My Tx port on my PIC18f uart appears to be at an undesired constant logic “0” value and mycomms are failing. My status Led is constantly illuminated. I have tried changing this pin to an output and blinking the LED to test it, again LED just stays high however when I use my meter to test the voltage from this pins it does blink from 0-0.25V, something is keeping this logic as 0 and I cant see anything wrong in the hardware. Traces look ok on my board etc no obvious shorts.


Any idea, could this be a dodgy PIC?

 

Possibly something on the Tx line is pulling it low. Posting a schematic would help.

 

...in addition to the schematic, post your code, there are numerous ways to mess up the initialization.

 

solved guys... i removed the 80pin SMD PIC and there was bridging underneath that i couldn't see.

 

Ammended post:

One thing I do with large tight SMD chips is use an ohmmeter to check adjacent pins for shorts.
Doesn't take too long to earn the warm fuzzy for this.



Orginal post:

Could be lots of things.

First check there is no other function "default enabled" on that pin (though since you see a ghost of a signal I doubt thia one).

Next divide and counque! Mechanically isolate that pin. If it is a DIP in a socket you just remove the chip and bend the pin out slightly. If it's a soldered down DIP think about cutting the trace(s), or cut the pin itself off, then later add a jumper for that pin. Even an SOIC can have one pin reflowed and pulled up a bit to isolate it.

I'm not making odds on this one: could be a flakey output pin drive or something being driven that is a near short to ground.