Debug pin with software serial

I have been intending to re-purpose the hardware serial interface on the JeeNode to run IR comms, because being hardware and buffered it won't miss data if it's doing something else other than listening, as it would with any software serial library.

But at the same time, while doing development, I like to have debug output. Until I actually start doing the IR thing, I can still use the serial port for this. But I would like to keep that ready to do at any time. Or I can send them over the RF12, but I would like to reduce my wireless usage as much as possible, and not clutter up my UI's text window. Again, until I have multiple cars going, wireless contention isn't as much of an issue, but I prefer to do things properly the first time as long as it isn't going to be harder than fixing it later.

Then I thought, I could use software serial. It's slower and ties up the CPU, but I don't have a lot of or very frequent debug output, so it should be fine. Furthermore, I can just turn off debug output when it's not useful (mainly, when the car is moving around). I added #define and #if-#endif statements to turn the serial output on or off entirely, in a method like this (from memory):

#define DEBUG_ENABLED 1

void debugln(char[] msg) {
  #if DEBUG_ENABLED
  debugSerial.println(msg);
  #endif
}

So if I turn off DEBUG_ENABLED, then calls in the code such as debugln("Car ready") won't have any impact. And including a library should only take up some memory.

First I tried NewSoftSerial, which I have tested out before and decided was too slow (with Arduino and XBee). This caused my steering servo to freak out at the same interval as the messages being sent, no matter what pin I tried to use for debug. I also didn't see any serial output. I think the freak out was because NSS uses interrupts, which is probably shared with either that servo output (it's probably a PWM pin, I didn't check) or the RF12 code. I don't need or want an interrupt-enabled library because this is going to be used sparingly and only for output.

Side note: it's annoying that every such library demands you give it both an Rx and Tx pin. If I only want to send, why do I have to give it a receive pin?

So I tried out SoftwareSerial instead, which is part of the core Arduino library. It didn't cause any apparent problems with servo or RF12, but it also didn't seem to work at all. No matter what pin I chose, from a digital to an analog, or a few other pins on the JeeNode ISP or other headers which are not used at the moment, nothing came out. I set it to 4800 baud (9600 being the max anyway) but the LEDs didn't even light up. I added Serial.println after the debug and that was still outputting.

In short: is the RF12, Ports, or Servo library interfering with SoftwareSerial?

Update: it turns out you need to set the Tx pin to OUTPUT mode. I would have thought the library would do that, I really would.

So that started working, but then I had a problem with debugging just after sending a status update: the output was scrambled. I guessed that the rf12 library was sending and that was throwing off the serial timings. I asked on the JeeLabs forums if there was a way to fix it, and jcw himself (creator of Jeenode) replied. If you add a rf12_sendWait(1); after the sendStart command, it waits for the send to complete before returning to the program flow. That's a double-edged sword; You can guarantee your timings won't be thrown off by the background sending, but at the same time you delay returning to the program, I'm not sure for how long. If I want to keep the loop very tight, it might be a problem. I could also reduce the size of my sends as much as possible. But so far it seems ok.

Forum post is at http://forum.jeelabs.net/node/259