If an external crystal (Primary Oscillator) is providing a clock source for your device, you will need to wait for the oscillator and PLL to stabilize before executing any code after a Power-on Reset (POR) or exit from sleep (see Oscillator Start-up Timer and System PLL Lock Status for more information). If you want to start executing code before the oscillator has stabilized, you can use the "Two-Speed Start-up" option.
Two-Speed Start-up uses the internal Fast RC Oscillator (FRC) as the system clock source until the Primary Oscillator (POSC) has stabilized. This allows the CPU to begin running code while the oscillator is stabilizing. After it has stabilized, the clock source will automatically be switched to use the Primary Oscillator (POSC).
If you want to determine the clock source currently in use, the MPLAB® Harmony function shown in the example below can be used. It will return the current oscillator (COSC) setting found in the OSCCON register.
// Enable Two-Speed Start-up (Internal/External switch over) #pragma config IESO = ON ... // variable to hold the state of the current clock source OSC_SYS_TYPE oscCurrent; // assign the current clock source to oscCurrent oscCurrent = PLIB_OSC_CurrentSysClockGet(OSC_ID_0); // 0 = FRC // 2 = POSC // 3 = POSC with PLL
The Watchdog Timer (WDT), if enabled, will continue to count at the same rate regardless of the SYSCLK frequency. Care must be taken to service the WDT during Two-Speed Start-up, taking into account the change in SYSCLK.
For more detail on the Two-Speed Start-up feature, please view the Oscillator family reference manual chapter for that device, for example:
- Section 6. Oscillators (PIC32MX795F512L)
- Section 42. Oscillators with Enhanced PLL (PIC32MZ2048EFG100)
The device data sheet should then be consulted to verify the specific features implemented in that device.