Imre, I think he have problems with load, mostly parasitical LED lighting
whithout any command.
A totem-pole output may have (if it's a correct design) greatest loads to
ground ( small resistors ) or smallest loads (large resistors) connected
to VCC accordingly to IOL and IOH specifications. If the load does not
meet with this, the output will be out of TTL requirements and the output
voltage levels may have unexpected values.
It's the situation with not only a microcontroller input is connected to a
TTL output.
But you knew that, I'm sure.
Cheers,
Vasile

On Thu, 4 Oct 2001, dr. Imre Bartfai wrote:

> 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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