demands it.

Regards

Mike