[StevenB]

Here's some code from my optical mouse project. mDataPin and mClockPin are just integers that refer to the specific pins being manipulated.

/**
 * Writes an 8-bit value to one of the optical mouse's registers.
 *
 * \param address  The register address to write to.  Valid registers for the ADNS-2610 are 0x00
 * through 0x11.
 * \param value  The value to write.
 */
void Mouse::writeRegister(uint8 address, uint8 value)
{
  address = address | 0x80; // Most significant bit is always 1, since we're doing a write (This only applies to the mouse chip)
  pinMode(mDataPin, OUTPUT); // Only necessary if mDataPin is sometimes used as an input

  // We start by writing the most significant bit first
  for(int i = 0; i < 8; i++)
  {
    digitalWrite(mClockPin, 0); // Set the clock pin low
    digitalWrite(mDataPin, address & 0x80); // Set the data to the next bit
    digitalWrite(mClockPin, 1); // Set the clock pin high
    // Now the chip reads the data pin while we go around the loop
    address = address << 1; // Shift to the next bit
  }

  // Now write the data in exactly the same way
  for(int i = 0; i < 8; i++)
  {
    digitalWrite(mClockPin, 0);
    digitalWrite(mDataPin, value & 0x80);
    digitalWrite(mClockPin, 1);
    value = value << 1;
  }

}