RE: [SI-LIST] : Why all reset signals are active low?

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From: Michael Nudelman (mnudelman@tellium.com)
Date: Mon Mar 05 2001 - 07:34:53 PST


Well....if you look at some PIC controllers, or Philips 8052 derivatives,
their resets are Hi; typically done with RC circuit.

I think, there are lots of factors, including listed here (noise); also,
since the Wired AND is needed on signals like Bus Handshake types, Resets
etc, and so they are done with open collector drivers, it is naturally to
bring them low.

-----Original Message-----
From: Doug McKean [mailto:dmckean@gte.net]
Sent: Monday, March 05, 2001 10:16
To: si-list@silab.eng.sun.com
Subject: RE: [SI-LIST] : Why all reset signals are active low?

Not all are. I've worked with some
communications signals whee the error
signals which end up causing a reset
are an active high. Simply because
and active high occurs when there is
a disconnect (an open) in the wire.

But most are acive low and this is simply
explained when considering when a system
boots up. On Boot-up, if the RESET is active
high, it is a valid possibility that the
RESET signal wil be indeterminate as power
comes up. This in turn can cause the
microprocessor to go into some wild state.
Much too long ago in the dim past of
microprocessors, there was one such device
which did have an active high for RESET and
caused much havoc and redesign in the circuit
of a young engineer

It's just easier to hold the RESET line low
throughout the entire bootup sequence until
power is established, release /RESET to a
high at some predetermined time, then start
the system.

- Regards, Doug McKean

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