Wisdom for the day.

Okay so I still haven't posted what I wanted to about my current project. I've been busy, please forgive me. I've got a little bit of a time constraint on this project and I haven't been taking as much time to document as I would like. I did however just fix a problem that was driving me nuts for a long time, so I'm going to take a brake and tell my tail.

My project is using a Propeller microcontroler from Parallax. I'm communicating with a piece of hardware using serial. I've been pulling my hair out for days, searching for a bug that didn't exist. Let me explain...

I'm using the full duplex 4 port serial driver from the propeller object exchange. I've used it many times and had lots of success with it. On my current project I am using it because I need to serial ports, I'm using one for a debug port and the other to communicate with a display. The problem was I was the commands I was sending to the display were not always working correctly. I blamed myself (quite correctly as it turned out). My initial tests indicated that I was successfully communicating at 115200 bps. I was wrong! I kept checking my code trying to figure out why some times the same function would work and other times it would not.

The thing finally made me realize what I was going wrong happened today. I'm working on a text interpreter for running scripts on the propeller. No mater what I did it kept reporting the wrong file size back to me over serial. For some reason the last bit of every byte was 1. A of 20 bytes was reporting the file size of $0080808A. (okay so the two highest nibbles were right). I then wrote test code to display the contents of the file. It worked perfectly.

This was were it got frustrating. I finally decided to trudge on with the code event though the file size was missing I could just wait till I got errors and assume I was at the end of the file. So I wrote more code, implementing handshaking as a form of flow control. That's when it became clear. Data after the ACK was corrupt. It turned out all this time that if I was transmitting while receiving the received (well it could even be the transmitted I don't know) was corrupt. I turned the communication speed down and everything is working the way it is supposed to.

So my new found wisdom is don't assume bidirectional communication is functional until you test them at the same time! I made a mistake weeks ago and didn't even look at it but it was the problem.