Wednesday, January 30, 2008

Programmable auto filter interface for C64 using Arduino

Demo video:


A software / hardware combo that allows for control of the analog filter inside a Commodore 64 that is running Cynthcart from a host computer (eg, Macbok, PC).

• 16-step filter sequencer
• Set the speed of the filter sequence (with intervals between values reaching 2ms)
• Set the length of the sequence from 1 to 16 beats
• Offset value allows for shifting the filter values up or down on a larger scale
• MIDI mode allows sending data directly to control the filter (this mode disables the sequencer)

Download the Max/MSP patch here:

It requires Max/MSP runtime which can be downloaded from


The hardware is very simple, and is based around an Arduino board that is using a bit-banging SPI technique to write data to a 100KΩ digital pot (model MCP42100). Approximately 7 of the 8 bits of data actually change the value of the filter (this could be due to the resistance value of the pot).

Any microcontroller that can either do SPI natively or through bit-banging should be able to handle this simple operation.

• J1 1 to 14 refers to Arduino digital pins 0 to 13
• The datasheet for the MCP42100 recommends a 0.1uF or similar capacitor to smooth out the power supply. However, this has not been an issue.

Arduino Code


int SS1 = 2; // set slave select 1 pin

int CLK = 3; // set clock pin

int MOUT = 4; // set master out, slave in pin

byte cmd_byte0 = B00010001; // command byte to write to pot 0, from the MCP42XXX datasheet

byte cmd_byte1 = B00010010; // command byte to write to pot 1, from the MCP42XXX datasheet

byte cmd_byte2 = B00010011; // command byte to write to pots 0 and 1, from the MCP42XXX datasheet

byte work; // setup a working byte, used to bit shift the data out

byte data;

/* SETUP */

void setup() { // setup function begins here


pinMode(SS1, OUTPUT); // set CS pin to output

pinMode(CLK, OUTPUT); // set SCK pin to output

pinMode(MOUT, OUTPUT); // set MOSI pin to output

digitalWrite(SS1, HIGH); // hold slave select 1 pin high, so that chip is not selected to begin with


void spi_transfer(byte working) {

; // function to actually bit shift the data byte out

for(int i = 1; i <= 8; i++) { // setup a loop of 8 iterations, one for each bit

if (working > 127) { // test the most significant bit

digitalWrite (MOUT,HIGH); // if it is a 1 (ie. B1XXXXXXX), set the master out pin high


else {

digitalWrite (MOUT, LOW); // if it is not 1 (ie. B0XXXXXXX), set the master out pin low


digitalWrite (CLK,HIGH); // set clock high, the pot IC will read the bit into its register

working = working << 1;

digitalWrite(CLK,LOW); // set clock low, the pot IC will stop reading and prepare for the next iteration (next significant bit



void spi_out(int SS, byte cmd_byte, byte data_byte) { // SPI tranfer out function begins here

digitalWrite (SS, LOW); // set slave select low for a certain chip, defined in the argument in the main loop. selects the chip

work = cmd_byte; // let the work byte equal the cmd_byte, defined in the argument in the main loop

spi_transfer(work); // transfer the work byte, which is equal to the cmd_byte, out using spi

work = data_byte; // let the work byte equal the data for the pot

spi_transfer(work); // transfer the work byte, which is equal to the data for the pot

digitalWrite(SS, HIGH); // set slave select high for a certain chip, defined in the argument in the main loop. deselcts the chip


void loop () {

if(Serial.available() > 0) {

data =;

spi_out(SS1, cmd_byte0, data); // send out data to chip 1, pot 0

delay(2); // set a short delay




philip said...

this is ideal! definitely my next project.

Sebastian Tomczak said...

im glad you like it.

philip said...

sebastian, do you have aim or msn? if so could you drop me an im next time you're online? aim: firebrandboy msn: i'd like to ask you a couple of questions if you dont mind.

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Nahuel said...

Wow! This is awesome! Can't wait to start building it.