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mxUnifiedSN76489

mxUnifiedIO device library for Arduino. Device specific driver for the SN76489 sound chip. Use SPI shift registers or I2C I/O expanders via the mxUnifiedIO API.

Introduction

This is a library for the SN76489 complex sound generator chip. The SN76489 sound chip is a low-cost 3+1 channel complex sound generator that provides three programmable tone generators and one noise generator, all with programmable attenuation (volume). All four channels can be active simultanously and are mixed to one output, allowing for three-voice music plus noise. The chip was quite popular in the early days of home computing and gaming, being used in consoles and computers such as the Sega Genesis, IBM PCjr and BBC Micro. See Wikipedia and the datasheet for more information.

This library requires the the mxUnifiedIO base-library and a suitable mxUnifiedIO expander driver. Additionally it is supported by the MML Music library. Instead of directly using MCU pins, the mxUnifiedIO libraries allow the sound chip to be driven via the expanded pins of an I2C I/O expander or a shift-register.

Pinouts & connections

The SN76489 sound generator requires power via VCC/GND, a clock signal up to 4MHz, instructions via eight data pins and one control pin (write enable, /WE). The output enable pin (/OE) can be tied low. Audio out requires a mono amplifier to filter noise and amplify the signal for a speaker or headphones.

This fine piece of ASCII-art shows the pinout of the SN76489. Of course you can also review the datasheet (PDF).

Pinout SN76489 sound generator:

     +--v--+          PINS 1-8       PINS 9-16
  1 -+     +- 16      1: D2          16: VCC
  2 -+     +- 15      2: D1          15: D3
  3 -+     +- 14      3: D0          14: Clock
  4 -+     +- 13      4: Ready       13: D4
  5 -+     +- 12      5: /WE         12: D5
  6 -+     +- 11      6: /OE         11: D6
  7 -+     +- 10      7: Audio Out   10: D7
  8 -+     +- 9       8: GND          9: N.C.
     +-----+

Note that D0 to D7 are in LSB-first order. This means that the proper connection to a shift-register is D7 to Qa, D6 to Qb, etc. Alternatively the output of the shift-register can be reversed using setBitOrder(LSB_FIRST);

Features & limitations

  • The current version of this library has been tested using the Arduino 1.8.2 IDE, one 74HC595 shift-register and an ATtiny85 MCU. It was also tested using an Arduino Pro Mini with an ATmega168 running at 8MHz/3v3, both with and without shift register.
  • It was also tested using the Arduino 1.6.10 IDE, with the PCF8575 16-bit I/O expander and a a WeMos D1 mini (ESP8266 ESP-12F). Some low volume ticking noise was heard, probably interference from the ESP. On occasion the control signal was also missed, causing a tone to hang, possibly due to bad reception of the volume-off data, most likely caused by noisy breadboard connections. Perhaps using pull-up resistors on the data and I2C could improve this.
  • The library should also work using two cascaded shift registers or the PCF8574 I2C I/O expander, as well as using other Arduino IDE supported MCUs such as the ESP32 and the ATmega328 and ATmega168 as found on the Arduino Uno and Nano.
  • The current version of the library assumes a setup using a 4MHz clock for the SN76489 chip. This can be an external oscillator, but the chip also works using a clock signal generated by the Arduino. However, the required code is not part of this library. You can use the FastPwmPin library to generate such signal (tested on ATmega328/168 and ATtiny85).
  • In case you want to run the chip at a different speed, you need to change the #define statement in the .h file. Note that using lower clock speeds will limit the ability to play high-pitched notes. Using an unstable clock will result in false notes.

Credits and references

Parts of this library are based on or inspired by the following information and examples:

Disclaimer

All code on this GitHub account, including this library is provided to you on an as-is basis without guarantees and with all liability dismissed. It may be used at your own risk. Unfortunately I have no means to provide support.