Thanks Re: Pullup resistors on 3-state buses

John V Fitzpatrick (John.Fitzpatrick@ln.cit.alcatel.fr)
Mon, 16 Jun 1997 17:57:35 +0200

Thanks to all who replied to my question regarding
pullup resistors. I've been listening to this list for
some time, and am always impressed by the quality of the replies.

from the rplies I got, the three main design rules are:

1) Pullup resistors can be dangerous if they pull up too
slowly. So calculate the RC constant, then compare it with the
input slew rate of the components on the bus (if
specuified!).

2) Bus-holds, which maintain the last state, are a safer choice.
If a definite hi-Z state is required, then the bus should
actively driven to this state before being put into hi-Z.

3) Most buses do not need pull-ups or bus-hold circuits
for data and address lines (e.g. PCI, memory bus, ...).
This is because they will not float for very long.
Not very long means <1us, but this is unlikely to
be guaranteed.

"Merci beaucoup" to all who replied,
John

-- 
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