32 LED fader with Arduino

Although it may not seem like it at first, this post is a follow up to my previous post.

The Arduino only has a maximum of 14 digital outputs (if serial communication is not used) or 12 (if serial communication is used). It is quite easy to extend the number of digital outputs using a 74hc595 serial to parallel shift register.

But the problem with using the 74hc595s is that firstly, the serial data must be clocked out one bit at a time and that in order to change only one of the registers, all of the registers must be written to (because they are all chained together). So there is a limitation in terms of speed and efficiency.

The 74hc374 IC can be used to perform the same function but in a different way. By clocking out data in parallel to all the 74hc374 IC data input lines and then pulsing the clock input of only one of them, it is possible to selectively write to only one of the sets of outputs.

The process is very quick - keep in mind that in order to change the output state of one the ICs, a PORTD or PORTB message can be used to quickly set the entire byte in parallel. Then, the clock input pin of the IC in question simply needs to set high for five microseconds (not milliseconds!) and then low for the same amount of time.

In this context, it is just like using a chip select line and writing to that chip.

This idea can be extended to execute pulse width modulation on, say, thirty-two outputs across four 74hc374 ICs. In the example below, the PWM has a bit depth of eight (as far as the duty cycle is concerned).

Example Video
http://www.youtube.com/watch?v=ioDDVLI9cpQ


Schematic
Notes about the schematic:
• R1 to R32 should be calculated based on your power supply voltage and the rating of the LEDs you're using.
• PORTD refers to Arduino digital pins 0 to 7.
• PORTB refers to Arduino digital pins 8 to 13.


Breadboard
Code
/*

3 LED Fader using Flip-Flops

by Sebastian Tomczak

27 January 2007

*/

int strobe_time = 5;

void setup() {

DDRD = B11111111;

DDRB = B00111111 | DDRB;

PORTB = B00000000;

}

void write374(byte clkPin, byte data) {

PORTD = data;

digitalWrite(clkPin, HIGH);

delayMicroseconds(strobe_time);

digitalWrite(clkPin, LOW);

delayMicroseconds(strobe_time);

}

void loop() {

for(int loop1 = 0; loop1 < 4; loop1++) {

randomSeed(analogRead(0));

int a = random(255);

randomSeed(analogRead(1));

int b = random(255);

randomSeed(analogRead(2));

int c = random(255);

randomSeed(analogRead(3));

int d = random(255);

for(int k = 0; k < 256; k++) {

for(int i = 0; i < 3; i ++) {

delayMicroseconds(k * 10);

write374(8, a);

write374(9, b);

write374(10, c);

write374(11, d);

delayMicroseconds(2551 - (k * 10) + 1);

write374(8, 0);

write374(9, 0);

write374(10, 0);

write374(11, 0);

}

}

}

for(int j = 0; j < 256; j++) {

int a = 255 | j;

int b = j;

int c = 255 & j;

int d = 255 - j;

for(int k = 256; k > 0; k--) {

for(int i = 0; i < 3; i ++) {

delayMicroseconds(k * 10);

write374(8, a);

write374(9, 0);

write374(10, c);

write374(11, 0);

delayMicroseconds(2551 - (k * 10));

write374(8, 0);

write374(9, b);

write374(10, 0);

write374(11, d);

}

}

}

}