PIC16F877A: DIP-40 pinout, MCLR reset, 20 MHz crystal and ICSP pins

A through-hole chip is marked PIC16F877A-I/P: I is the −40 to +85 °C industrial range and P the 40-pin PDIP. Other suffixes are other packages (PT for the 44-lead TQFP, L for PLCC, ML for QFN), which number their pins differently.

PIC16LF877A is the low-voltage grade (2.0 to 5.5 V, 10 MHz at most). The PIC16F874A has the same pins with half the memory. The older PIC16F877 without the A has a separate datasheet and programming specification, so read the marking and pick the exact device in the programming software.

Microchip is the only maker, so there is one table. The figures to design around are the current ones: 25 mA per pin is the absolute limit, not a working figure, and the output voltages are specified only at 8.5 mA (sinking) and 3.0 mA (sourcing). Pins share two 200 mA groups, and the whole chip may pass 300 mA out of VSS and 250 mA into VDD.

PORTA and PORTE carry the eight analog inputs AN0 to AN7, and they come up as analog after reset, so a pin you want as digital I/O needs ADCON1 set first. RA4 is open drain, so as an output it needs a pull-up. PORTB has weak pull-ups you can switch on in software, interrupt-on-change on RB4 to RB7, and the programming pins RB3, RB6 and RB7. PORTC holds the peripherals: the two PWM outputs (RC1, RC2), SPI and I2C (RC3 to RC5) and the UART (RC6 TX, RC7 RX). PORTD is plain I/O, or the 8-bit parallel slave port.

• Course and lab work built around the mid-range PIC instruction set, MPLAB and a PICkit-style programmer, on a chip that sits in a breadboard.
• 5 V control boards that need a lot of pins at once: 33 I/O lines, eight 10-bit analog inputs, two PWM outputs, a UART and SPI or I2C.
• Keeping an existing PIC16F877A design or project kit running.

For a new design on 3.3 V, with USB, or with more RAM than 368 bytes, a newer chip is the better start. The ATmega328P covers similar ground in 28 pins.

A 5 V supply, a 20 MHz crystal, the MCLR network from Figure 14-5 and both supply pairs decoupled: the least the chip needs to run its program, plus what that clock gives the UART, PWM and A/D.

• Sink LEDs rather than source them: the low output is specified at 8.5 mA, the high one only at 3.0 mA. For a red LED of about 2 V, (5 − 2 − 0.6) V / 8.5 mA = 282 Ω, so 330 Ω: 7.27 mA at the VOL limit and 9.09 mA with the pin at 0 V. All 8 on PORTD come to 72.73 mA, well inside the 200 mA PORTC + PORTD group (Figure 5).
• Set ADCON1 before using PORTA or PORTE as digital pins. After reset they are analog inputs, and a digital read of an analog pin returns 0.
• Wire both VDD pins and both VSS pins, with a 100 nF capacitor across each pair at the chip.
• Erasing the whole chip through ICSP needs VDD at 4.5 to 5.5 V (p. 158). On a board that runs at 4.0 V, raise the supply for that step.
• Keep the crystal and its two capacitors within a couple of centimetres of pins 13 and 14, and select HS (not XT) in the configuration word for a 20 MHz crystal.

Both share the 40-pin layout and the same datasheet. The 874A loses half the memory; the LF grade trades top speed for a 2.0 V minimum supply.

1. Measure 5 V from pin 11 to pin 12 and from pin 32 to pin 31.
2. MCLR (pin 1) should read close to 5 V (4.94 V or more with these resistors). Near 0 V means the reset button or capacitor is shorted.
3. RB3 (pin 36) should read 0 V if LVP is still on. If it sits high, the chip may be in programming mode and will not run code.
4. Load a program that toggles a pin and watch it on a scope or a logic probe. A frequency counter on the crystal pins can stop a weak oscillator, so probe the output pin instead.
5. If the programmer cannot find the chip, check that it is set for the PIC16F877A (not the 877 or 874A), that MCLR, PGC and PGD reach pins 1, 39 and 40, and that the board is powered at 4.5 V or more if the programmer erases the chip first.

Run the PIC16F877A-I/P at 5 V on a 20 MHz crystal with 22 pF capacitors, use the 10 kΩ / 1.5 kΩ / 100 nF MCLR network, decouple both supply pairs, put 10 kΩ from RB3 to ground unless LVP is switched off, and bring MCLR, PGC and PGD to a 6-pin ICSP header. Set BRGH = 1 for the UART, set ADCON1 before touching PORTA, and sink LEDs at about 7 to 9 mA each.

The PIC16F877A-I/P is stocked by most Indian component shops and comes in many PIC starter kits. Read the full marking, since an 877 and an 877A look alike, and buy a 40-pin socket with it so the chip can come out for a programmer that has no ICSP lead. Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes.