Configure PIC24E to run at 70 MIPS

Setting the configuration bits

Macro for setting configuration bits must be placed somewhere at the top of the source file, right after the #include lines. For portability sake, I recommend this method rather than setting them in IDE.

Regardless of which clock source you will be using afterwards, the system is configured to use FRC at startup.

_FOSCSEL(FNOSC_FRC);

Next, use this macro for FRC case

_FOSC(FCKSM_CSECMD & OSCIOFNC_ON & POSCMD_NONE);

and for the external crystal case

_FOSC(FCKSM_CSECMD & OSCIOFNC_OFF & POSCMD_XT);

The latter is used for both cases in Microchip document. Consult the PIC24EPdatasheet and family reference manual for the meaning of each definition.

I/O Port Definitions

As shown in figure 1 above, only one input port (RB2) is used for the push-button switch, and one output port (RB15) for LED. Define them accordingly

#define SW1 !_RB2      // switch is active-low
#define LEDB  _LATB15  // blue LED 

Then put these lines in main() before the infinite loop, or in an init( ) function called by main.

ANSELA=0x0000;  // use all ports RA and RB as digital
ANSELB=0x000;
_TRISB2 = 1;   // RB2 as input 
_CNPUB2 = 1;   // enable internal weak pull up
_TRISB15 = 0;   // RB15 as output

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