// default PBCLK = SYSCLK/2
#pragma config FPBDIV = DIV_2    
...
// PBCLK=SYSCLK/2
PLIB_OSC_PBClockDivisorSet(OSC_ID_0, OSC_PERIPHERAL_BUS_1, 2);
// enable PBCLK
PLIB_OSC_PBOutputClockEnable(OSC_ID_0, OSC_PERIPHERAL_BUS_1);
// disable PBCLK
PLIB_OSC_PBOutputClockDisable(OSC_ID_0, OSC_PERIPHERAL_BUS_1);