Lunchbox computer

The latest iteration of my 'lunchbox' computer is powered by a BBC micro:bit. It has a monochrome 128 x 64 LCD display, and a keyboard recycled from an XBox 360 chatpad. The software consists of the micro:bian operating system and a threaded interpreter for a language I'm calling Morf.

The hardware

Power supply

The board is powered from two AA batteries. A 5V rail is provided by a small boost converter board, and the 3.3V supply for the micro:bit is derived from it by an LDO linear regulator.

Display

The display consists of a MIKROE–4 monochrome graphic LCD of 128 x 64 pixels, with a controller based on a pair of NT7108 chips. An ATMega328P AVR microcontroller is used as an interface between the display and the micro:bit.

Both the display electronics and its backlight are powered from the 5V rail. Communication between the micro:bit and the AVR chip is over an I²C bus, with the micro:bit end operating at 3.3V and the AVR end operating at 5V. A level shifter connects the two, consisting of two BS270 MOSFETS configured as described in a famous Philips application note.

The display protocol contains the following commands:

CLEAR

Clear the LCD and reset the cursor to (0, 0).

WRITE str

Write the string str to the display at the cursor position. The display scrolls as necessary, and newline (\n) and backspace (\b) characters in the string are interpreted appropriately.

GOTOXY col row

Move the cursor to (col, row).

GETXY

Return the cursor position as two bytes row col (sic).

CLRLINE

Clear from the cursor to the end of the current line.

CLREND

Clear from the cursor to the end of the display.

BACKLIGHT f

Set the backlight intensity to f/255.

MOVELEFT n

Move the cursor n positions to the left.

MOVERIGHT n

Move the cursor n positions to the right.

RVIDEO flag

Set the reverse video flag.

In addition to the above commands, the protocol provides access to the 1KB EEPROM of the ATMega328P via commands GETBYTE, GETWORD, PUTBYTE and PUTWORD.

Keyboard

The keyboard is recycled from a chatpad accessory for the XBox 360. This communicates over 19200-baud serial using a somewhat obscure proprietary protocol that is documented (to the extent I understand it) on the linked page. The chatpad is managed by a micro:bian device driver that is layer on top of a driver for the raw serial device.

The software

Morf

Morf (Mike's Own Reimplementation of Forth) runs as a process under micro:bian. As the name suggests, it is a dialect of Charles H. Moore's Forth language, but it does not aim for compatibility with ANSI Forth or any other standard dialect.