12 OSC - Oscillator Module (With Fail-Safe Clock Monitor)

The oscillator module contains multiple clock sources and selection features that allow it to be used in a wide range of applications while maximizing performance and minimizing power consumption.

Clock sources can be supplied either internally or externally. External sources include:

External clock oscillators
Quartz crystal resonators
Ceramic resonators
Secondary Oscillator (SOSC)

Internal sources include:

High-Frequency Internal Oscillator (HFINTOSC)
Low-Frequency Internal Oscillator (LFINTOSC)
Analog-to-Digital Converter RC Oscillator (ADCRC)

Special features of the oscillator module include:

Oscillator Start-up Timer (OST): Ensures stability of quartz crystal or ceramic resonators
4x Phase-Locked Loop (PLL): Frequency multiplier for external clock sources
HFINTOSC Frequency Adjustment: Provides the ability to adjust the HFINTOSC frequency
Clock switching: Allows the system clock to switch between internal or external sources via software during run time
Fail-Safe Clock Monitor (FSCM): Designed to detect a failure of the system clock (F_OSC), primary external clock (EXTOSC) or secondary external clock (SOSC) sources. The FSCM automatically switches to an internal clock source upon detection of a F_OSC failure.

The Reset Oscillator (RSTOSC) Configuration bits determine the type of oscillator that will be used when the device runs after a Reset, including when the device is first powered up (see the table below).

Table 12-1. RSTOSC Selection Table
RSTOSC	SFR Reset Values			Clock Source
RSTOSC	NOSC / COSC	NDIV / CDIV	OSCFRQ	Clock Source
`111`	`111`	`0000` (1:1)	`0010` (4 MHz)	EXTOSC per FEXTOSC
`110`	`110`	`0010` (4:1)		HFINTOSC @ 1 MHz
`101`	`101`	`0000` (1:1)		LFINTOSC
`100`	`100`	`0000` (1:1)		SOSC
`011`	Reserved
`010`	`010`	`0000` (1:1)	`0010` (4 MHz)	EXTOSC + 4x PLL⁽¹⁾
`001`	Reserved
`000`	`000`	`0000` (1:1)	`1000` (64 MHz)	HFINTOSC @ 64 MHz

Note:

EXTOSC must meet the PLL specifications (see the data sheet Electrical Specifications).

If an external clock source is selected by the RSTOSC bits, the External Oscillator Mode Select (FEXTOSC) Configuration bits must be used to select the External Clock mode. These modes include:

ECL: External Clock Low-Power mode
ECM: External Clock Medium-Power mode
ECH: External Clock High-Power mode
LP: 32 kHz Low-Gain Crystal mode
XT: Medium-Gain Crystal or Ceramic Resonator mode
HS: High-Gain Crystal or Ceramic Resonator mode

The ECH, ECM and ECL modes rely on an external logic-level signal as the device clock source. The LP, XT and HS modes rely on an external quartz crystal or ceramic resonator as the device clock source. Each mode is optimized for a specific frequency range. The internal oscillator block produces both low-frequency and high-frequency clock signals, designated LFINTOSC and HFINTOSC, respectively. Multiple system operating frequencies may be derived from these clock sources.

The figure below illustrates a block diagram of the oscillator module.

Figure 12-1. Clock Source Block Diagram