There's supposed to be a wiki page for it, but it seems that the URL is broken.

R&D mode ('Research and Development mode') is a feature built into the N900 for the sake of developers, but which is sometimes useful for other purposes.

Basically, there are a few fairly technical features that the N900 can turn on/off, and by default, the way it does that is that one of the early bootup scripts reads the R&D mode string from the CAL area of the N900 (the CAL area is a special partition/section of the small nand flash chip, on which the rootfs resides), and then the shell scripts see what, if any, R&D mode features to turn on depending on what, if any, flags it reads in the string.

By default, you can only change these settings with the Nokia Flasher utility, which has to run on another computer, and can't do the needed changes locally.

One of the things that turning R&D mode does by default (without actually toggling on any of the R&D flags), is that the N900 turns on a thing where the LEDs of the keyboard flicker in response to system activity - this was meant to let developers see if their apps were causing system activity in the background.


Another thing you can do is disable a few features on the N900 that will reboot the phone if they think they detect a problem (those are the watchdogs you might hear about occasionally), there's also some 'lifeguard' thing that does a similar thing. In rare cases being able to disable these things is good, though admittedly that can be rare.

Then there's some other R&D mode flags that deal with an STI console and disabling sleep on the USB port, etc.