doc.txt

SpiffChorder - Spaceman Spiff's Chording Keyboard Experiment
Copyright (C) 2006-2012  Mikkel Holm Olsen

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.



Spaceman Spiff's Chording Keyboard Experiment
=============================================

This document contains more detailed documentation regarding the 
structure of the software and hardware, useful for people who want
to make modifications to the system.


Hardware overview
-----------------

The standard configuration of the SpiffChorder hardware is shown in
circuit/chorder.png and circuit/chorder.pdf. Component values are
described in the bill of materials circuit/bom.txt.

The design of the SpiffChorder allows 8 chord buttons as well as 3
modifiers. The idea is that the modifiers could be bound to a key
such as shift, and affect all chords. In a future version of the
firmware, which supports a trackball (or other pointing device), the
modifiers will be used for mouse buttons. The 8 chord buttons (or
7 in the standard configuration) are connected to PC0-PC5 and
PD6-PD7). On the 28-pin devices, only PORTD has all 8 bits. But we
need some of the bits in PORTD for other purposes (such as the
interrupt for USB communication), so we must combine more than one
port. By selecting the low 6 bits from PORTC and the high 2 bits
from PORTD, the code to combine the inputs into a single byte
representing the active chord is greatly simplified.

SpiffChorder uses a USB reference design where the microcontroller
is powered by 5V from the bus, and the signal levels on the USB
lines are limited by 3.6V zener diodes. Although some people have
made this circuit work without the zener diodes, this is not 
recommended, because a fairly large percentage of motherboards will
not tolerate the higher signal levels on the bus lines. The reason
for choosing a 3.6V instead of 3.3V (which is the specified bus
voltage level) is that this gets much closer to the specified voltage
due to the current running through the zener. For this to work, 
remember that the zeners should be low-power types (typically the
glass-bead types, looking like 1N4148), in order to be sufficiently 
fast. 


Copyright (C) 2006-2012 by Mikkel Holm Olsen
$Id: doc.txt 26 2012-01-22 18:27:08Z spacemanspiff $