On 8/7/2012 1:29 PM, Harold Hallikainen wrote:
> Following up on what someone else mentioned, a pin can be used as an open
> drain output by driving the TRIS register instead of the PORT or LAT
> register. You set the PORT or LAT bit low. When TRIS is low, the pin is
> pulled low. When TRIS is high, the pin floats. If the pin has a 5V
> tolerant input, it can be pulled up to 5V, making this a 5V output.
>
> But, I think I recall the original problem being a UART interface. Since
> you can't make the UART drive the TRIS bits, that would not work here. In
> that case, I still think the 74HCTxxx is a good solution.
>
> Harold
>
That's a good idea and Microchip has implemented it on some of the newer=20
parts. The PIC18F27J13 datasheet says:

20.3.3 OPEN-DRAIN OUTPUT OPTION
The drivers for the SDOx output and SCKx clock pins
can be optionally configured as open-drain outputs.
This feature allows the voltage level on the pin to be
pulled to a higher level through an external pull-up
resistor, provided the SDOx or SCKx pin is not multiplexed
with an ANx analog function. This allows the
output to communicate with external circuits without the
need for additional level shifters. For more information,
see Section 10.1.4 =93Open-Drain Outputs=94.
The open-drain output option is controlled by the
SPI2OD and SPI1OD bits (ODCON3<1:0>). Setting an
SPIxOD bit configures both the SDOx and SCKx pins for
the corresponding open-drain operation.
--=20
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