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'[EE]: Need 74HC165 with tristate output'
2001\01\19@111618 by Brian Aase

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I'm starting a project in which a number of inputs (>16) need to
be periodically read via the SSP (SPI) bus.  In the beginning it
looked like using a few 74HC165A's would be just the thing,
until I realized that the outputs are not tristate.  Thus I can't
tie multiple outputs together onto the SDI line :-(
Can anyone suggest a chip with a similar function that DOES
have a tristate output?
Brian Aase

PS. I'm using a 16C62B, if anyone is curious.

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2001\01\19@112631 by Robert Young

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From: "Brian Aase" <spam_OUTulcsaTakeThisOuTspamESKIMO.COM>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Friday, January 19, 2001 10:12 AM
Subject: [EE]: Need 74HC165 with tristate output


> I'm starting a project in which a number of inputs (>16) need to
> be periodically read via the SSP (SPI) bus.  In the beginning it
> looked like using a few 74HC165A's would be just the thing,
> until I realized that the outputs are not tristate.  Thus I can't
> tie multiple outputs together onto the SDI line :-(
> Can anyone suggest a chip with a similar function that DOES
> have a tristate output?
> Brian Aase

If you have the room to add a chip besides the HC165's, how about HC125.
Pull G* low and it is transparent, G* high makes the gate output 3-state.

Rob Young
rwyoungspamKILLspamieee.org

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2001\01\19@112932 by Alan B. Pearce

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>I'm starting a project in which a number of inputs (>16) need to
>be periodically read via the SSP (SPI) bus.  In the beginning it
>looked like using a few 74HC165A's would be just the thing,
>until I realized that the outputs are not tristate.  Thus I can't
>tie multiple outputs together onto the SDI line :-(
>Can anyone suggest a chip with a similar function that DOES
>have a tristate output?

Why do they need to tri-state? these are parallel load serial out registers
which are surely what you need. I would have thought that you would hook Q7 (or
is it Qh on your datasheet) to the serial in line of the next chip. The last
chip has its Q? pin hooked to the SPI in pin on the PIC. Once you move the
shift/load pin on the 165's from load to shift, then the clock of the SPI will
serially shift the data into the pic.

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2001\01\19@120707 by Alan B. Pearce

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>Why do they need to tri-state?

Whoops, sorry I did not read your original piece close enough.

You could use something like a CMOS multiplexor to select between outputs if you
have more than two, or if other devices can already tri state a single 4066 gate
could be used.

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2001\01\19@140822 by rottosen

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Brian:
What about using a 74HC299? All eight I/O pins change direction as a
group.

-- Rich


Brian Aase wrote:
{Quote hidden}

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2001\01\19@151736 by Dwayne Reid

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At 08:12 AM 1/19/01 -0800, Brian Aase wrote:
>I'm starting a project in which a number of inputs (>16) need to
>be periodically read via the SSP (SPI) bus.  In the beginning it
>looked like using a few 74HC165A's would be just the thing,
>until I realized that the outputs are not tristate.  Thus I can't
>tie multiple outputs together onto the SDI line :-(
>Can anyone suggest a chip with a similar function that DOES
>have a tristate output?

Couple of ideas:  are other SPI devices other than the '165s connected to
the SDI line?  If so, just use a series resistor from the Dout pin of the
last shift register to the SDI line - 4k7 is OK for medium clock
rates.  When all of the other SPI devices are de-selected, SDI sees the
logic level present at the Dout pin of the last HC165.  Selecting any other
SPI device turn on output of that device which swamps the signal from the 165s.

As far as having multiple HC165 shift registers is concerned - no
problem.  You just cascade them.  The Dout line of each SR connects to the
Din line of the next device.  The Dout of the last device is what feeds
SDI.  And, of course, take the Din pin of the 1st SR to whatever logic
level you want - usually ground.  Just read in as many bits as you have in
the chain.

dwayne



Dwayne Reid   <.....dwaynerKILLspamspam.....planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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2001\01\19@155305 by Brian Aase

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Same problem -- the serial outputs QA & QH cannot tristate.
It seems overkill to use a separate data selector chip as some
have suggested (though it would certainly work).  Maybe I'll end
up using the daisy-chain solution, but it would be neater to find an
SPI-friendly part.
Thanks to all,
Brian Aase

> Brian:
> What about using a 74HC299? All eight I/O pins change direction
as a
> group.
>
> -- Rich

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