Michael Watterson wrote:
> I need to drive a 3.3V  chip from 5V PIC (PIC18F4550). The PCB with the 
> 3.3V chip already has 10K pullups to +3.3V on the chip inputs. (the 3.3V 
> chip is an off the shelf  iTrip  type module using A7303B with  5 
> parallel pins to set FM Radio TX channel, I'll only be using 3 or 4 
> pins, instead of the normal 4 way switch, giving me 7 channels minimum)
>
> So I thought maybe drive with Open Drain. I thought  did this about 7 
> years ago in Assembler on 16F877. But I don't know how to do it 
> exclusively in JAL.
>
> Also I can find NO mention of "open drain" output mode on any port on 
> 18F4550 data sheet. maybe I imagined I did this before once.
>   
You can make any IO pin on a pic act in an effectively open drain mode 
by leaving the data output (port or lat) bit permanently at zero then 
controlling the output through the tris bit.
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