Scanless keyboard decoding

If your reading this, you likley are interested in a solution to identify keys from a keyboard made of an XY switch matrix. (I just thought you might want to know that, I suppose you might just like my drawings and have no interest in keyboards at all)


Welcome to scanless keyboard decoding, the idea is that, with a minimal amount of circuitry, you get a unique binary code from your keyboard for each key pressed. No resistors and ADCs, no scanning, no delays,  just a unique code.





The core of the idea, is to use a transistor as a current detector. Two things happen when the button is pushed,

• The "row" is pulled down to "0.7V" by the BE junction of the transistor, and, due to that current flow,
• The collector of the transistor is pulled down. (0.2v? 0.7??)

The system as a whole usually consists of 3 parts.

• The keyboard,
• The row/column detector,
• An encoder.

As a good person once said, you can skip any one part of a project, but skipping all of them with leave you with no project at all. (thanks Tony)




Here is an implementation with just a buffer chip:

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The row / column detector can be one of two types*,
- Normally high output
- Normally low output

This option can help with the optional decoder that comes next.

 * you can mix these too, and even have a row/column with no output, I'm not going there right now :)



To expand this circuit past 1 row and 1 column, just duplicate the circuit per row/column, quick example:

*There are all sorts of wonderful optimizations I didn't do here.

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ENCODERS:

 So the row/column detectors have lots of signals, adding an encoder reduces them. The encoder can help remove parts from the row/column detector too, here, 74LS is used for its built in pullup resistors on the inputs. Here is 7->4 with an optimization:

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Here a few things are done.




When is a transistor not a transistor?
When its two diodes. The general idea was to use a transistor instead of two diodes, but *sometimes* its trouble. Here is a redraw with diodes.

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No decoder, I said optional right? Here, a shift register is used to grab the detector lines. Again, a "key down" circuit was added, monitor the serial line, it will go high when a key is pressed, then clock the value in!

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Here, again, is an example of no decoder, The microcontroller is programmed with the pullups enabled on the input lines. Just read the value, no scanning!

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ROM decoder, this seems practical up to about an 8x9 matrix, after all, the encoding process is just a lookup table, right?
Here I'v shown with a 4x4 matrix, cause my pencil arm is getting tired. One of the advantages to this is 'key valid' signals are free. & uP interface!




I need to use git more often!, anyhow, here is the code I used to generate the binary image for the 8x8 keyboard decoder I made yesterday! I made if from the new framework I'm using for LUTs and state machines.

https://github.com/ruenahcmohr/KeyBoardLUTGen
































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Rue N. Mohr.