Michael Noland's Blog

If you’re using LPD8806 LED strips and you can’t use the hardware SPI port (e.g., when using an Ethernet board), there are two other options in the Adafruit library: the default mode and ‘slowmo’ mode. The default mode is decent, but the flexibility of being able to choose the pins at runtime comes with a cost.

However, you can still get a decent speedup by defining your pin usage at compile time in a replacement show() function. I measured the time required to update an 86 LED strip using each method on an EtherTen board (Atmega328 @ 16 MHz, same as the Uno):

30.23 ms - Adafruit 'slowmo' method (digitalWrite)
7.76 ms - Adafruit default method (port pointers)
1.54 ms - Compile-time method
1.43 ms - Adafruit hardware SPI method

The timing was done using micros() around the show() call with strip.pause = 0.

I’ve tried to make this method as minimally hardcoded as possible. To use, throw the code from CompileTimeLEDs.h into the LPD8806 class in LPD8806.h and replace:

int ClockPin = 3;
int DataPin = 2;
...
strip.show();

with:

const int ClockPin = 3;
const int DataPin = 2;
...
strip.showCompileTime<ClockPin, DataPin>();

This overload is only available on ATmega168 or ATmega328 boards; on the Arduino Mega or other random boards, you need to specify the port register and use pin offsets within the port instead of the Arduino board pin number (e.g., showCompileTime<0..7, 0..7>(PORTD, PORTD))

CompileTimeLEDs.h (Download)

template<unsigned int ClockPin>
void PulseClockLine(volatile uint8_t& ClockRegister)
{
  const byte LED_CLOCK_MASK = 1 << ClockPin;
  ClockRegister |= LED_CLOCK_MASK;
  ClockRegister &= ~LED_CLOCK_MASK;
}
 
template<unsigned int ClockPin, unsigned int DataPin>
void TransmitBit(byte& CurrentByte, volatile uint8_t& ClockRegister, volatile uint8_t& DataRegister)
{
  // Set the data bit
  const byte LED_DATA_MASK = 1 << DataPin;
  if (CurrentByte & 0x80)
  {
    DataRegister |= LED_DATA_MASK;
  }
  else
  {
    DataRegister &= ~LED_DATA_MASK;
  }
 
  // Pulse the clock line
  PulseClockLine<ClockPin>(ClockRegister);
 
  // Advance to the next bit to transmit
  CurrentByte = CurrentByte << 1;
}
 
#if defined(__AVR_ATmega328P__) || defined(__AVR_ATmega168__)
  #define MAP_ARDUINO_PIN_TO_PORT_PIN(ArduinoPin) \
    ( ArduinoPin & 7 )
 
  #define MAP_ARDUINO_PIN_TO_PORT_REG(ArduinoPin) \
    ( (ArduinoPin >= 16) ? PORTC : (((ArduinoPin) >= 8) ? PORTB : PORTD) )
 
  // Specify Arduino pin numbers
  template<unsigned int ClockPin, unsigned int DataPin>
  void showCompileTime()
  {
    showCompileTime<MAP_ARDUINO_PIN_TO_PORT_PIN(ClockPin), MAP_ARDUINO_PIN_TO_PORT_PIN(DataPin)>(
      MAP_ARDUINO_PIN_TO_PORT_REG(ClockPin),
      MAP_ARDUINO_PIN_TO_PORT_REG(DataPin));
  }
 
  #undef MAP_ARDUINO_PIN_TO_PORT_PIN
  #undef MAP_ARDUINO_PIN_TO_PORT_REG
#else
  // Sorry: Didn't write an equivalent for other boards; use the other
  // overload and explicitly specify ports and offsets within those ports
#endif
 
// Note: Pin template params need to be relative to their port (0..7), not Arduino pinout numbers
template<unsigned int ClockPin, unsigned int DataPin>
void showCompileTime(volatile uint8_t& ClockRegister, volatile uint8_t& DataRegister)
{
  // Clock out the color for each LED
  byte* DataPtr = pixels;
  byte* EndDataPtr = pixels + (numLEDs * 3);
 
  do
  {
    byte CurrentByte = *DataPtr++;
 
    TransmitBit<ClockPin, DataPin>(CurrentByte, ClockRegister, DataRegister);
    TransmitBit<ClockPin, DataPin>(CurrentByte, ClockRegister, DataRegister);
    TransmitBit<ClockPin, DataPin>(CurrentByte, ClockRegister, DataRegister);
    TransmitBit<ClockPin, DataPin>(CurrentByte, ClockRegister, DataRegister);
 
    TransmitBit<ClockPin, DataPin>(CurrentByte, ClockRegister, DataRegister);
    TransmitBit<ClockPin, DataPin>(CurrentByte, ClockRegister, DataRegister);
    TransmitBit<ClockPin, DataPin>(CurrentByte, ClockRegister, DataRegister);
    TransmitBit<ClockPin, DataPin>(CurrentByte, ClockRegister, DataRegister);
  }
  while (DataPtr != EndDataPtr);
 
  // Clear the data line while we clock out the latching pattern
  const byte LED_DATA_MASK = 1 << DataPin;
  DataRegister &= ~LED_DATA_MASK;
 
  // All of the original data had the high bit set in each byte.  To latch
  // the color in, we need to clock out another LED worth of 0's for every
  // 64 LEDs in the strip apparently.
  byte RemainingLatchBytes = ((numLEDs + 63) / 64) * 3;
  do 
  {
    PulseClockLine<ClockPin>(ClockRegister);
    PulseClockLine<ClockPin>(ClockRegister);
    PulseClockLine<ClockPin>(ClockRegister);
    PulseClockLine<ClockPin>(ClockRegister);
 
    PulseClockLine<ClockPin>(ClockRegister);
    PulseClockLine<ClockPin>(ClockRegister);
    PulseClockLine<ClockPin>(ClockRegister);
    PulseClockLine<ClockPin>(ClockRegister);
  } while (--RemainingLatchBytes);
 
  // Need a bit of a delay before clocking again, but ideally this
  // is set to 0 and meaningful work is done instead
  if (pause)
  {
    delay(pause);
  }
}

License: CC0 (Placed into the public domain).

The avr-gcc compiler seems to do fine at recognizing the compile time constants and doing the right thing in codegen, as this method is just as fast as my original macrotastic implementation. You can probably eke out some more performance (would be nice to beat the hardware SPI implementation ^_^), but this was enough to get the headroom I needed.