Alt-BEAM Archive
Message #01977
To: "'Steven Bolt'" sbolt@xs4all.nl, beam beam@corp.sgi.com
From: Sean Rigter rigter@cafe.net
Date: Thu, 01 Apr 1999 23:10:26 -0800
Subject: [alt-beam] RE:74HC240
Steven,
I tested my thought experiment for reducing cmos oscillator power
consumption and it appears to be correct. It turns out that with the
addition of the 5x "isolation" resistor often recommended for cmos
oscillator designs, the input spends more time in the saturated/digital
state and proportionally less time in the linear region for a net gain
in efficiency. However, the 5x resistor and stray capacitance between
inverter output and input can lead to the Miller effect which slows the
transition. This is common in 74HC04 inverters with adjacent input and
output pins plugged into popular "breadboards" with relatively high pin
to pin capacitance and is less of a problem in gates that seperate input
and output pins ie 74HC240. Good layout and "guarding" the input will
eliminate this problem and gives up to 20% lower current consumption at
5V. I will test at different supply voltages and I believe there are
some other tricks to speed the transistion through the linear input
region for ultra low power applications. I will report back with the
results.
regards
wilf
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