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// Arduino bridge code between host computer and LPD8806-based digital
// addressable RGB LEDs (e.g. Adafruit product ID #306). LED data is
// streamed, not buffered, making this suitable for larger installations
// (e.g. video wall, etc.) than could otherwise be contained within the
// Arduino's limited RAM. Intended for use with USB-native boards such
// as Teensy or Adafruit 32u4 Breakout; also works on normal serial
// Arduinos (Uno, etc.), but speed will be limited by the serial port.
// LED data and clock lines are connected to the Arduino's SPI output.
// On traditional Arduino boards (e.g. Uno), SPI data out is digital pin
// 11 and clock is digital pin 13. On both Teensy and the 32u4 Breakout,
// data out is pin B2, clock is B1. On Arduino Mega, 51=data, 52=clock.
// LEDs should be externally powered -- trying to run any more than just
// a few off the Arduino's 5V line is generally a Bad Idea. LED ground
// should also be connected to Arduino ground.
// Elsewhere, the WS2801 version of this code was specifically designed
// to avoid buffer underrun conditions...the WS2801 pixels automatically
// latch when the data stream stops for 500 microseconds or more, whether
// intentional or not. The LPD8806 pixels are fundamentally different --
// the latch condition is indicated within the data stream, not by pausing
// the clock -- and buffer underruns are therefore a non-issue. In theory
// it would seem this could allow the code to be much simpler and faster
// (there's no need to sync up with a start-of-frame header), but in
// practice the difference was not as pronounced as expected -- such code
// soon ran up against a USB throughput limit anyway. So, rather than
// break compatibility in the quest for speed that will never materialize,
// this code instead follows the same header format as the WS2801 version.
// This allows the same host-side code (e.g. Adalight, Adavision, etc.)
// to run with either type of LED pixels. Huzzah!
// --------------------------------------------------------------------
// This file is part of Adalight.
// Adalight is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as
// published by the Free Software Foundation, either version 3 of
// the License, or (at your option) any later version.
// Adalight is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// GNU Lesser General Public License for more details.
// You should have received a copy of the GNU Lesser General Public
// License along with Adalight. If not, see
// <>.
// --------------------------------------------------------------------
#include <SPI.h>
// A 'magic word' precedes each block of LED data; this assists the
// microcontroller in syncing up with the host-side software and latching
// frames at the correct time. You may see an initial glitchy frame or
// two until the two come into alignment. Immediately following the
// magic word are three bytes: a 16-bit count of the number of LEDs (high
// byte first) followed by a simple checksum value (high byte XOR low byte
// XOR 0x55). LED data follows, 3 bytes per LED, in order R, G, B, where
// 0 = off and 255 = max brightness. LPD8806 pixels only have 7-bit
// brightness control, so each value is divided by two; the 8-bit format
// is used to maintain compatibility with the protocol set forth by the
// WS2801 streaming code (those LEDs use 8-bit values).
static const uint8_t magic[] = { 'A','d','a' };
#define MAGICSIZE sizeof(magic)
static uint8_t
buffer[HEADERSIZE], // Serial input buffer
bytesBuffered = 0; // Amount of data in buffer
// If no serial data is received for a while, the LEDs are shut off
// automatically. This avoids the annoying "stuck pixel" look when
// quitting LED display programs on the host computer.
static const unsigned long serialTimeout = 15000; // 15 seconds
static unsigned long lastByteTime, lastAckTime;
void setup() {
byte c;
int i, p;
Serial.begin(115200); // 32u4 will ignore BPS and run full speed
// SPI is run at 2 MHz. LPD8806 can run much faster,
// but unshielded wiring is susceptible to interference.
// Feel free to experiment with other divider ratios.
SPI.setClockDivider(SPI_CLOCK_DIV8); // 2 MHz
// Issue dummy byte to "prime" the SPI bus. This later simplifies
// the task of doing useful work during SPI transfers. Rather than
// the usual issue-and-wait-loop, code can instead wait-and-issue --
// with other operations occurring between transfers, the wait is
// then shortened or eliminated. The SPSR register is read-only,
// so this flag can't be forced -- SOMETHING must be issued.
SPDR = 0;
// Issue initial latch to LEDs. This flushes any undefined data that
// may exist on powerup, and prepares the LEDs to receive the first
// frame of data. Actual number of LEDs isn't known yet (this arrives
// later in frame header packets), so just latch a large number:
// Issue test pattern to LEDs on startup. This helps verify that
// wiring between the Arduino and LEDs is correct. Again not knowing
// the actual number of LEDs, this writes data for an arbitrarily
// large number (10K). If wiring is correct, LEDs will all light
// red, green, blue on startup, then off. Once you're confident
// everything is working end-to-end, it's OK to comment this out and
// re-upload the sketch to the Arduino.
const uint8_t testColor[] = { 0x80, 0x80, 0xff, 0x80, 0x80, 0x80 },
testOffset[] = { 1, 2, 0, 3 };
for(c=0; c<4; c++) { // for each test sequence color...
