Electronic Art


I had been wanting to do this for such a long time…

… do something more artistic and less technical, something just for the sake of it looking good.

So when I realised that my 7 years old Peggy 3 LED board and an old IKEA picture frame were wasting space in my loft, I decided to combine them into something more artistic.


On the wall

I knew from the beginning that I wanted to have the installation be wireless for interactive updates, but I was initially thinking more along the lines of a simple serial to Bluetooth adaptor, then an ESP8266 and then realised that I also had an older Raspberry Pi lying around… and it fits so nicely on the Peggy 2 board, one would think it was custom made !

The only way to connect the 2 was using I2C. SPI would have been much better as higher speeds, but the SPI port on the Peggy’s Atmega 328 is already used for driving the LEDs.



For this configuration I simply used Jay Clegg’s really nice post, which not only describes in detail how it works and the necessary optimisations, but also offers the Atmega328 code, which I paste below just as a reference (I used it without any modification):

* Copyright 2008 Jay Clegg.  All rights reserved.
*    This program is free software: you can redistribute it and/or modify
*    it under the terms of the GNU General Public License as published by
*    the Free Software Foundation, either version 3 of the License, or
*    (at your option) any later version.
*    This program is distributed in the hope that it will be useful,
*    but WITHOUT ANY WARRANTY; without even the implied warranty of
*    GNU General Public License for more details.
*    You should have received a copy of the GNU General Public License
*    along with this program.  If not, see <http://www.gnu.org/licenses/>.

	Peggy2-i2c interface, Copyright 2008 by Jay Clegg.  All rights reserved.

	This code is designed for an unmodified Peggy 2.0 board sold by evilmadscience.com

	The code configures the Peggy as an TWI (I2C) slave.

	Companion code for an Arduino allows it to act as an TWI master, so that it can transmit
	frames to the peggy.  

	Please see http://www.planetclegg.com/projects/Twi2Peggy.html for explanation of how all
	this is supposed to work.

	Credits goes to:
		Windell H Oskay, (http://www.evilmadscientist.com/)
			for creating the Peggy 2.0 kit, and getting 16 shades of gray working
		Geoff Harrison (http://www.solivant.com/peggy2/),
			for proving that interrupt driven display on the Peggy 2.0 was viable.

#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/sleep.h>
#include <avr/pgmspace.h>
#include <stdio.h>

// FPS must be high enough to not have obvious flicker, low enough that main loop has
// time to process one byte per pass.
// ~140 seems to be about the absolute max for me (with this code on avr-gcc 4.2, -Os),
// but compiler differences might make this maximum value larger or smaller.
// if the value is too high errors start to occur or it will stop receiving altogether
// conversely, any lower than 60 and flicker becomes apparent.
// note: further code optimization might allow this number to
// be a bit higher, but only up to a point...
// it *must* result in a value for OCR0A in the range of 1-255

//#define FPS 144
#define FPS 100

// 25 rows * 13 bytes per row == 325
#define DISP_BUFFER_SIZE 325

#define TWI_SLAVE_ID 34

uint8_t frameBuffer[DISP_BUFFER_SIZE];

uint8_t *currentRowPtr = frameBuffer;
uint8_t currentRow=0;
uint8_t currentBrightness=0;

// Note: the refresh code has been optimized heavily from the previous version.
	// there are 15 passes through this interrupt for each row per frame.
	// ( 15 * 25) = 375 times per frame.
	// during those 15 passes, a led can be on or off.
	// if it is off the entire time, the perceived brightness is 0/15
	// if it is on the entire time, the perceived brightness is 15/15
	// giving a total of 16 average brightness levels from fully on to fully off.
	// currentBrightness is a comparison variable, used to determine if a certain
	// pixel is on or off during one of those 15 cycles.   currentBrightnessShifted
	// is the same value left shifted 4 bits:  This is just an optimization for
	// comparing the high-order bytes.
	if (++currentBrightness >= MAX_BRIGHTNESS)
		if (++currentRow > 24)
			currentRow =0;
			currentRowPtr = frameBuffer;
			currentRowPtr += 13;

