Need 74HC165 with tristate output
Dwayne Reid email (remove spam text)
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
Dwayne Reid <planet.eon.net> dwayner
Trinity Electronics Systems Ltd Edmonton, AB, CANADA
(780) 489-3199 voice (780) 487-6397 fax
Celebrating 17 years of Engineering Innovation (1984 - 2001)
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