Gyro PID User drive - How we did it

[ay2b]

Originally Posted by Bongle

255 can do some wierd stuff. Way back in 2003, I couldn't figure out why our robot wouldn't drive straight. Another team pointed out that 255 was some sort of null character. I changed it to 254, and the thing started driving perfectly.

I believe the limit today is actually 255. "Back in the day", when the user microprocessor was a BASIC stamp, instead of a PIC, it used the serial i/o from the stamp to communicate with the master processor. Each packet had a 0xFF 0xFF header (i.e. 255, 255). The PWM out values were interlaced with the relays and the digital outs, and we were told not to run PWMs above 254. If a PWM was run at 255, then being next to some other byte, which could be 255, it could look like a packet header, and throw things off. Now, however, with the use of a PIC instead of the BASIC stamp, the problem of getting this false header no longer exists, so it's safe to run PWM outs at 255. (Tangentially, I think the user and master processors now communicate via shared memory, but I'm not certain of the details.)

The difference in output between 255 and 254 is not going to be noticeable, and Victors have a small center deadband, so both 127 and 128 are going to give you a "full off" from the Victor, so it really doesn't matter which way you do it. Just don't try to use 256 as your "full on" value.