The 20afTopOctave chip provides square waves pitched with good approximations to the equal-tempered scale. The thirteen pitched outputs are derived from a master clock input (pin 2) using individual dividers. Divisors are arranged so that A440Hz can be synthesized from integer divisors of a 4MHz input clock.

The clock input may be divided by 2, 4, 8 using the LSB and MSB inputs (pins 3,4).

Pin 20 outputs the internal clock divided by two. This is intended for daisy-chaining multiple 20afTopOctave chips to provide a complete range of pitches.

Pin 19 outputs a sequence of bits synthesized by an LFSR (Linear Feedback Shift Register) to provide a white noise source. In organ designs this source is commonly used for percussion sound synthesis.

Power (pin 1) and Ground (pin 11) follow the unusual arrangement of all the 20af series.

Canvas 1 Layer 1 20afTopOctave ÷2 12 13 14 15 16 17 18 20 11 Vdd 2 1 3 4 6 8 Vss 9 10 5 7 19 Clock In ÷ 2^N ÷253÷2 ÷239÷2 LFSR ÷268÷2 ÷284÷2 ÷301÷2 ÷319÷2 ÷338÷2 ÷478÷2 ÷451÷2 ÷426÷2 ÷402÷2 ÷379÷2 ÷358÷2 low C C# D D# E F F# G Noise G# A A# B C LSB MSB

Maximum Input Frequency

8MHz at 5V


20-pin STQFN: 2 x 3 x 0.55 mm
20-pin TSSOP: 6.5 x 6.4 x 1.2 mm

Operating Conditions

–40 to 85°C


  • Sound Design
  • Electronic Organs
  • Circuit Bending and Experimentation
  • Rhythm Sequencers
  • DIY Synthesizers
  • Synthesis voice sources
  • From the Designer's Desk

    A MOS Top Octave Generator IC was one of my most precious possessions as a teenager: it was the most expensive single electronic component I owned and it saved me from the dauntingly large amount of wiring or the separate oscillator tuning that would have been required for the electronic organ I was building.

    Finding this much functionality in a single chip in the early 1970s was a wonderful magic for me that can be appreciated today by undestanding that this was the same decade that produced the first single-chip alarm clocks, calculators and eventually microprocessors, DRAM and SRAM.

    It is possible to simulate top octave generators with microprocessors but unfortunately it is very difficult with the usual cheap microcontroller instruction rates to avoid introducing jitter.

    The popular clock generators of the 1970s were in 16-pin packages and the pin assignments are unsurprising: power, ground, clock in, the twelve pitches and an octave. For an implementation in the 20af series packages this leaves 4 pins to assign.

    I use one pin to output a white noise generator - they are commonly used in organ designs for percussion generators. Production stopped long ago of standalone noise generator chips so the 20afTopOctave has value in designs even when you don't need the Top Octave.

    The assignment of a pin to the output of a simple divide by 2 flip-flop allows for daisy chaining 20afTopOctave chips providing for as many lower octave pitches as desired. The usual away of creating lower octaves notes is to add a divider chip per pitch of a top divider. The price difference between divider chips and top octave chips is now very small so the advantages of having fewer part numbers and a simpler design may favor multiple 20afTopOctave chips.

    The remaining two pins are inputs to define the prescaling of the input clock by powers of 2.

    Although organs are not as popular as they were in their boom decades from the 1950s I am confident that this generation of designers will explore interesting applications afforded by a handy set of pitched square waves.

    Here are a few ideas to start with:

  • Mixing the top octave outputs to generate shaped noise for percussion instrument sounds
  • Slowing the clock frequency and using the outputs to generate polymetric triggers
  • Building chord generators and polyphonic drones

    Here are some inspirational designs incorporating top octave generators:

  • paia chord egg
  • paia oz
  • vibrato
  • Copyright 2019-2022. Adrian Freed. All Rights Reserved.