	////////////////////  Parse a row of data and write out the bits via spi
	uint8_t currentBrightnessShifted = currentBrightness <<4; 	 	uint8_t *ptr = currentRowPtr + 12;  // its more convenient to work from right to left 	uint8_t p, bits=0; 	  	// optimization: by using variables for these two masking constants, we can trick gcc into not  	// promoting to 16-bit int (constants are 16 bit by default, causing the  	// comparisons to get promoted to 16bit otherwise)].  This turns out to be a pretty 	// substantial optimization for this handler 	uint8_t himask = 0xf0;   	uint8_t lomask = 0x0f; 	 	// Opimization: interleave waiting for SPI with other code, so the CPU can do something useful 	// when waiting for each SPI transmission to complete 	 	p = *ptr--; 	if ((p & lomask) > currentBrightness)  			bits|=1;
	SPDR = bits;

	p = *ptr--;
	if ((p & lomask) > currentBrightness)  			bits|=64;
	if ((p & himask) > currentBrightnessShifted)	bits|=128;
	p = *ptr--;
	if ((p & lomask) > currentBrightness)  			bits|=16;
	if ((p & himask) > currentBrightnessShifted)	bits|=32;
	p = *ptr--;
	if ((p & lomask) > currentBrightness)  			bits|=4;
	if ((p & himask) > currentBrightnessShifted)	bits|=8;
	p = *ptr--;
	if ((p & lomask) > currentBrightness)  			bits|=1;
	if ((p & himask) > currentBrightnessShifted)	bits|=2;

	while (!(SPSR & (1<<SPIF)))  { } // wait for prior bitshift to complete 	SPDR = bits; 	 	 	bits=0; 	p = *ptr--; 	if ((p & lomask) > currentBrightness)  			bits|=64;
	if ((p & himask) > currentBrightnessShifted)	bits|=128;
	p = *ptr--;
	if ((p & lomask) > currentBrightness)  			bits|=16;
	if ((p & himask) > currentBrightnessShifted)	bits|=32;
	p = *ptr--;
	if ((p & lomask) > currentBrightness)  			bits|=4;
	if ((p & himask) > currentBrightnessShifted)	bits|=8;
	p = *ptr--;
	if ((p & lomask) > currentBrightness)  			bits|=1;
	if ((p & himask) > currentBrightnessShifted)	bits|=2;

	while (!(SPSR & (1<<SPIF)))  { } // wait for prior bitshift to complete 	SPDR = bits; 	 	 	bits=0; 	p = *ptr--; 	if ((p & lomask) > currentBrightness)  			bits|=64;
	if ((p & himask) > currentBrightnessShifted)	bits|=128;
	p = *ptr--;
	if ((p & lomask) > currentBrightness)  			bits|=16;
	if ((p & himask) > currentBrightnessShifted)	bits|=32;
	p = *ptr--;
	if ((p & lomask) > currentBrightness)  			bits|=4;
	if ((p & himask) > currentBrightnessShifted)	bits|=8;
	p = *ptr--;
	if ((p & lomask) > currentBrightness)  			bits|=1;
	if ((p & himask) > currentBrightnessShifted)	bits|=2;

	while (!(SPSR & (1<<SPIF)))  { }// wait for prior bitshift to complete
	SPDR = bits;

	////////////////////  Now set the row and latch the bits

	uint8_t portD;

	if (currentRow < 15)
		portD = currentRow+1;
		portD = (currentRow -14)<<4;

	while (!(SPSR & (1<<SPIF)))  { } // wait for last bitshift to complete

	//if (currentBrightness == 0)
	PORTD = 0;				// set all rows to off
	PORTB |= (1<<PB1); //  latch it, values now set
	//if (currentBrightness == 0)
	PORTD = portD;     // set row
	PORTB &= ~((1<<PB1)); // reset latch for next time

	// notes to self, calculations from the oscope:
	// need about minimum of 6us total to clock out all 4 bytes
	// roughly 1.5ms per byte, although some of that is
	// idle time taken between bytes.  6=7us therefore is our
	// absolute minimum time needed to refresh a row, not counting calculation time.
	// Thats just if we do nothing else when writing out SPI and toggle to another row.
	//Measured values from this routine
	// @ 144 fps the latch is toggled every 19us with an actual 4byte clock out time of 12-13us
	// @ 70 fps the latch is toggle every 39us, with a clock out time of 13-14us
	// times do not count setup/teardown of stack frame

	// one byte @ 115k takes 86us (max) 78us (min) , measured time
	// one byte @ 230k takes 43us (max) 39us (min) , measured time
	// so 230k serial might barely be possible, but not with a 16mhz crystal (error rate to high)
	// 250k might just barely be possible

void displayInit(void)
	// need to set output for SPI clock, MOSI, SS and latch.  Eventhough SS is not connected,
	// it must apparently be set as output for hardware SPI to work.
	DDRB =  (1<<DDB5) | (1<<DDB3) | (1<<DDB2) | (1<<DDB1);
	// set all portd pins as output
	DDRD = 0xff; 

	PORTD=0; // select no row

	// enable hardware SPI, set as master and clock rate of fck/2
	SPCR = (1<<SPE) | (1<<MSTR);
	SPSR = (1<<SPI2X); 

