PIC16F84A — Historic 18-Pin 8-Bit CMOS Flash Microcontroller (DIP-18)

Overview

The PIC16F84A (along with its iconic forebears the PIC16C84 and PIC16F84) is the historic 18-pin 8-bit CMOS Flash microcontroller manufactured by Microchip Technology. Housed in a through-hole DIP-18 package, it is widely acknowledged as the single component that ignited the hobbyist and DIY microcontroller revolution in the 1990s and early 2000s.

Before the PIC16F84, microcontrollers were expensive windowed ceramic chips requiring UV erasers (UV-EPROMs) or one-time-programmable (OTP) devices. By introducing electronically re-programmable Flash program memory and an inexpensive serial programmer interface (ICSP), the PIC16F84 made microcontroller programming accessible to students, hobbyists, amateur radio operators, and hardware hackers worldwide.

Featuring 1K words ( $1.75\text{ KB}$ ) of Flash memory, 68 bytes of RAM, 64 bytes of EEPROM, an 8-bit Timer0, and 13 digital I/O lines, it served as the basis for thousands of classic tutorials, textbook curricula, smart-card programmers (Gold Cards), keyers, and retro hobby projects before being succeeded by the enhanced PIC16F628A.

Quick reference


CPU Core	8-bit PIC Mid-Range RISC (Harvard architecture, 35 instructions)
Package	18-pin DIP (DIP-18 / PDIP-18) / 18-pin SOIC / 20-pin SSOP
Max Clock Frequency	$20\text{ MHz}$ ( $4\text{ MHz}$ on -04 grade, $20\text{ MHz}$ on -20 grade)
Flash Program Memory	1024 words ( $1.75\text{ KB}$ , 14-bit words, 10,000 write cycles)
Data RAM	68 bytes
Data EEPROM	64 bytes true non-volatile EEPROM (10,000,000 write cycles)
Operating Voltage (`VDD`)	$4.0\text{ V}$ to $5.5\text{ V}$ DC ( $2.0\text{V} \dots 5.5\text{V}$ on low-voltage PIC16LF84A)
GPIO Count	13 I/O pins (Port A: 5 pins, Port B: 8 pins)
Timers	1x 8-bit Timer0 with 8-bit programmable prescaler
Programming Interface	In-Circuit Serial Programming (ICSP via `RB6`, `RB7`, `MCLR/VPP`)

Pinout (DIP-18 Package)

text

                            ┌───┴───┐
                      RA2  1│ 1   18│ RA1
                      RA3  2│       │17 RA0
             (T0CKI)  RA4  3│PIC16F │16 OSC1 / CLKIN
         (Active-Low) RA5/ 4│  84A  │15 OSC2 / CLKOUT
                 MCLR/VPP   │ DIP-18│14 VDD (+5V)
                      VSS  5│       │13 RB7 (PGD / Data)
              (INT)   RB0  6│       │12 RB6 (PGC / Clock)
                      RB1  7│       │11 RB5
                      RB2  8│       │10 RB4
                      RB3  9│       │9  (Bottom Notch)
                            └───────┘

Pin	Name	Type	Description
1, 2	`RA2`, `RA3`	GPIO / Digital	Port A bi-directional I/O pins
3	`RA4 / T0CKI`	GPIO / Open-Drain	Port A Bit 4 / Timer0 External Clock Input (Open-drain output)
4	`MCLR / VPP`	Digital Input	Master Clear (Reset, Active LOW) / Programming Voltage Input ( $+12\text{V}$ – $13\text{V}$ )
5	`VSS`	Power	Common Ground reference ( $0\text{ V}$ )
6	`RB0 / INT`	GPIO / Interrupt	Port B Bit 0 / External Interrupt input pin
7, 8, 9, 10, 11	`RB1`-`RB5`	GPIO / Digital	Port B bi-directional I/O pins (with software pull-ups)
12	`RB6 / PGC`	GPIO / Digital	Port B Bit 6 / ICSP Serial Programming Clock
13	`RB7 / PGD`	GPIO / Digital	Port B Bit 7 / ICSP Serial Programming Data
14	`VDD`	Power	Positive Power Supply ( $+4.0\text{ V}$ to $+5.5\text{ V}$ )
15	`OSC2 / CLKOUT`	Clock Output	Crystal oscillator output or instruction clock output ( $F_{OSC}/4$ )
16	`OSC1 / CLKIN`	Clock Input	Crystal oscillator input or external clock input
17, 18	`RA0`, `RA1`	GPIO / Digital	Port A bi-directional I/O pins

Minimal Crystal Circuit & ICSP Connections

text

                                +5.0V Regulated DC
                                       │
                  ┌────────────────────┴───────────────┐
                  │                                    │
            [Pin 14: VDD]                        [ 10kΩ Pull-Up ]
                  │                                    │
            [ 100nF Cap ]                        [Pin 4: MCLR/VPP]
                  │                                    │
            [Pin 5: VSS] ──────────────────────────────┼─── Common GND
                  │                                    │
  4MHz Crystal Setup:                                  │
    OSC1 (Pin 16) ───┬───[ 4MHz Crystal ]───┬─── OSC2 (Pin 15)
                     │                      │
               [ 22pF Cap ]            [ 22pF Cap ]
                     │                      │
                    GND                    GND

Common mistakes

Missing external oscillator components: Unlike modern microcontrollers (and the successor PIC16F628A), the PIC16F84A does not have an internal RC oscillator. It will not run without an external crystal, resonator, or external RC resistor/capacitor network connected across pins 15 and 16.
Forgetting that RA4 is open-drain: Pin 3 (RA4) cannot source current to drive high. To output a logic HIGH, connect an external pull-up resistor (e.g. $4.7\text{ k}\Omega$ ).
No hardware UART or ADC: The PIC16F84A possesses no built-in ADC and no hardware USART. Serial communication must be implemented via software delay-loop bit-banging.

Notes

Modern Successor: The PIC16F628A is pin-compatible and provides an internal 4MHz oscillator, hardware USART, 2 comparators, and 3.5KB Flash.