Nuts and Volts - February 2009

counts, SELECT cycles through those error screens that have non-zero counters. When all counters have been reset, the global error flag is cleared. Individual error screens are exited by allowing the display timeout to return you to the Home Screen or by choosing to reset all counters.

The LCD backlight brightness is controlled in Screen 8. Nine levels — from Off to Maximum — are available.

Finally, we return to the Splash Screen and then the Home Screen. Figure ⁴also shows shortcuts to the backlight and error screens using the SET key from the Home Screen.

All option settings (including backlight level) are saved in non-volatile memory and reloaded when mistralXG is switched on.

Error Flags and Counters

mistralXG tracks a number of different errors that may occur. The second line of Screen 8 (the error flags) shows the status of the 16 error types. An underscore (“_”) in a position indicates that the related error has not occurred. Each position has its own error character. If every error type had occurred, the flags would look like this:

BFOD12012SE459DM

Here’s a quick summary of the error types. More detailed information is in the source code:

BFO These flags indicate errors receiving MIDI data from the inputs (receive buffer overflow, EUSART framing error, EUSART overflow error).

D Unexpected data byte (MIDI command byte expected).

12 Unexpected channel command byte (commands

expecting one and two data bytes, respectively).

012 Unexpected System Control command byte (commands expecting zero, one, and two data byte, respectively).

S Unexpected Sysex command byte.

E Unexpected End of Sysex command byte.

459D Invalid command byte (0xF4, 0xF5, 0xF9, 0xFD).

M USB-MIDI input buffer overflow.

Errors are rare, but you can induce them by switching the input stream between “M” and “W” while a track is playing.

Next Time: Technical Details

Okay, that about wraps things up for this month. I hope you can see that mistralXG offers a lot of flexibility, and that you are interested enough to want to try either building it for yourself or using it as a base for your own project.

Next time, I’ll take you through the details of the hardware and software that bring mistralXG to life.

NV

About the author

Steve Russell started out his working life as a hardware design engineer. Somewhere along the way, he discovered computers and began designing subsystems of various types for them, working for a multi-national computer company before moving into software development. Currently developing code in Java and Python, he still misses the baleful green glow of an oscilloscope in a darkened lab, late at night ...