PIC16F628A — 18-Pin 8-Bit Flash Microcontroller with Hardware USART (DIP-18)

Overview

The PIC16F628A is an 18-pin Flash-based 8-bit CMOS microcontroller manufactured by Microchip Technology. Available in a DIP-18 through-hole and SOIC-18 package, it was engineered as the direct modernized drop-in replacement for the legendary PIC16F84A.

Operating from $2.0\text{V}$ to $5.5\text{V}$ DC at up to $20\text{ MHz}$ (or via its internal $4\text{ MHz}$ calibrated RC oscillator without external crystals), the PIC16F628A vastly outperforms the older 16F84A by providing double the Flash memory ( $2\text{K words} / 3.5\text{ KB}$ ), more than triple the RAM ( $224\text{ Bytes}$ ), double the EEPROM ( $128\text{ Bytes}$ ), a full addressable hardware USART (RX/TX), dual analog comparators with programmable internal voltage reference ( $V_{REF}$ ), and a Capture/Compare/PWM (CCP) module. It is a workhorse for hobbyist digital clocks, frequency counters, and serial display modules.

Quick reference


CPU Core	8-bit PIC Mid-Range RISC (35 instructions)
Package	18-pin DIP (DIP-18 / PDIP-18) / 18-pin SOIC / 20-pin SSOP
Max Clock Frequency	$20\text{ MHz}$ ( $4\text{ MHz}$ factory internal RC)
Flash Program Memory	2048 words ( $3.5\text{ KB}$ , 14-bit words)
Data RAM	224 bytes
Data EEPROM	128 bytes
Operating Voltage (`VDD`)	$2.0\text{ V}$ to $5.5\text{ V}$ DC ( $3.0\text{V} \dots 5.5\text{V}$ for $20\text{MHz}$ )
GPIO Count	16 I/O pins (Ports A and B)
Hardware USART	Full duplex addressable USART (`RB1/RX`, `RB2/TX`)
Analog Peripherals	2x Analog Comparators + Programmable internal $V_{REF}$ module
Timers / PWM	3 Timers (Timer0/1/2) + 1x CCP module (10-bit PWM)

Pinout (DIP-18 Package)

text

                            ┌───┴───┐
      (AN2/VREF)      RA2  1│ 1   18│ RA1 (AN1)
      (AN3/CMP1)      RA3  2│       │17 RA0 (AN0)
      (T0CKI/CMP2)    RA4  3│       │16 RA7 (OSC1/CLKIN)
      (Active-Low)   RA5/ 4│PIC16F │15 RA6 (OSC2/CLKOUT)
                 MCLR/VPP   │ 628A  │14 VDD (+5V)
                      VSS  5│DIP-18 │13 RB7 (T1OSI/PGD)
              (INT)   RB0  6│       │12 RB6 (T1OSO/T1CKI/PGC)
             (RX/DT)  RB1  7│       │11 RB5
             (TX/CK)  RB2  8│       │10 RB4 (PGM)
             (CCP1)   RB3  9│       │9  (Center-notch bottom)
                            └───────┘

Pin	Name	Type	Description
1, 2	`RA2`, `RA3`	GPIO / Analog	`AN2` / `VREF` / `AN3` / Comparator 1 Input
3	`RA4`	GPIO / Digital	Timer0 Clock Input `T0CKI` / Open-drain output
4	`RA5 / MCLR`	Digital Input	Active-LOW Reset / `VPP` / General Input
5	`VSS`	Power	Common Ground reference ( $0\text{ V}$ )
6	`RB0`	GPIO / Digital	External Interrupt `INT` / `PCINT`
7	`RB1`	GPIO / Digital	Hardware UART Receive `RX` / Data `DT`
8	`RB2`	GPIO / Digital	Hardware UART Transmit `TX` / Clock `CK`
9	`RB3`	GPIO / Digital	Capture/Compare/PWM Output `CCP1`
10, 11	`RB4`, `RB5`	GPIO / Digital	Port B GPIOs / Low-Voltage ICSP `PGM`
12	`RB6`	GPIO / Digital	In-Circuit Serial Programming Clock `PGC` / `T1CKI`
13	`RB7`	GPIO / Digital	In-Circuit Serial Programming Data `PGD` / `T1OSI`
14	`VDD`	Power	Positive Supply Voltage ( $+2.0\text{ V}$ to $+5.5\text{ V}$ )
15, 16	`RA6`, `RA7`	GPIO / Clock	`OSC2`/`CLKOUT` and `OSC1`/`CLKIN` (usable as GPIO with internal RC)
17, 18	`RA0`, `RA1`	GPIO / Analog	`AN0`, `AN1` / Comparator Inputs

Comparison: PIC16F628A vs PIC16F84A

Parameter	PIC16F628A	PIC16F84A
Pinout / Socket	Identical DIP-18	Identical DIP-18
Internal Oscillator	Yes ( $4\text{ MHz}$ calibrated RC)	No (Requires crystal or RC network)
Flash Memory	$2048\text{ Words}$ ( $3.5\text{ KB}$ )	$1024\text{ Words}$ ( $1.75\text{ KB}$ )
Data RAM	$224\text{ Bytes}$	$68\text{ Bytes}$
Hardware USART	Yes (Pins RB1/RB2)	No (Software Bit-Bang Only)
Timers / PWM	3 Timers + CCP (PWM)	1 Timer (Timer0 only, no PWM)

Common mistakes

PORTA analog comparator defaults: On power-up, PORTA pins default to analog comparator inputs. To use PORTA as general digital I/O, write CMCON = 0x07; at the beginning of the program to turn off the comparators.
Pin RA4 open-drain behavior: Pin 3 (RA4) has an open-drain output. When driven HIGH, it floats; it requires an external pull-up resistor to output a logic HIGH voltage.

Notes

Programming: Compatible with PICkit 2, 3, 4, SNAP, and standard ICSP programmers using MPLAB X IDE and the XC8 compiler.