5.1 Overview

The oscillator module has 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. Figure 5-1 illustrates a block diagram of the oscillator module.

Clock sources can be supplied from external oscillators, quartz-crystal resonators and ceramic resonators. In addition, the system clock source can be supplied from one of two internal oscillators and PLL circuits, with a choice of speeds selectable via software. Additional clock features include:

  • Selectable system clock source between external or internal sources via software.
  • Fail-Safe Clock Monitor (FSCM) designed to detect a failure of the external clock source (LP, XT, HS, ECH, ECM, ECL) and switch automatically to the internal oscillator.
  • Oscillator Start-up Timer (OST) ensures stability of crystal oscillator sources.

The RSTOSC bits of Configuration Word 1 determine the type of oscillator that will be used when the device runs after Reset, including when it is first powered up.

If an external clock source is selected, the FEXTOSC bits of Configuration Word 1 must be used in conjunction with the RSTOSC bits to select the External Clock mode.

The external oscillator module can be configured in one of the following clock modes, by setting the FEXTOSC[2:0] bits of Configuration Word 1:

  • ECL – External Clock Low-Power mode
(below 1 MHz)
  • ECM – External Clock Medium Power mode
(1 MHz to 16 MHz)
  • ECH – External Clock High-Power mode
(above 16 MHz)
  • LP – 32 kHz Low-Power Crystal mode
  • XT – Medium Gain Crystal or Ceramic Resonator Oscillator mode (between 500 kHz and 4 MHz)
  • HS – High Gain Crystal or Ceramic Resonator mode (above 4 MHz)

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

Figure 5-1. Simplified PIC® MCU Clock Source Block Diagram