Alt-BEAM Archive
Message #02429
To: beam@corp.sgi.com
From: Wilf Rigter Wilf.Rigter@powertech.bc.ca
Date: Sat, 17 Apr 1999 11:03:58 -0700
Subject: [alt-beam] Slave BiCore operation
content-type: text/plain
Hello to all BiCore fans,.
I wrote this description of the operation of a slave BiCore a little while
ago but forgot to post it. I know that some readers already know this stuff
but I thought you might enjoy reading it and especially the surprise ending
in the next post.
The attached GIF of a Master-Slave BiCore shows the relevant waveforms. The
description of the master BiCore was previously posted. Because of the
symmetry of circuit operation, the waveforms on the bottom side of the
circuit would be identical but "upside/down".
In normal operation the familiar Slave BiCore is a kind of complementary Nu
Neuron with feedback.
Assume a stable condition where:
The outputs of the master are 1 and 0,
the inputs of the slave are 1 and 0,
the outputs of the slave are 0 and 1.
Therefore the voltage across the slave coupling resistors (Vsr) = 0V and the
voltage across the slave capacitors (Vsc) = 0V. Now assume the master BiCore
has just flipped it's bits.
The master outputs are now 0 and 1,
the slave inputs are still 1 and 0,
the slave outputs are still 1 and 0.
Therefore Vsr=Vcc and Vsc=0V and the slave capacitors start to charge /
discharge through the "coupling" resistors. So far, no different from two Nu
neurons (integrators) delaying a step input by their RC time constant :
The slave inputs 1 and 0 charge towards opposite values and when either of
the slave inputs reaches the switching threshold at approximately 1/2 Vcc,
the corresponding slave output starts to switch from 1 or 0 to 0 or 1. Now
feedback occurs, which is quite different from the Nu neuron and more like a
Nv neuron:
When the first output changes, this change is capacitively coupled into the
other slave input and causes that input, already near 1/2 Vcc to cross it's
threshold which in turn causes the second output to change, which is
capacitively coupled into the first input. The second RC node with the
larger time constant plays no role in the timing of the slave BiCore and
the RC components can be eliminated. This positive feedback results in an
rapid voltage change at both slaves input towards the value as the
corresponding master outputs. During this rapid change, each slave
capacitor charge is "dumped" through the slave input protection diodes so
that the voltage across the caps and resistors rapidly changes to 0V. At
that point the following stable condition exists:
The master outputs are 0 and 1,
the slave inputs are 0 and 1,
the slave outputs are 1 and 0
Vsr = 0V and Vsc = 0V
The process repeats when the master BiCore again flips it's bits in the
opposite direction.
The formula for the delay time of a 74HC/ACxx Slave BiCore is approximately
0.7RC.
The time constant of the master BiCore is much trickier to calculate
especially when components are closely matched since the switching threshold
is close to 0V across the suspended resistor (ie on the flat part of the
exponential discharge curve). This is what makes the master BiCore time
constant long for a given RC and quite sensitive to preemptive triggering by
"feedback" from the load. A rule of thumb used for determining the 74HC/AC04
or 74HC/AC240 type master BiCore time constant is approximately 1.4RC. Based
on the requirement for 90 degree phase delay between Master and Slave
BiCores the same RC components can be used in both. The 90 degree phase
shift means that the Slave BiCore output changes occur half way in time
between the Master BiCore output changes.
enjoy
Wilf Rigter mailto:wilf.rigter@powertech.bc.ca
tel: (604)590-7493
fax: (604)590-3411
> <>
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