	// setup the interrupt.
	TCCR0A = (1<<WGM01); // clear timer on compare match
	TCCR0B = (1<<CS01); // timer uses main system clock with 1/8 prescale 	OCR0A  = (F_CPU >> 3) / 25 / 15 / FPS; // Frames per second * 15 passes for brightness * 25 rows
	TIMSK0 = (1<<OCIE0A);	// call interrupt on output compare match

	for (uint8_t i=0; i < 4; i++)
		SPDR = 0;
		while (!bit_is_set(SPSR, SPIF)) {}

// I2C  routines

// TWI Slave Receiver staus codes, from Atmel notes
#define TWI_SRX_ADR_ACK            0x60
#define TWI_SRX_GEN_ACK            0x70
#define TWI_SRX_ADR_DATA_ACK       0x80
#define TWI_SRX_ADR_DATA_NACK      0x88
#define TWI_SRX_GEN_DATA_ACK       0x90
#define TWI_SRX_GEN_DATA_NACK      0x98
#define TWI_SRX_STOP_RESTART       0xA0
#define TWI_NO_STATE               0xF8
#define TWI_BUS_ERROR              0x00 

void initTwiSlave(uint8_t addr)

    PORTC |=  (1<<PC5) | (1<<PC4); // enable pullups

    TWAR = (0<<TWGCE) |((uint8_t) (0xff & (addr<<1)));    // set slave address, no general call address
    TWDR = 0xff; // Default content = SDA released

    TWCR = (1<<TWINT) |   // "clear the flag"  (hate this backward terminology)
          (1<<TWEA) |     // send acks to master when getting address or data
          (0<<TWSTA) |    // not a master, cant do start
          (0<<TWSTO) |    // doc says set these to 0
          (0<<TWWC) |
          (1<<TWEN) |   // hardware TWI  enabled
          (0<<TWIE);   // do NOT generate interrupts.

    while (TWCR & (1<<TWIE)) { }


uint8_t getTwiByte(void)
	uint8_t result=0;


 	// wait for an state change
  	while (!(TWCR & (1<<TWINT))) { } // wait for TWINT to be set

  	//uint8_t sr = TWSR;
  	switch (TWSR)
	    case TWI_SRX_ADR_DATA_ACK:  // received a byte of data data
			result = TWDR;
	      	TWCR = (1<<TWEN)|(1<<TWINT)|(1<<TWEA);

//        case TWI_SRX_GEN_ACK_M_ARB_LOST:
//        case TWI_SRX_ADR_ACK_M_ARB_LOST:
    	case TWI_SRX_GEN_ACK:
    	case TWI_SRX_ADR_ACK:      // receive our address byte
		      TWCR = (1<<TWEN)|(1<<TWINT)|(1<<TWEA);
		      goto keepListening;

	    case TWI_SRX_STOP_RESTART:       // A STOP or repeated START condition was received
	    	TWCR = (1<<TWEN)|(1<<TWINT)|(1<<TWEA);
		    goto keepListening;

	    case TWI_SRX_ADR_DATA_NACK:   // data received, returned nack
	     	//result = TWDR;
	      	TWCR = (1<<TWEN)|(1<<TWINT); //|(1<<TWEA);
	      	goto keepListening;

    	case TWI_NO_STATE:
    		goto keepListening;

//	    case TWI_BUS_ERROR:
	    default:     			 // something bad happened. assuming a bus error, we try to recover from this
	      //state = TWSR;
	      //TWCR = (1<<TWEN)|(0<<TWINT)|(0<<TWEA);    // Don't ack any further requests, will stop receiving
	      // alternate handling: reset state and continue
	      TWCR = (1<<TWEN)|(1<<TWINT)|(1<<TWEA)|(1<<TWSTO);    // ignore
	      // wait for stop condition to be exectued; TWINT will not be set after a stop
		  while(TWCR & (1<<TWSTO)){ }
		  result = 0xff;
	return result;

// MAIN LOOP: handle the input data stream  and stuff bytes into the framebuffer

void serviceInputData(void)
	uint8_t *ptr = frameBuffer;
	uint8_t state = 0;
	int counter = 0;
	while (1)
		uint8_t c = getTwiByte();

