= updated clock prescaler setup to support the following external oscillator / cpu frequency combinations:
1. XTAL=16MHz F_CPU=16MHz
  ok for 5v, slightly "out of spec" for 3.3v (like the rx3s)
2. XTAL=16MHz F_CPU=8Hz
  ok for 3.3v, but running at half speed
3. XTAL=8MHz F_CPU=8MHz
  for devices with an 8MHz oscillator