PIC16F877A

Fig. 1. PIC16F877A, PDIP-40. Hover or tap a signal to light up every pin that carries it.

• Both VDD pins (11 and 32) and both VSS pins (12 and 31) must be connected. Wiring only one pair gives you a chip that half works and resets randomly.
• MCLR needs a 10k pull-up to VDD. There is no internal one by default, and a floating MCLR means a chip that never comes out of reset or resets on a sneeze.
• RA4 is open drain. It can sink but not source, so it cannot drive an LED to ground the way every other pin can. This is the single most asked question about this chip.
• Port A and Port E default to ANALOG at power-on. Until you write the ADCON1 register, digital reads on those pins return zero. The classic first-project bug.
• RB3 is the low-voltage programming pin. Tie it low if you are not using LVP, or noise can drop the chip into programming mode mid-run.
• RB4 to RB7 have interrupt-on-change, which is how keypads and wake-from-sleep are normally done.
• RB6 and RB7 are the ICSP clock and data. Anything hung on them must not fight the programmer, so keep loads light or add series resistors.
• Superseded for new designs by the PIC16F1939 and the PIC18F family, but still in production and still what a lot of university courses teach.

• Set up the ports, and stop them being analog

  XC8

  #include <xc.h>
  
  /* Port A and Port E come up ANALOG. Until this runs, a digital
     read on those pins returns 0 no matter what you wired. */
  ADCON1 = 0x06;                 /* PCFG = 0110: all digital */
  
  TRISB  = 0x00;                 /* 0 = output, the opposite of AVR's DDR */
  PORTB  = 0xFF;
  
  TRISA  = 0xFF;                 /* 1 = input */
  if (PORTAbits.RA0 == 0) {
  }
  
  /* RA4 is open drain. It can sink but never source, so an LED
     to ground will not light from this pin. */

  TRIS is inverted against AVR's DDR: 0 means output here. And Port A defaults to analog, which is the single most common first-project failure on this chip.

Table 2. Pin assignments.