00058 [alt-beam] BiCores (suspended, isolated, Schmitt)

Alt-BEAM Archive

Message #00058

To: beam@corp.sgi.com
From: Wilf Rigter Wilf.Rigter@powertech.bc.ca
Date: Sun, 31 Jan 1999 12:54:08 -0800
Subject: [alt-beam] BiCores (suspended, isolated, Schmitt)

Hello everyone,

Well, I was a tad confused in my earlier post but without that I wouldn't
have had as much fun discovering just what goes on under the hood of Beam
Core Circuits!

The "pretty amazing, right?" bit, in my previous post should have alerted
you that there is more between Vcc and Ground than meets the eye!

However it make little sense to suggest a formula to calculate the time
constant of NvCore and "suspended" BiCores : it must be done by experiment
(like my first experience in electronics: probing the Galena cube in my
crystal set with a "cat's wisker"- God, I must be old!)

Perhaps a guide line for calculating the time constant of a NvCore is
useful:

The nominal lower threshold of a National and Philips 74HC14 Schmitt trigger
is about 1.8V which makes the time constant of a NvCore stage T=R*C .
(Rather easy to calculate) Because the voltage across the capacitor is
referenced to V (via the output of the previous Nv), the cap charges
exponentially through R towards 0V. At 63% of the applied voltage across the
cap, it crosses the lower threshold (Vt-=1.8V @ Vcc=5V) at which point the
output of the Schmitt trigger goes high. I checked this with several 74HC14s
and it is correct to within 5%! In short, the process duration for these
makes of 74HC14 NvCore is R*C seconds. ie 1M and 0.1ufd = 100ms pulse
duration. I understand that TI parts have different Vt- threshold and will
have a longer timeconstant for the same R/C component values as a result.
Similarly the 74HCT14 would have a lower Vt- threshold and therefore a
longer time constant. I will repeat the experiment when I get some samples
and post some more guide lines. Also see
http://www.serve.com/heretics/discus/messages/223/45.html?FridayJanuary29199

The "suspended" BICORE however is a different engine:

The "suspended" BiCore is a remarkable circuit that at first glance cannot
possibly work. It has 2 processes which are active at the same time with the
shorter process always in control. The "suspended" BiCore circuit operation
bears little or no resemblance to the operation of a 74xx14 BiCore despite
claims to the contrary! Since we use digital devices as linear amplifiers
without a well defined voltage gain and since the trigger thresholds are
dependent on the voltage gain, the pulse duration of a bicore is notoriously
difficult to predict and ranges from 0.2RC for a 74HC00 to 5RC for a 74HC04
depending very much on gain, the level of supply ripple and spurious
oscillations. Even differences in manufacturers and families (I have tested
this with 74HC04, HCT04 and AC04 devices) can result in a 4 to 1 variation
in time constants.

In digital electronics, we think of of an inverter as a device operating
with input and output voltages at discrete logic levels, usually 0V and Vcc.
In analog electronics, we think of an inverter as a linear amplifier with
the ratio of input to output voltage directly proportional to the gain of
the circuit and the linear input range = Vcc/gain. When we think
"suspended" BiCore, we enter the world of mixed analog and digital
electronics.

In the "suspended" BiCore Oscillator circuit both upper and lower threshold
voltages are used and for a "standard" HC inverter input these are the
points at which the inverter output voltage starts to change (the linear
region). The switching thresholds therefore are the upper and lower edges of
the linear input region which depend on the gain of the linear inverter. For
the high gain 74HC04 or 74HC240, it is a narrow region of ~100mV close to
the center of Vcc. In the case of the low voltage gain 74HC00, the linear
region is about 1V giving upper and lower thresholds of 3V and 2V compared
to the 2.65V and 2.45V of the 74HC04. I won't give the whole description of
operation of a BiCore (the suspended kind of course, more on this later) but
suffice it to say that the timing network consists of 2 (nominally equal)
capacitors in series with a series resistor and that the initial charge
across both capacitors is (nominally) 0V and that the charge across both
capacitors rises exponentially towards Vcc, while the voltage across the
resistor decays exponentially towards 0V. In the case of the high gain
inverters (74HC04) the linear region is near the center of Vcc, so the
voltage across the resistor must be close to 0V, ergo the charge across the
capacitors must be nearly Vcc. The important difference between the Schmitt
NvCore and the "suspended" BiCore is the fact that NvCore processes are
independed sequential edge trigger time constants while in the "suspended"
BiCore both proceses are triggered simultaneously and only the shorter of
the two time constants determines the combined "BiCore process"
timeconstant.

I think using the term BiCore for both circuits is a tad of a misnomer.

Here is some good news! A 74HC14 BiCore is indeed possible! (Forget about
"crosstalk" 8^) Just use two capacitors with a ratio of values greater than
1/2 for Schmitt triggers that have thresholds at 33% and 66% ie National
74HC14 . Since the smaller capacitor (the shorter process) sets the time
constant of the BiCore you can make the ratio arbitralily large (make it 1
to 4 to be on the safe side given the variety of thresholds out there) The
larger cap has no effect on timing other that to provide positive feedback
during switching. Interestingly enough the larger cap transfers a larger
charge and this can be used in the next circuit to provide +/- 5V
powersupply. Amazing what you can do with the BiCore or maybe this should
be called the SCore!

