The delay effect is limited by the amount of SRAM memory. With 8KB or RAM (in Arduino MEGA/ADK) we can store up to 4096 samples of 16 bits each.
With the default parameters, the PWM signal is generated at 33KHz, that means that every sample is generated every 30us (1/33KHz) so with 2KB of ram we have a delay of:
4096 x 30us = 120ms
120ms is not a massive delay, more like a "doubler" effect, but there are some tricks to make this delay longer (store 8 bits instead of 16)
This is the complete delay code:
// CC-by-NC-www.Electrosmash.com/rights
// Based on OpenMusicLabs previous works.
// pedalshield_mega_delay.ino reads the ADC, saves it into the DelayBuffer[] array and plays it into the PWM output.
// pressing the pushbutton_1 or 2 makes the delay longer or shorter
#include "U8glib.h"
U8GLIB_SH1106_128X64 u8g(U8G_I2C_OPT_NO_ACK); // Display which does not send ACK
//defining harware resources.
#define LED 13
#define FOOTSWITCH 12
#define TOGGLE 2
#define PUSHBUTTON_1 A5
#define PUSHBUTTON_2 A4
//defining the output PWM parameters
#define PWM_FREQ 0x00FF // pwm frequency - 31.3KHz
#define PWM_MODE 0 // Fast (1) or Phase Correct (0)
#define PWM_QTY 2 // 2 PWMs in parallel
//other variables
int input, vol_variable=512;
int counter=0;
unsigned int ADC_low, ADC_high;
#define MAX_DELAY 3500
unsigned int DelayBuffer[MAX_DELAY];
unsigned int DelayCounter = 0;
unsigned int Delay_Depth = MAX_DELAY;
void setup() {
//setup IO
pinMode(FOOTSWITCH, INPUT_PULLUP);
pinMode(TOGGLE, INPUT_PULLUP);
pinMode(PUSHBUTTON_1, INPUT_PULLUP);
pinMode(PUSHBUTTON_2, INPUT_PULLUP);
pinMode(LED, OUTPUT);
pinMode(6, OUTPUT); //PWM0 as output
pinMode(7, OUTPUT); //PWM1 as output
// setup ADC
ADMUX = 0x60; // left adjust, adc0, internal vcc
ADCSRA = 0xe5; // turn on adc, ck/32, auto trigger
ADCSRB = 0x00; // free running mode
DIDR0 = 0x01; // turn off digital inputs for adc0
// setup PWM
TCCR4A = (((PWM_QTY - 1) << 5) | 0x80 | (PWM_MODE << 1)); //
TCCR4B = ((PWM_MODE << 3) | 0x11); // ck/1
TIMSK4 = 0x20; // interrupt on capture interrupt
ICR4H = (PWM_FREQ >> 8);
ICR4L = (PWM_FREQ & 0xff);
DDRB |= ((PWM_QTY << 1) | 0x02); // turn on outputs
sei(); // turn on interrupts - not really necessary with arduino
}
void loop()
{
//Turn on the LED and write the OLED if the effect is ON.
if (digitalRead(FOOTSWITCH))
{
digitalWrite(LED, HIGH); //light the LED
u8g.firstPage();
do {
u8g.setFont(u8g_font_helvR14r);
u8g.drawStr( 0, 16, " DELAY ");
u8g.setPrintPos(10, 50);
u8g.setFont(u8g_font_helvR24r);
u8g.print(Delay_Depth);
} while( u8g.nextPage() );
}
else
{
digitalWrite(LED, LOW); // switch-off the LED
u8g.firstPage();
do {
u8g.setFont(u8g_font_helvR24r);
u8g.drawStr( 0, 32, "EFFECT ");
u8g.drawStr( 0, 64, " OFF ");
} while( u8g.nextPage() );
}
}
ISR(TIMER4_CAPT_vect)
{
// get ADC data
ADC_low = ADCL; // ADC_low always 0 to save space
ADC_high = ADCH;
//store the high bit only for
DelayBuffer[DelayCounter] = (((ADC_high << 8) | ADC_low) + 0x8000);
counter++; //to save resources, the pushbuttons are checked every 100 times.
if(counter==100)
{
counter=0;
if (!digitalRead(PUSHBUTTON_1)) {
if (Delay_Depth<MAX_DELAY)Delay_Depth=Delay_Depth+1; //increase the vol
digitalWrite(LED, LOW); //blinks the led
}
if (!digitalRead(PUSHBUTTON_2)) {
if (Delay_Depth>0)Delay_Depth=Delay_Depth-1; //decrease vol
digitalWrite(LED, LOW); //blinks the led
}
}
//Increse/reset delay counter.
DelayCounter++;
if(DelayCounter >= Delay_Depth) DelayCounter = 0;
//the output signal is = (delayed signal + current signal)/2
input = (DelayBuffer[DelayCounter] +(((ADC_high << 8) | ADC_low) + 0x8000))>>1;
//write the PWM signal
OCR4AL = ((input + 0x8000) >> 8); // convert to unsigned, send out high byte
OCR4BL = input; // send out low byte
}
