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Belt and Suspenders BiCore Circuits

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(Redirected from The "belt and suspenders" BiCore Circuits)

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The following is a copy of an article originally posted by Wilf Rigter to the BEAM email list and as such represent

an important historical and technical reference

Thank you

by wilf rigter (c) Jan 1999 (used by permission)

The suspended BiCore circuit consists of 2 inverters, typically from a HC240 (or HC14 Schmitt triggers), 2capacitors, 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 theseare 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

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and Suspenders BiCore Circuits - BEAM Robotics Wiki http://www.beam-wiki.org/wiki/The_"belt_and_suspenders"_BiCore_C...

3 2013-05-21 4:45 PM GP
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(http://en.wikipedia.org/wiki/Exponential_function ) 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 power supply, greatly easing the problem of sensing

signals near ground and Vcc.

-

Also See: "BiCores (suspended, isolated, Schmitt)"

-

Ascii Schematics

The original asci schematics have been converted to GIFs to eliminate any difficult in viewing them.


3 2013-05-21 4:45 PM GP
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