[sdiy] Microchip PIC 5 x 4 matrix

[Jean-Pierre Desrochers]

Hi Tom,

The PIC has no internal pullups on port A.
You were right. I uploaded a corrected picture
on the same link shown.
And yes the 100 ohms is for the LED's current limiting.
The voltage drop of the led + the series diode gives around
2.2 + .7 = 2.9v leaving 2.1v (@ 5vdc) for the limiting resistor.
At 20mA that gives around 100R.
Thanks for your good reply (as usual).
JP


Le 2015-10-07 10:07, Tom Wiltshire a écrit :
> What does the 100R do? Is that a current limiting resistor for the LED?
> What about pull-ups for the buttons? Does the 16F88 have internal
> pull-ups on Port A?
> 
> Aside from those questions, it looks fine to me, JP. Nice and simple.
> 
> Personally, I'd have done it with serial-parallel and parallel-serial
> chips, but that's because I've got code for that already. 74HC594/595
> and 74HC166/165 can both be driven from 6 pins total (or even five if
> you use the same clock for both in and out), and you can extend the
> chain as long as you want.
> 
> Your way saves the six(!) other chips I'd have used though, so it's
> probably to be preferred.
> 
> Tom
> 
> On 7 Oct 2015, at 14:52, Jean-Pierre Desrochers <jpdesroc at oricom.ca> 
> 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
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