Adafruit_NeoPixel_External_Timer.ino

#include <Adafruit_NeoPixel.h>
// Which pin on the Arduino is connected to the NeoPixels?
#define PIN            3
// How many NeoPixels are attached to the Arduino?
#define NUMPIXELS      128
Adafruit_NeoPixel strip = Adafruit_NeoPixel(NUMPIXELS , PIN, NEO_GRB + NEO_KHZ800);;


void setup(void) 
{
  Serial.begin(115200);
  strip.begin();
  strip.show(); // Initialize all pixels to 'off'
  delay(2000);
}
 
byte Start = 0; // start byte is dec 0
byte Stop = 1; // stop byte is dec 1
int maxLoops = 100;
int postDelay = 100; //delay after the Stop byte gets sent in order to give Processing time to be ready for next Start signal
void loop(void) 
{
  Serial.write(Start);
  for(int loopCount = 0; loopCount < 100; loopCount++){
    for(int i = 0; i < NUMPIXELS; i++){
       strip.setPixelColor(i, color(map(i,0,NUMPIXELS,1,384),1));
    }
    strip.show();
  }
  Serial.write(Stop);
  delay(postDelay);
}


// Color 1 from 384; brightness 0.0 to 1.0.
uint32_t color(uint16_t color, float brightness)  {
  byte r, g, b;
  int range = color / 128;
  switch (range) {
    case 0: // Red to Yellow (1 to 128)
      r = 127 - color % 128;
      g = color % 128;
      b = 0;
      break;
    case 1: // Yellow to Teal (129 to 256)
      r = 0;
      g = 127 - color % 128;
      b = color % 128;
      break;
    case 2: // Teal to Purple (257 to 384)
      r = color % 128;
      g = 0;
      b = 127 - color % 128;
      break;
  }
  r *= brightness;
  g *= brightness;
  b *= brightness;
  return strip.Color(r, g, b);
}

FastLED_External_Timer.ino

#include <FastLED.h>

#define LED_PIN     3
#define NUM_LEDS    128
#define BRIGHTNESS  255
#define LED_TYPE    WS2811
#define COLOR_ORDER GRB
CRGB leds[NUM_LEDS];

#define UPDATES_PER_SECOND 100

#define WAIT_FOR_KEYBOARD 0

CRGBPalette16 currentPalette;
TBlendType    currentBlending;

extern CRGBPalette16 myRedWhiteBluePalette;
extern const TProgmemPalette16 myRedWhiteBluePalette_p PROGMEM;


void setup() {
  Serial.begin(115200);
  if (WAIT_FOR_KEYBOARD) {

    // Wait for serial to initalize.
    while (!Serial) { }

  	Serial.println("Strike any key to start...");

  	// Wait for the next keystroke.
  	while (!Serial.available()) { }

  	// Clear the serial buffer.
    Serial.read();
  }
  FastLED.addLeds<LED_TYPE, LED_PIN, COLOR_ORDER>(leds, NUM_LEDS).setCorrection( TypicalLEDStrip );
  FastLED.setBrightness(  BRIGHTNESS );
  
  currentPalette = RainbowColors_p;
  currentBlending = BLEND;
}

static uint8_t startIndex = 0;
byte Start = 0; // start byte is dec 0
byte Stop = 1; // stop byte is dec 1
int maxLoops = 100;
int postDelay = 100; //delay after the Stop byte gets sent in order to give Processing time to be ready for next Start signal


void loop()
{
  Serial.write(Start);
  for(int loopCount = 0; loopCount < 100; loopCount++){
    FillLEDsFromPaletteColors(startIndex);
    FastLED.show();
  }
  Serial.write(Stop);
  delay(postDelay);
}

void FillLEDsFromPaletteColors( uint8_t colorIndex)
{
  uint8_t brightness = 255;
  
  for( int i = 0; i < NUM_LEDS; i++) {
    colorIndex = map(i,0,NUM_LEDS,0,255);
    leds[i] = ColorFromPalette( currentPalette, colorIndex, brightness, currentBlending);
  }
}