Pullup resistors on 3-state buses

John V Fitzpatrick ([email protected])
Tue, 03 Jun 1997 10:53:18 +0200

Hello all,

A question:

When must pullup resistors (or bus-hold circuits) be used
on 3-state buses, and when they can be dropped?

It is a generally accepted rule that CMOS inputs should never be
left floating. So for unused inputs or 3-state buses, we use
a pullup or pulldown resistor, or some form of bus-hold circuit.

However, very often we see chip sets which have 3-state buses,
but pullups are not recommended, and no indication of internal
pullups or bus-hold circuitry is given. Is this safe design?

Another example:
The PCI bus does not require pullups, but has a paragraph which
suggests that there may be problems if pullups aren't used.
So should resistors be used? Are the risks real?

As you may have guessed, I don't like specifying resistors
whose function may not be necessary.....

Any pointers to useful information very welcome

Thanks in advance,

John Fitzpatrick   <[email protected]>    
Alcatel Telecom, 4 rue de Broglie, 22304 Lannion, France
Tel: +33(0)  Fax: +33(0)