Saturday, May 22, 2010

Embedded Keyboard Project, Part 3

Today, I put several hours into the embedded project. My lab partner extended the 15-wire band by soldering on longer wires, then running the wires outside of the keyboard case, where they're much easier to access in a bread board.


In the picture, the black wires are for the key matrix rows, and the red wires are for the key matrix columns. We also broke out controls for the power switch on the keyboard and for the two speakers, and we think we probably will try to incorporate them into our design.

After probing the 15-wire keyboard bus, we determined the overall schematic for the keyboard matrix:


This matrix design is exactly as I had speculated in an earlier post. I got keyboard drivers working, which as of now, detects when any of the keys have changed state, and which keys changed. The driver works by driving one of the rows high and the others low, then reading in the columns. This happens repeatedly for each of the seven rows. The old values of the columns are saved and compared to the new values, and when a change is detected, that is when we process a key change, and send MIDI commands to the Atmel synthesizer chip. Here is a PIC16F877a microprocessor wired up:



As you could see, we found we needed pull-down resistors for the column pins. We found without them, we got misread key strokes, since the voltage on the column pins wasn't falling fast enough. We speculate the diodes introduce capacitance to the column pin nodes. We found 10 kΩ resistors are small enough to discharge the nodes in time for the next read.

The keyboard driver is currently written to toggle the LED on or off every time a key event is detected. The idea is, you press down one key, the LED turns on; press down another key, it turns off; release a key, it turns on again; etc. The current drivers for the keyboard controller can be found using the following links:

After pounding around for a while, we're now pretty certain that every key event is being processed correctly, since after all keys are released, the LED always ends up in the off state.


Pressing down one key turns on the LED

As of now, we're still using a PIC16F processor, which runs off a supply voltage of at least 4 volts. Our synthesizer chip runs off of 3.3 volts, and we don't really want to deal with level shifters, voltage regulators, or use multiple logic supply voltages. So we requested some free PIC18LF samples from Microchip, which should be arriving early next, which can run off of a 3.3 volt power supply. Once those arrive, we could begin to try to send key commands to the synthesizer chip.

So I feel the project is falling together pretty well so far. Next week, we will begin trying to interface the ATSAM2195 chip, and we're really hoping that we figure it out fairly quickly. As of now, I think all the soldering and modifications to the keyboard are complete. Just out of curiosity, I'm thinking about having my blood lead levels tested.

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