12.4.3.1 Oscillator Start-up Time

As the device voltage increases from V_SS, the oscillator will start its oscillations. The time required for the oscillator to start oscillating depends on these factors:

• Crystal and resonator frequency
• Capacitor values used (C1 and C2 in Figure 12-9)
• Device V_DD rise time
• System temperature
• Series resistor value and type if used
• Oscillator mode selection of device (selects the gain of the internal oscillator inverter)
• Crystal quality
• Oscillator circuit layout
• System noise

Figure 12-10 illustrates a plot of a typical oscillator and resonator start-up.

To ensure that a crystal oscillator (or ceramic resonator) has started and stabilized, an Oscillator Start-up Timer (OST) is provided with the POSC. The OST is a simple, 10-bit counter that counts 1024 cycles before releasing the oscillator clock to the rest of the system. This time-out period is denoted as T_OST.

The amplitude of the oscillator signal must reach the V_IL and V_IH thresholds for the oscillator pins before the OST can begin to count cycles. The T_OST interval is required every time the oscillator restarts (that is, on POR, BOR and wake-up from Sleep mode) when XT or HS mode is selected in the Configuration Words. The T_OST timer does not exist when EC mode is selected.

After the POSC is enabled, it takes a finite amount of time to start oscillating. This delay is denoted as T_OSCD. After T_OSCD, the OST timer takes 1024 clock cycles (T_OST) to release the clock. The total delay for the clock to be ready is: T_OSCD + T_OST. If the PLL is used, an additional delay is required for the PLL to lock. For more information, see Phase-Locked Loop (PLL).

POSC start-up behavior is illustrated in Figure 12-11, where the CPU begins toggling an I/O pin when it starts execution after the T_OSCD + T_OST interval.