Hi,

I did not understand the question at all. An output may be either
totem-pole or open-collector. Even if it is not totem-pole (i. e. open
collector) it is definitely capable to pull down the connected pin (the
common mistake when using RA4 in PIC).

Regards,
Imre



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On Thu, 4 Oct 2001, Bob Barr wrote:

> Jeethu Rao wrote:
> >
> >Bob,
> >       Thanks for the quick reply.The problem is that I'm using
> >       the 74HC164 and a 74HC238 for multiplexing 4 7seg displays.
> >       with this arrangement, I end up using only 5 pins on the PIC.
> >
> >       But the problem was that 238s are not available in my little Indian
> >       town of Mangalore. So, I thought of using 74HC138 which is the same as
> >       138 except for the fact that its outputs are inverted. So, if I changed
> >       the software to shift out the complement of the actual pattern to the
> >       display, So, to clear out the displays, I would load 0xFF into the shift
> >       register rather than 0x00 . Hope that makes this issue clear.
> >
> >       I had thought that this would be ok. But I just wanted to be sure.
> >       The PCB guy has not yet got the PCBs. So I'll have to wait for a few days
> >       before trying this out.
> >
> >       BTW, The Philips Data Sheet I've got mentions nothing about OH. It only
> >mentions
> >       the timings in ns. Maybe they've got a family datasheet for the entire
> >74HCxxx series.
> >
>
> Yep, the Philips datasheet "HCMOS Family Characteristics" lists all of the
> DC specs. It has specs for V sub OH and V sub OL at a few different current
> values ranging up to several mA.
>
> Regards, Bob
>
> >
> > > -----Original Message-----
> > > From: Bob Barr [mailto:bob_barr@hotmail.com]
> > > Sent: Tuesday, October 02, 2001 2:36 PM
> > > To: jeethur@sancharnet.in
> > > Subject: Re: [EE]: Totem Pole Outputs in 74HC164
> > >
> > >
> > > Jeethu Rao wrote:
> > > >
> > > >         I've got a small little doubt about the outputs of
> > > >         74HC164. Are the outputs of these ICs totem pole ?
> > > >         In other words, Does the IC pull down the output to
> > > >         ground when one of its outputs is low ?
> > > >
> > >
> > > What raised this doubt? Check the data sheet for the part's I sub OL
> > > (current with output low) value. That spec will tell you how hard
> > > the output
> > > drives the pin to ground.
> > >
> > > An open drain output will have a spec for I sub OL but it won't
> > > have one for
> > > I sub OH (current with output high).
> > >
> > > AFAIK the '164 pulls down as well as any HC output.
> > >
>
>
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