fvrbor.c

/* Code for the PIC18F25K50 that causes a brown-out reset by dumping a
 * high voltage onto the FVR from the ADC's holding capacitor.
 *
 * Connections:
 *   RA0: Tie this pin to an LED that turns on when the pin is high.
 *   RA1/AN1: Don't connect anything to this pin.
 *   RA2/AN2: Don't connect anything to this pin.
 *   VDD: Connect a power source between 3 V and 5 V.
 *   GND: Connect to the GND of the power source.
 *
 * Expected behavior:
 *   On power up, LED blinks twice, pauses, then turns on.
 *
 * Actual behavior:
 *   On power up, LED blinks twice, pauses, blinks 4 times, pauses,
 *   blinks 4 times, pauses, blinks 4 times, pauses...
 *   This indicates that a brownout reset happens when the ADC
 *   is connected to the FVR.
 */

#pragma config FOSC = INTOSCIO
#pragma config CFGPLLEN = OFF
#pragma config CPUDIV = NOCLKDIV
#pragma config nPWRTEN = ON
#pragma config BOREN = ON
#pragma config BORV = 285
#pragma config nLPBOR = OFF

#define _XTAL_FREQ 1000000

#include <xc.h>

#define LED_ENABLE() (TRISA0 = 0)
#define LED(on) (on ? LATA0 = 1 : LATA0 = 0)

void delay_ms(unsigned int delay)
{
    while(delay--) { __delay_ms(1); }
}

void blink()
{
    LED(1);
    delay_ms(100);
    LED(0);
    delay_ms(300);
}

void main(void)
{
    LED_ENABLE();

    /* Make RA1/AN1 be an analog input and drive it high. */
    ANSELAbits.ANSA1 = 1;
    TRISA1 = 0;
    LATA1 = 1;

    /* Make RA2/AN2 be an analog input and drive it low. */
    ANSELAbits.ANSA2 = 1;
    TRISA2 = 0;
    LATA2 = 0;

    /* Enable the FVR for 1.024 V operation. */
    VREFCON0 = 0b10010000;
    while(!VREFCON0bits.FVRST) { }

    /* Check for a brown-out reset. */
    RCONbits_t initialRCON = RCONbits;
    RCONbits.POR = 1;
    RCONbits.BOR = 1;
    if (initialRCON.POR && !initialRCON.BOR)
    {
        /* A brown-out reset occurred. */
        blink();
        blink();
    }

    /* Charge up the ADC's holding capacitor by selecting AN1, which
     * is driving high. */
    ADCON0 = 0b00000101;

    blink();

    /* Uncommenting the line below fixes the problem, because it
     * allows the ADC's holding capacitor to discharge by selecting
     * AN2, which is driving low. */
    // ADCON0 = 0b00001001;

    blink();

    delay_ms(400);

    /* Select the FVR, connecting its output to the ADC's holding
     * capacitor.  This causes a brown-out reset if the capacitor is
     * charged. */
    ADCON0 = 0b01111101;

    while(1)
    {
        LED(1);
    }
}