View Full Version : BS2 pin13 not working

04-17-2007, 08:56 PM
Hi ,

BS2 pin 13 on the Professional Dev Board·does give an output, but it does accept inputs. Why is that.


04-17-2007, 09:23 PM
"does give an output, but does accept inputs" is a good thing. What's your problem?

04-17-2007, 09:28 PM
sorry, wrong posting....does accept inputs but does not give output

Chris Savage
04-17-2007, 09:44 PM
It is possible you have damaged the output driver. Meanwhile the input register could still function. How are you confirming this?

Chris Savage
Parallax Tech Support

04-17-2007, 09:49 PM
Right. The PIC I/O pin (which the BS2 uses) is a very clever little device. 'Behind' the pin (on the PIC chip itself) there's a transistor which will 'short' the pin to Vdd, another transistor which will 'short' the pin to Vss (ground), and a register which will read the state of the pin.

The result of this is that you can set the pin as OUTPUT HIGH, OUTPUT LOW, or INPUT. When you set it to input, both transistors are turned 'off'. In the "INPUT" state, the pin has a 'high-impedance' to ground (like 1 to 10 MegOhm) -- which allows any device external to the PIC to set the pin to +5 or to ground without interference from the PIC chip.

Now, if, inadvertently, you tie a PIC I/O pin to ground through a low-impedance path (less than 220 ohms or so) and THEN make the PIC pin an OUTPUT HIGH, the driver transistor on that pic pin will basically destroy itself. It will act like a fuse. It will conduct as much current as it can, get hot, and melt. After that, you can command the BS2 to OUTPUT HIGH on that pin all you want -- there's no transistor there anymore to make the internal connection to Vdd.

This doesn't damage the input register, so you can still use that pin for input.

Now, what can you do next? Well, you can mark that BS2 as 'damaged, but useable'. Or you can return the BS2 to Parallax, and for a fee they'll send you a new one. And probably, when you're prototyping, you should put 220 ohm resistors in series with I/O pins to prevent this damage in the future.