I tested an "isolated/suspended" BiCore which uses large resistors in series
with the inverter inputs to avoid "diode clamping" of the input waveforms.
As a result, the time constant is roughly doubled and some interesting
characteristics were observed. The voltage across the resistor is now
initially 10V for a Vcc of 5V and the optional addition of some diodes and
capacitors turn the bicore into a symmetric voltage doubler circuit.
Moreover the + and - timeconstants are dependent on the loading of these
+7.5V and -2.5V supplies. This opens up a whole new way to provide feedback
for an "isolated " BiCore circuit or maybe this should be called the ICore!

The description of operation of the "suspended" and the "isolated" BiCore is
shown below:

The 74HC14 "suspended" BiCore waveforms are a little bit complicated and not
easy to do in ASCII so you will have to wait a bit for those details.
However, I will have the detailed description of operation of all these
circuits and GIFs up later today or tomorrow at my new website which I will
open up despite the fact that it is just bare bones right now.

In the mean time:

enjoy

wilf

The "belt and suspenders" BiCore Circuits
by wilf rigter (c) Jan 1999

The suspended BiCore circuit consists of 2 inverters, typically from a HC240
(or HC14 Schmitt triggers), 2 capacitors, and a resistor. In addition, there
are 2 reverse biased input protection diodes between each inverter input and
the chip supply pins. These diodes are not normally shown but they play a
role in the operation of the circuit.

Here is a brief description of a suspended BiCore oscillator operation:

Given a +5V supply , a 74HC240 and equal capacitors C1 and C2, assume that
the BiCore is oscillating with the input of A1 at +5V and the input of A2 at
0V. Each inverter amplifies and inverts the input and therefore the output
of A1 is 0V and the output of A2 is +5V.

Since each capacitor is connected between the output of one and the input of
the other inverter and since these are at the same potential, the voltage
across each cap is 0V. However the 2 caps are also connected in series with
the resistor across 0V and +5V (the two opposite outputs).

This initial +5V across the resistor causes an exponentially decaying
current which charges the caps so that the voltage approaches 0V across the
resistor and 2.5V across each cap (and at each inverter input) As the A1
input approaches 2.5V, the A1 output starts to switch state from 0V to 5V.
This A1 output signal is coupled via C1 to the input of A2 which, already at
2.5V, causes A2 output to switch rapidly from +5V to 0V. In turn, the A2
output coupled via C2, now drives the input of A1 from 2.5V to 0V. The high
gain of the two inverters and the positive feedback cause both outputs to
flip to the opposite digital states.

Theoretically , the voltage at each input should swing between 7.5V and
-2.5V, the sum of the input voltage (2.5V) and the coupled output signal
(+/-5V) at the moment of switching and this is the case in the "isolated"
BiCore. However in the "suspended" BiCore each input is protected internally
by the equivalent of 2 diodes connected in reverse bias from the input to
each supply, so that the voltage at each input is clamped and swings between
+5.6V and -0.6V respectively as shown below. The real waveform has
exponential slopes approximated here with diagonal lines. The A2 waveforms
A2 are identical but phase reversed (upside / down). The third waveform
shows sum of the 2 input voltage waveforms across the resistor which is also
proportional to the capacitor charging currents.

Note that in the case of the isolated BiCore, the over and under voltage
generated by the capacitor charge pump can be put to good use: for example
for a source side driver for MOSFETs or to overdrive the inputs of a
saturated Emitter Follower bridge or as an +/- Opamp/Comparator powersupply,
greatly easing the problem of sensing signals near ground and Vcc.

5.6V _ _
A1 _ |\ |\ |\ | _
input 2.5V _ | \ | \ | \ | _ __
_ / | / | / | / _ __|A1 \___
-.6V _ |/ |/ |/ _ | |__ / |
| |
5.0V_ _ __ __ __ _ | ===
A1 _ | | | | | | | _ | | 0.1
output _ | | | | | | | _ |---[1M]---|
_ | | | | | | | _ 0.1| |
0V_ |__| |__| |__| | _ === |
| __ |
6.2V _ _ |___/ A2|__|
voltage _ |\ |\ |\ |\ |\ |\ | _ \ __|
across R _ | \| \| \| \| \| \| _
_ | /| /| /| /| /| /| _
0V _ |/ |/ |/ |/ |/ |/ | _

| | | | | | | | |

Suspended BiCore switching waveforms Supended BiCore Circuit

7.5V_ _
_ |\ |\ |\ _ __
_ | \ | \ | \ _ __|A1 \___
TP(A1) 2.5V_ | \ | \ | \ _ | |__ / |
_ / | / | / | _ | |
_ | / | / | _ [10M] |0.1
-2.5V_ |/ |/ |/ _ | ===
TP(A1)--|---[1M]---|--TP(A2)
| | | | | | === |
5.0V_ _ ___ ___ _ _ 0.1| [10M]
A1 _ | | | | | | _ | __ |
output _ | | | | | | _ |___/ A2|__|
_ | | | | | | _ \ __|
0V_ |___| |___| |___| _

Isolated BiCore waveforms Isolated BiCore

+7.5V +7.5V
| |
|----|<-----+ +----->|---+----|<-----+
|+ | | |+ |
=== 10uF | | === 10uF |
| | | | |
Vcc | | Vcc |
|---TP(A2) TP(A1)--| |--TP(A2)
0V | | 0V |
|+ | | |+ |
=== 10uF | | === 10uF |
| | | | |
|---->|-----+ +-----|<---+---->|-----+
| |
-2.5V -2.5V

Isolated Bicore Doubler Isolated BiCore Bridge Doubler

Wilf Rigter mailto:wilf.rigter@powertech.bc.ca
tel: (604)590-7493
fax: (604)590-3411

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