		// very simple state machine to look for 6 byte start of frame
		// marker and copy bytes that follow into buffer
		if (state  <6) 		{ 			// must get a 0xdeadbeef to start frame. 			// I look for two more bytes after that, but 			// they are reserved for future use.  			// so send a 1 followed by a 0 for now. 			if (state == 0 && c == 0xde) state++; 			else if (state ==1 && c == 0xad) state++; 			else if (state ==2 && c == 0xbe) state++; 			else if (state ==3 && c == 0xef) state++; 			else if (state ==4 && c == 0x01) state++; 			else if (state ==5)  // dont care what 6th byte is  			{ 				state++; 				counter = 0; 				ptr = frameBuffer; 			} 			else state = 0; // error: reset to look for start of frame 		} 		else  		{ 			// inside of a frame, so save each byte to buffer 			*ptr++ = c; 			counter++; 			if (counter >= DISP_BUFFER_SIZE)
				// buffer filled, so reset everything to wait for next frame
				//counter = 0;
				//ptr = frameBuffer;
				state = 0;

int main(void)
	// Enable pullups for buttons/i2c
	PORTB |= (1<<PB0);
	PORTC = (1<<PC5) | (1<<PC4) | (1<<PC3) | (1<<PC2) | (1<<PC1) | (1<<PC0);

	UCSR0B =0; // turn OFF serial RX/TX, necessary if using arduino bootloader 



 	sei( );

	// clear display and set to test pattern
	// pattern should look just like the "gray test pattern" from EMS

	uint8_t v = 0;
	for (int i =0; i < DISP_BUFFER_SIZE; i++)
		v = (v+2) % 16;
	    // set to 0 for blank startup display
		// low order bits on the left, high order bits on the right
			frameBuffer[i]= v + ((v+1)<<4);
		//	frameBuffer[i]=0;

	serviceInputData();  // never returns


However, where Jay had to use a laptop and an Arduino to do the serial to I2C conversion, I have the luxury of a Raspberry Pi which can do both these things !

Below you can see a pretty basic Python script that can display either a still image, a pre-recorded movie or just stream what the camera sees, using :

  • “smbus” for I2C communication (needs to be enabled in raspi-config, and then configured to use a speed <50000 (edit “/boot/config.txt” and add/update “dtparam=i2c_baudrate=45000”)
  • OpenCV to do some basic image manipulation (mainly scaling and grayscale transform)
  • PiCamera to read from Raspberry Pi’s camera


import sys
import cv2
import smbus
import picamera
import numpy as np
import io

# standard header is 0xdeadbeef (sent in big endian order),
# followed by a 1 and a 0 (currently these are reserved values)
HEADER = [0xde, 0xad, 0xbe, 0xef, 1, 0]
# the RaspberryPi B V2 has the bus #1 exposed on the pins 3/SDA and 5/SCL
_bus = smbus.SMBus(1)

def writeBlockIgnoreError(data):
    error = True
            _bus.write_i2c_block_data(PEGGY_ADDR, data[0], data[1:])
            error = False
        except IOError as err:
            print err

# write an array of packed bytes to the I2C port, pre-pending the header bytes first
def writeFrame(frameArray):
    # write the magic header

    # write the actual data
    # Can't send more than 32 bytes at once: "OverflowError: Third argument must be a list of at least one, but not more than 32 integers"
    for i in xrange(0, len(frameArray), 33):
        end = min(i + 33, len(frameArray))

def displayImage(img):
    # convert to grayscale if it's colour
    if(len(img.shape) > 2):
        img = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
    height, width = img.shape[:2]
    if(height != PEGGY_SIZE or width != PEGGY_SIZE):
        img = cv2.resize(img, (PEGGY_SIZE, PEGGY_SIZE), interpolation = cv2.INTER_CUBIC)

    peggyFrame = []
    for row in xrange(PEGGY_SIZE):
        # we combine the brightness of 2 consecutive pixels in the same byte
        val = 0

        for col in xrange(PEGGY_SIZE):
            # the Peggy only uses 16 levels of gray, not 256
            brightness = img[row, col] >> 4

            if (col % 2 == 0):
                val = brightness
                val |= brightness << 4

        # add the last half-byte. Because we know PEGGY_SIZE is odd

    # not send the frame to the Peggy

#### MAIN ###

fileName = sys.argv[1]

if(fileName == "camera"):
    stream = io.BytesIO() #create a new in memory stream
    with picamera.PiCamera() as camera:
        camera.resolution = (25, 25)
        camera.framerate = 10
        for unused in camera.capture_continuous(stream, "png", use_video_port=True):
            # nmpy array from the stream
            data = np.fromstring(stream.getvalue(), dtype=np.uint8)
            # decode the image from array as gray scale
            img = cv2.imdecode(data, cv2.CV_LOAD_IMAGE_GRAYSCALE)
    cap = cv2.VideoCapture(fileName)
    ret = True
    while(cap.isOpened() and ret):
        ret, frame = cap.read()
    image = cv2.imread(fileName, cv2.CV_LOAD_IMAGE_GRAYSCALE)

Final result


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