DescriptionThe 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.