GP4020 Cheat Sheet

Here are some things we learned the easy way (we explored it first) or the hard way (we banged our heads against until we finally figured it out).

GPIO

No, really, they mean it: you have to use a full 32 bit access to get to the GPIO registers. Do not use a byte access (e.g., ldrb or strb) because it won't work.

Boot ROM

Translation: "Pull MULTI_FNIO high during a reset in order to start the internal boot ROM instead of the external flash".

The 1.3% error on the 56K UART speeds doesn't seem to be a problem, but it might be, especially if your computer's UART is fast. I think, but I'm not sure, that putting a few bits worth of delay between each charater will help.Since the UART syncs it's 16x receive clock on the start bit of each byte, having a small gap between bytes prevents timing problems from bleeding across received bytes.

Watchdog Timer

Don't forget, if your GP4020 turns on the watchdog timer AND has a backup battery, the watchdog timer will keep going until you disconnect the batter AND power supply.