Oscillator Start-Up Timer (OST)

The Oscillator Start-up Timer (OST) ensures that the oscillator circuit has started and is providing a stable system clock to the oscillator module. Quartz crystals or ceramic resonators do not start immediately, and may take a few hundred cycles before the oscillator becomes stable. The oscillations must build up until sufficient amplitude is generated to properly toggle between logic states. The OST counts 1024 oscillation periods from the OSC1 input following a Power-on Reset (POR), Brown-out Reset (BOR), or wake-up from Sleep event to ensure that the oscillator has enough time to reach stable and accurate operation. Once the OST has completed its count, module hardware sets the External Oscillator Ready (EXTOR) bit, indicating that the oscillator is stable and ready to use.