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'[EE]: Shift Registers Question'
2004\01\16@082126 by Lucian Copat

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Hello to all Piclisters,

I am designing a circuit which uses 3 shift-registers to spare some
microcontroller inputs. I actually use 74HC165 as a shift register, but
I found also CD4021. In practice, I found out that the data pin of each
register cannot be tied to a common data bus. Is there a solution to do
this ? (for 3 shift-registers, tie the Q7 data out pin to the same data
bus D0 pin, which is also common to other circuits).
Is it worth to have an address/data/command bus for a PIC project, which
uses one latch, 3 shift-registers, one analog multiplexer and an ADC ?
Thank you for your answers.

Lucian Copat

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2004\01\16@083619 by Wouter van Ooijen

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> In practice, I found out that the data
> pin of each
> register cannot be tied to a common data bus.

It can, but it probably won't have the effect you want.

> Is there a solution to do
> this ? (for 3 shift-registers, tie the Q7 data out pin to the
> same data bus D0 pin, which is also common to other circuits).

You seem to have found the solution. What is your question?

> Is it worth to have an address/data/command bus for a PIC
> project, which
> uses one latch, 3 shift-registers, one analog multiplexer and an ADC ?

Probably, if you need it for some purpose. But for the latch, 3 SR's,
mux and ADC you do not realy need it. And the normal way to operate a
PIC is not to create a data bus as found in a microprocessor system, but
to use the PIC pins to interace directly to your peripherals, using
tricks like SR's and serial ADC's to limit the number of pins used.

Wouter van Ooijen

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2004\01\16@090418 by Lucian Copat

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Tying the Q7 pin to a common D0 data pin would be the solution, but in
my practical circuit, this doesn't works and I've read on some pages on
the internet that this pin must be tied separately to the
microcontroller. The question is: is this true, or there was something
else wrong with my schematic ?

Lucian

{Original Message removed}

2004\01\17@060612 by dr. Imre Bartfai

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Hello,

use CD4014, which is to be daisy-chained having appropriate serial input
pin.

Regards,
Imre


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2004\01\18@000542 by Dwayne Reid

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At 05:15 AM 1/16/2004, Lucian Copat wrote:
>Hello to all Piclisters,
>
>I am designing a circuit which uses 3 shift-registers to spare some
>microcontroller inputs. I actually use 74HC165 as a shift register, but
>I found also CD4021. In practice, I found out that the data pin of each
>register cannot be tied to a common data bus. Is there a solution to do
>this ? (for 3 shift-registers, tie the Q7 data out pin to the same data
>bus D0 pin, which is also common to other circuits).

The shift registers are daisy-chained: the serial data out of one device
connects to the serial data input of the next.  You mention CD4021: ground
the 1st device's DS input (pin 11), connect Q8 (pin 3) of that chip to the
DS input of the 2nd chip, and so on.  The last chip's Q8 pin goes to the
PIC.  Connect all the clock pins together and to the PIC, do the same for
the P/S lines.  To use: pulse the P/S line high to load the shift registers
with the parallel data, then clock the data out.

dwayne

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