20afStanley

Description

The 20afStanley integrates circuit blocks used by the experimental music community and often associated with Stanley Lunetta who shared them widely in the early years of TTL and CMOS in the 1970s. Also available are some circuits that were 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).

The digitalinput pins (2,4-9,20) 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 audio experimental community.

The low input threshold voltage and availability of an open-drain nMOS output (pin 10) allows the 20afStanley to be integrated into designs with mixed 3.3volt and 5v rails.

Pin 8 sources a clock for a linear-feedback shift register configured to produce a pseudorandom bit sequence on pin 14.

Pin 2 clocks an 8-bit binary counter connected to a DAC. The output on pin 19 is derived from an on-chip 1V bandgap reference. If pin 20 is grounded, the DAC will output a sawtooth wave. When pin 20 is pulled high the counter is clocked up and down resulting in a triangle wave at the output.

A rising edge on the Reset pin 4 resets the counter Interesting and useful waveshapes can be created by orchestrating the timing relationship between the clock and Reset pins.

The counter is set to 255 at power on. If the clock and reset are left grounded the output at pin 19 can be used as a 1V voltage reference.

Pin 18 outputs 0.5v from the bandgap reference and can be used to provide a half rail to a voltage differencer to center the sawtooth/triangle output.

Pin 3 is a 1V maximum PWM modulation input which is compared to the DAC output. This PWM output is on pin 17. Connect Pin 3 to the 0.5v reference to produce a square wave synchronized to the saw/triangle wave. The Sine clock (pin 5) drives a sine wave approximator that uses Pulse Position Modulation (PPM). An external reconstruction filter is required to obtain a smooth sinusoidal output.

Pin 15 outputs the XOR function of the inputs pins 6 and 7.

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.

Packages

20-pin TSSOP: 6.5 x 6.4 x 1.2 mm
20-pin STQFN: 2 x 3 x 0.55 mm
20-pin TSSOP: 6.5 x 6.4 x 1.2 mm 3.3V Automotive Qualification

Operating Conditions

1.8V-5V
–40 to 85°C

Applications

Sound Design

Circuit Bending and Experimentation

Companion for 40106/4093 Relaxation Oscillators

Rhythm Sequences

DIY Synthesizers

Random and Noise music

Chaos Oscillators

Experimental Music

Synthesis voice sources

VCOs

Level Shifting

Signal Generators

Logic Probes

LED flashers and lighting effects

Application Notes

The 20afStanley is a swiss-army knife of handy CMOS circuits used in audio applications. Here are the pins organized by function:

Function	Input	Output	Discrete	Audio	LF
LFSR	8	14	CD4006, CD4070	Noise	Rhythm
0.5V		18	LM4140	Bandgap	Calibration
Staircase	2,4,20	19	CD4520, DAC08	Sawtooth	LFO
PWM	3	17	LM339	Pulse	Modulation
Sine	5	16		Modulation	LED
Ring	6,7	15	CD4070	XOR	Modulation
by 2	9	13	CD4013	Square Wave	LED
Buffer	9	10,12	CD4007, CD40107	Oscillators	Relays Motors
Power	1
Ground	11

As with all CMOS circuits unused inputs should not be left to float. Tying them to ground is the most common 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?". Answers were drawn mostly from the long history of audio applications of CMOS circuits and from new designs developed by nOmni that take advantage of mixed signal functions unavailable to early designers.

Oscillators

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.

Voltage 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 was achieved with the addition of a 2uS delay from the input pin to the output drive transistors. In the diagrams below, the usual Inverter symbol has been enhanced to represent the delay with a Greek delta and the split drain outputs above and below the inversion symbol.

The 2uS timing 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 flip-flop divider output (pin 13) may be used.

Current Control

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.

Dual Inverter

The well-known two-inverter astable is realized by connecting the open drains of the buffer and adding an inverter created from the XOR gate. 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 flashing LEDs.

LED driving

Flasher

LEDs can be added in both orientations between ground and the RC node of the dual inverter to produce brief flashes, brief pauses in light or alternating flashes.

Pulsating LED

Driving an LED from the Sine wave output produces a pulsating effect or at lower frequencies an accelerating/decelerating flashing effect.

Flickering LED

The flickering LED effect usually used with yellow LEDs to simulate candle light is easily obtained by driving an LED from the noise output.

PLL

By using the XOR gate as a phase component, all the elements to create a PLL are available.