[sdiy] Microchip PIC 5 x 4 matrix

[Richie Burnett]

Multiplexing is fine as long as you keep the track or wiring lengths short. Otherwise capacitance can cause ghosting in the LEDs. Stray capacitance also lengthens the settling time for reading the switches. Obviously you don't want to take quickly changing multiplex lines outside of a screened enclosure because they will radiate significant interference.

The switch reading times in the multiplex only need to be as long as is necessary for the voltage to settle and give an accurate switch reading. Probably only a few 100s of microseconds at most. This stops the switch reading time significantly eating into the LED  multiplex time, reducing the LED drive duty ratio further, dimming the LEDs unnecessarily. You want the switch reading periods to be tiny compared to the LED drive periods. 

You can also probably dispense with the signal diodes in series with the LEDs. Most LEDs can tolerate 5V reverse voltage without damage.

-Richie,

Sent from my Xperia SP on O2

---- Jean-Pierre Desrochers wrote ----

>I'm about to build a MIDI foot controller
>using around 20 normally open footswitches
>with there respective state LEDs.
>I have a bunch of PIC16F88 micros and want
>to use one for this project using as less pins as possible.
>(9 pins for the matrix)
>I figured out a way to read and feed all switches
>and LEDs this way:
>
>http://www.arcenson.com/public/PIC16F88_5x4_matrix.JPG
>
>All the switches and LEDs will be processed one after each other
>in an endless loop..
>RB5 will send the MIDI data.
>
>Does anybody see something I forgot in that drawing ?
>I'm not home and will try it tonight.
>
>Thanks. JP
>_______________________________________________
>Synth-diy mailing list
>Synth-diy at dropmix.xs4all.nl
>http://dropmix.xs4all.nl/mailman/listinfo/synth-diy