The Primary Oscillator (POSC) uses the OSC1 and OSC2 pins. POSC can be configured for an External Clock Input (EC mode) or an External Crystal or Resonator (HS mode).

#pragma config POSCMOD = EC  //default = external clock#pragma config POSCMOD = HS  //default = high-speed crystal

External Clock Mode with Output Clock

When POSC is in the EC mode, the OSC1 pin is a high-impedance input that can be driven by a CMOS driver.

The EC mode also disables the internal feedback buffer allowing the OSC2/CLKO pin to be used for other functions (general purpose I/O or as a clock out). The output clock provided on the CLKO pin is the peripheral bus clock number 1(PBCLK1) divided by 2.

#pragma config OSCIOFNC = ON  // clock out enabled#pragma config OSCIOFNC = OFF // clock out disabled

USB Input Clock Requirements

If using the USB peripheral, POSC must be 12 MHz or 24 MHz. You must also configure the USB PLL for a 12 MHz or 24 MHz input frequency.

#pragma config UPLLEN = ON            // Enable USB PLL#pragma config UPLLFSEL = FREQ_12MHZ  // USB PLL input clock = 12MHz#pragma config UPLLFSEL = FREQ_24MHZ  // USB PLL input clock = 24MHz

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