NP-41 Emulator (may be)

[Chris Chung]

(04-06-2016 11:43 AM)emece67 Wrote:  

(03-09-2016 06:21 PM)Chris Chung Wrote:  Here is a simplified schematic for a 5 pin key multiplexer to illustrate the connection and scanning, it yields 4 + 3 + 2 + 1 keys.



In the firmware I start by placing pin 0 (say from top) as output set to high, pin 1, 2, 3, 4 as input w/ pull-down to low. And I would scan for the 4 input pins to see any of the 1st row keys has been hit.

That's interesting. I've searched a bit and have found another approach, named "charlieplexing" that allows scanning a 42 switch keypad with only 7 pins. It also uses 7 diodes. In general, such approach, when using n pins, can scan up to \(n*(n-1)\) switches, requiring n diodes.

I'm now studying it to see if it can handle (with the aid of software) the simultaneous activation of 2 switches.

It's interesting to introduce diodes expand keys. There are also implementations that uses resistor series w/ switches and use ADC to "read" which switch has been depressed based on how it alter a voltage divider via shorting out resistors.
In my case, there are enough I/O lines and I would avoid additional components.