1: // Configure for 20MHZ using the 8MHz internal oscillator
2: #pragma config FNOSC=FRCPLL, FPLLIDIV=DIV_2, FPLLMUL=MUL_20, FPLLODIV=DIV_4
3:
4: #include <plib.h> // include PIC32 peripheral library
5:
6: main() {
7: int i;
8:
9: SYSTEMConfigPerformance(20000000); // optimise for speed
10: mPORTDSetPinsDigitalOut(BIT_1); // make RD1 (LED) an output
11:
12: while(1) {
13: mPORTDToggleBits(BIT_1); // flip the LED off/on
14: for(i=0; i<416000; i++); // 250mS delay at 20MHz
15: }
16: }

Line 1 Comments start with a double slash (//)

Line 2 This sets the configuration parameters for the chip (sometimes called the
"fuses"). The first entry (FNOSC=FRCPLL) sets the clock source to the internal
oscillator (8MHz) via the phase locked loop (PLL). The second entry causes the
oscillator to be divided by 2 before being applied to the PLL. The third entry sets
the PLL multiply ratio to 20. This means that the internal oscillator after being
divided by 2 will be multiplied by 20 thereby giving an output from the PLL of
80MHz. The last entry causes the PLL output to be divided by 4 before being used by
the core processor, which therefore runs at 20MHz. By varying this last entry you
can change the core speed with DIV_1 giving 80MHz and DIV_2 giving 40MHz.

Line 4 This includes standard code that defines the library functions that we will
use.

Line 6 The program starts running at the beginning of the function main(). The
curly bracket marks the beginning of the function and the closing bracket on line 16
marks the end.

Line 7 We define an integer for later use. Note that all integers default to a
signed 32 bit number.

Line 9 This calls a library function to optimize the chip for the clock speed that
we are running at (20000000 Hz). The optimisations include setting up the
instruction cache and wait states for memory access.

Line 10 This calls another library function to set RD1 as an output. RD1 is pin 49
on a 64 pin chip.

Line 12 This sets up an infinite loop so the LED will keep flashing forever.

Line 13 We call another library routine to toggle the state of the RD1 output from
high to low or vice versa. This is where the LED is turned on or off.

Line 14 This is a delay routine to prevent the LED from flashing too fast. Counting
to 416000 takes about 250mS when running at 20MHz.