for(p=0; p<10000; p++) { // for each pixel...
for(i=0; i<3; i++) { // for each R,G,B...
while(!(SPSR & _BV(SPIF))); // Wait for prior byte out
SPDR = testColor[testOffset[c] + i]; // Issue next byte
if(c < 3) delay(250);
Serial.print("Ada\n"); // Send ACK string to host
lastByteTime = lastAckTime = millis(); // Initialize timers
// Program flow is simpler than the WS2801 code. No need for a state
// machine...instead, software just alternates between two conditions:
// a header-seeking mode (looking for the 'magic word' at the start
// of each frame of data), and a data-forwarding mode (moving bytes
// from serial input to SPI output). A proper data stream will
// consist only of alternating valid headers and valid data, so the
// loop() function is simply divided into these two parts, and repeats
// forever.
// LPD8806 pixels expect colors in G,R,B order vs. WS2801's R,G,B.
// This is used to shuffle things around later.
static const uint8_t byteOrder[] = { 2, 0, 1 };
void loop() {
uint8_t i, hi, lo, byteNum;
int c;
long nLEDs, remaining;
unsigned long t;
// HEADER-SEEKING BLOCK: locate 'magic word' at start of frame.
// If any data in serial buffer, shift it down to starting position.
for(i=0; i<bytesBuffered; i++)
buffer[i] = buffer[HEADERSIZE - bytesBuffered + i];
// Read bytes from serial input until there's a full header's worth.
while(bytesBuffered < HEADERSIZE) {
t = millis();
if((c = >= 0) { // Data received?
buffer[bytesBuffered++] = c; // Store in buffer
lastByteTime = lastAckTime = t; // Reset timeout counters
} else { // No data, check for timeout...
if(timeout(t, 10000) == true) return; // Start over
// Have a header's worth of data. Check for 'magic word' match.
for(i=0; i<MAGICSIZE; i++) {
if(buffer[i] != magic[i]) { // No match...
if(i == 0) bytesBuffered -= 1; // resume search at next char
else bytesBuffered -= i; // resume at non-matching char
// Magic word matches. Now how about the checksum?
hi = buffer[MAGICSIZE];
lo = buffer[MAGICSIZE + 1];
if(buffer[MAGICSIZE + 2] != (hi ^ lo ^ 0x55)) {
bytesBuffered -= MAGICSIZE; // No match, resume after magic word
// Checksum appears valid. Get 16-bit LED count, add 1 (nLEDs always > 0)
nLEDs = remaining = 256L * (long)hi + (long)lo + 1L;
bytesBuffered = 0; // Clear serial buffer
byteNum = 0;
// DATA-FORWARDING BLOCK: move bytes from serial input to SPI output.
// Unfortunately can't just forward bytes directly. The data order is
// different on LPD8806 (G,R,B), so bytes are buffered in groups of 3
// and issued in the revised order.
while(remaining > 0) { // While more LED data is expected...
t = millis();
if((c = >= 0) { // Successful read?
lastByteTime = lastAckTime = t; // Reset timeout counters
buffer[byteNum++] = c; // Store in data buffer
if(byteNum == 3) { // Have a full LED's worth?
while(byteNum > 0) { // Issue data in LPD8806 order...
i = 0x80 | (buffer[byteOrder[--byteNum]] >> 1);
while(!(SPSR & _BV(SPIF))); // Wait for prior byte out
SPDR = i; // Issue new byte
} else { // No data, check for timeout...
if(timeout(t, nLEDs) == true) return; // Start over
// Normal end of data. Issue latch, return to header-seeking mode.
static void latch(int n) { // Pass # of LEDs
n = ((n + 63) / 64) * 3; // Convert to latch length (bytes)
while(n--) { // For each latch byte...
while(!(SPSR & _BV(SPIF))); // Wait for prior byte out
SPDR = 0; // Issue next byte
// Function is called when no pending serial data is available.
static boolean timeout(
unsigned long t, // Current time, milliseconds
int nLEDs) { // Number of LEDs
// If condition persists, send an ACK packet to host once every
// second to alert it to our presence.
if((t - lastAckTime) > 1000) {
Serial.print("Ada\n"); // Send ACK string to host
lastAckTime = t; // Reset counter
// If no data received for an extended time, turn off all LEDs.
if((t - lastByteTime) > serialTimeout) {
long bytes = nLEDs * 3L;
latch(nLEDs); // Latch any partial/incomplete data in strand
while(bytes--) { // Issue all new data to turn off strand
while(!(SPSR & _BV(SPIF))); // Wait for prior byte out
SPDR = 0x80; // Issue next byte (0x80 = LED off)
latch(nLEDs); // Latch 'all off' data
lastByteTime = t; // Reset counter
bytesBuffered = 0; // Clear serial buffer
return true;
return false; // No timeout
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