[sdiy] mcu project with too few I/O lines :(

[Tim Ressel]

The way Agilent handles this on their front panels
surprised me. The use long chans of shift registers
for both switch scanning and for displays. This method
makes for tedious programming, but it saves many I/O
lines.

--Tim

--- Ingo Debus <debus at cityweb.de> wrote:
> Rainer Buchty wrote:
>  > As for the 16 LEDs, you probably want to have
> some LED drivers. One
>  > 74245 per 8 LEDs should do the job as the data
> sheet claims Iol to be
>  > 24mA, so attach them with the cathode facing the
> 74245 output which 
> means
>  > that pulling the corresponding data line low will
> lit the LED.
> 
> Easier: use low current LEDs. 2 or 4 milliamps isn't
> too much for most
> uC port pins.
> 
> Here's another idea to save port pins: in my last
> project one pin
> drives one LED and scans one momentary switch at the
> same time. LED
> (in series with current limiting resistor) goes from
> pin to V+, switch
> goes from pin to ground. When the switch is scanned,
> the LED is turned
> off for this short time. The only downside is, the
> LED always lights
> up when the switch button is pressed, but this
> doesn't matter in my
> application.
> 
> Ingo
> 
> 
> 


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