The 20afStanley+ integrates many common circuits used by the experimental
music community and often associated with Stanley Lunetta who shared them widely in the
early days of TTL and CMOS. Also available
are some circuits unavailable to designers
of the venerable 4000 and 74HC series ICs.
Power (pin 1) and Ground (pin 11) follow the unusual arrangement of all the 20af series.
A 100nF decoupling capacitor is recommended as close to the power supply pins as possible. Additional
decoupling may be valuable when large currents are switched by the buffer transistors (pins 10 and 12).
All the input pins (2-9) feature a
low voltage input threshold: around 1V.
This is so that the chip can be clocked from external oscillators which are being voltage-starved - a common
modulation technique in the CMOS audio experimental community.
Pin 2 (HF) sources a clock for a linear-feedback shift register configured to produce
a pseudorandom bit sequence on pin 20.
HF also clocks an 8-bit binary counter connected to a DAC. The 1 Volt maximum amplitude output from the
DAC (Saw) is buffered on pin 19 and derived from an on-chip bandgap reference.
The overflow from this counter is output on pin 18. With Reset unused this output provides
a short pulse every 256 counts of the counter. A rising
edge on the Reset pin resets the counter. This outputs a pulse on 18 and also restarts
the sawtooth waveform. Interesting and useful waveshapes can be created
by orchestrating the timing relationship between the HF clock and Reset pins.
The Clock input (pin 4) drives sine wave approximator that uses Pulse Position Modulation (PPM).
An external reconstruction filter is required to obtain a smooth sinusoidal output.
Pin 4 also clocks a divider chain consisting of 7 divide-by-2 circuits that are multiplexed in
to the Divide pin (14) according to the 3-bit encoding established on I4 (pin 5),
I2 (pin 6) and I1 (pin 7).
I3 (pin 8) inserts a 50% duty cycle divide-by-3 circuit into the output.
The multiplexor control pins (5,6, and 7) are shared by two other functions.
Pin 15 outputs the XOR
Pin 16 outputs the result of chain of NOR gates applied to these inputs.
Pin 9 is the input to a CMOS inverter built from high-current complementary transistors. Their
drains are separately available on pin 10 (Pull Down) and pin 12 (Pull up).
Pin 13 provides the divide by 2 output of a flip-flop clocked by pin 9. This is intended to provide
a square wave output in relaxation oscillator applications using the buffer transistors in
their capacity to discharge and/or recharge a timing capacitor. A 2uS delay is provided from the input
to control rapid recharge and discharge rates.
20-pin STQFN: 2 x 3 x 0.55 mm
20-pin TSSOP: 6.5 x 6.4 x 1.2 mm
–40 to 85°C
Circuit Bending and Experimentation
Companion for 40106/4093 Relaxation Oscillators
Random and Noise music
Synthesis voice sources
The 20afStanley+ is a swiss army knife of handy CMOS circuits used in audio applications.
Just like swiss army knives it may not be possible to use all the functions at once. The
colors of the table below describe functions that share input pins.
As with all CMOS circuits unused inputs should not be left to float. Tying them to ground
is the most useful solution for the 20afStanley+.
The guiding question driving the design of this chip was "what are the most useful functions involving
a small number of inputs and outputs?". The answer was drawn mostly from the long history of audio
applications of CMOS circuits and from new designs developed by nOmni to take advantage of the availability
of mixed signal functions.
Most of the 20afStanley+ circuit blocks are intended to be clocked externally. Many interesting sounds are obtained
from asynchronous clocks, so a hex schmitt inverter (CD40106 or 74HC14) is a convenient source
of relaxation oscillators. Voltage control of the frequency of these inverters can be achieved
by changing their power supply voltage. The inputs of the 20afStanley+ were tuned to a low 1 Volt
threshold to maximize the dynamic range of power supply modulation ("Voltage Starvation").
For these situations the 74LV14 is worth considering as it operates from 1 to 5.5 volts.
CMOS versions of the 555 timer may be useful as they afford moderate amounts of frequency modulation via their pin 5.
The inverting buffer of the 20afStanley+ has been tuned for building relaxation oscillators using
circuits that favor voltage or current control.
If the resistor of the usual schmitt trigger relaxation oscillator is replaced by a diode the charging
rate of the capacitor can be controlled by an externally controlled current source. A constant current source
will result in a sawtooth wave at the capacitor node. An approximate sawtooth is obtained with a resistor
sourcing current from the power supply or an external voltage.
Although the inverting buffer of the 20afStanley+ is not a Schmitt trigger, the same circuit can be
used as long as the capacitor is reliably discharged below the 1 volt input threshold. This is achieved
with a 2uS delay from input to output integrated into chip. This is long enough to discharge capacitors up
to 0.01uF. The low threshold 1V gives a good dynamic range of voltage control: 4:1 in the case of 5V VDD.
With Schottky diode protection of the input node higher voltages (e.g., Eurorack +12v) may be
used for wider frequency range.
A narrow pulse output can be obtained by adding a pull-down resistor to the p-mos drain output (pin 12). Alternatively
the square wave divider output (pin 13) may be used.
Instead of rapidly discharging a capacitor, the p-channel pull-up transistor can be used to recharge the capacitor. The
discharge rate is then controlled by a current sink. This produces a sawtooth ranging in amplitude from the power rail down
to the 1V switching threshold of the inverter. This circuit is suitable for control by
common exponential converter designs to create a 1V/Octave oscillator core. This circuit allows for a very wide range
of oscillation frequencies from infra to ultrasonic.
well-known two-inverter astable is realized by connecting the open drains of the buffer
and adding an inverter created from the divider or logic sections.
This oscillator has the disadvantage that all the timing components
are floating so it is not easy to frequency modulate. On the other hand it is useful to obtain low oscillation frequencies
with small capacitors, e.g., for LED flashing.
Copyright 2019. Adrian Freed. All Rights Reserved.