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?
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 <John.Fitzpatrick@ln.cit.alcatel.fr> Alcatel Telecom, 4 rue de Broglie, 22304 Lannion, France Tel: +33(0)2.96.04.79.33 Fax: +33(0)2.96.04.85.09