Hi Dwayne,

Thanks for the nice suggestion that enables me to save one more I/O pin.

Incidentally, in my experiments with the 4021, I found a method to use the
IC for 9 PB switches instead of only 8. The sequence is like this:

 (The inputs of 4021 for the 9 switches are connected +5V through pull-up
 resistors. When a PB switch is pressed, the corresponding input goes low.)

 Connect switches 1 - 8 to P7 - P0 input pins of 4021 respectively.
 Connect switch 9 to pin 11 (Serial Data In) of 4021.

 1. Make pin 9 (PL input of 4021) high to parallel load the
    8 input bits into the shift register. Make pin 9 low again.
 2. If pin 3 (output of the last stage) is low, switch 1 has been
    pressed; take appropriate action.
 3. Serially shift 8 bits one by one and check each bit.
    If a bit is low, it means the corresponding switch (one of the remaining

    switches from 2 to 9) has been pressed; take appropriate action.
 4. Go to step No.1.

This may be helpful to those who are not already aware of it.

Regards,
Bala

> -----Original Message-----
> From: Dwayne Reid [mailto:dwayner@PLANET.EON.NET]
> Sent: Saturday, December 15, 2001 12:17 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE]: Which is the ideal IC?
>
>
> At 11:01 PM 12/12/01 -0600, Bala Chandar wrote:
>
> >I like the suggestion by Tim McDonough of using 4021 that will give 8
> >inputs. (74HC165 can also be used, I think, as it looks a functional
> >equivalent of 4021.) Since I already have two pins working
> as common Clock
> >and Data outputs, I can use them for 4021 as Clock and Load
> outputs. This
> >means, by using only one pin to receive the serial data, I can get 8
> >additional inputs.
>
> You can avoid using that extra pin by using the data pin used
> to talk to the output SRs as an input pin.  Just add a single 10K
> resistor in series with the data out pin of the 4021 and feed the
> serial data pin on the PIC.  Set the pin as input, read the 4021,
> set the pin as output, write the output SRs, assert strobe.
>
> I do this in most of my products: 3 pins to talk to all my SR
> based inputs and outputs.  Add 1 more pin if I am also talking to my
> watchdog chip of choice (Xicor X25043).
>
> dwayne

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads