Config bits and configuration word

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The configuration word (also called config bits or fuses) sets hardware options the PIC applies at power-on, before running your program: oscillator source, watchdog, MCLR behavior, whether ports start digital, and more.

This page completes the Foundations block with MPASM assembly: there you saw CONFIG at the top of the .ASM and the sandwich layout; here you learn what the configuration word is, what each bit means, and how it ties to the UNEXPO lab minimum system.

In MPASM

CONFIG directives do not generate executable code. The assembler turns them into bits the programmer writes to the PIC’s configuration memory (separate from the Flash where your GOTO programa lives).

What is the configuration word

In the course you will hear configuration word, config bits, and fuses — the same idea with different names: a fixed block of bits that tells the PIC how to start before your programa: runs.

Step 1 — A group of bits, not a program

The configuration word is not instructions the CPU runs with MOVLW, GOTO, or CALL. It is a hardware switch pack stored in a special chip area:

	Configuration word	Your Flash program
What it is	Startup bits (oscillator, WDT, MCLR…)	Instructions the CPU executes
When it acts	At power-on or after reset	Line by line while running
You define it with	CONFIG directives in .ASM	Code under ORG / labels
Changes at runtime	No (unless reprogramming)	Yes — your code modifies SFRs

Think of it as factory setup for the stage: crystal, reset, and watchdog are chosen before main; then your assembly works on registers (TRISB, LATB…).

Step 2 — You write it with CONFIG in the .ASM

Each CONFIG line sets one part of the word. MPASM maps the name (FOSC, WDT…) and value (HS, OFF…) to the actual PIC18F4550 bit:

        CONFIG  FOSC = HS     ; part of the word: high-speed oscillator
        CONFIG  WDT  = OFF    ; part of the word: watchdog off
        CONFIG  LVP  = OFF    ; part of the word: RB5 free

This is not a literal inside an instruction (like MOVLW D'10'). It is a directive: it talks to the assembler, not to the CPU at run time.

Step 3 — MPASM packs it into the .HEX

When you build, the .HEX carries two things:

  .HEX file
  ├── Program code (executable Flash)
  └── Configuration word (startup bits)

That is why in MPLAB and Proteus you load one .HEX: Proteus and the programmer store program and configuration word together. Without a CONFIG block, the chip may use defaults different from the lab.

Step 4 — The PIC applies it at power-on

Sequence when you apply 5 V:

  Power applied (POR)
       ↓  PWRT (if CONFIG PWRT = ON)
  PIC reads configuration word
       ↓  FOSC, WDT, PBADEN, MCLRE… are fixed
  PC = 0000H → runs ORG 0x0000
       ↓
  GOTO    programa          ; jump to main
; ...
programa:                     ; destination — your code starts here

If the program “does not start”, check word + hardware before debugging LATB: supply → crystal/FOSC → MCLR/MCLRE → LVP/PBADEN → only then code.

Configuration word vs SFR register

• Configuration word → hardware at power-on (FOSC = HS).
• SFR register → hardware while running (BCF TRISB, 0).

CONFIG WDT = OFF does not replace CLRWDT in a program that later enables WDT another way — different layers.

Key concepts — equivalent names

Concept	Explanation
Configuration word	The full block of startup bits
Config bits / fuses	Each bit (or group) inside that word
CONFIG directive	How you write those bits in .ASM
Configuration memory	Where they are stored on chip (not executable Flash)

Think of config bits as factory switches you choose once when building/programming. If WDT = ON and your program never runs CLRWDT, the PIC will reset on its own even when your logic looks fine.

Where it goes in the sandwich .ASM

It belongs in the top layer, after LIST and #include, before the reset vector and code — same as Assembly — anatomy:

; ══ TOP — header + configuration word ════════════════
        LIST    P=18F4550
        #include <P18F4550.INC>

        CONFIG  FOSC = HS
        CONFIG  WDT  = OFF
        CONFIG  LVP  = OFF
        CONFIG  MCLRE = ON
        CONFIG  PBADEN = OFF
        CONFIG  BOR  = OFF
        CONFIG  PWRT = ON

; ── Reset vector ──────────────────────────────────────
        ORG     0x0000
        GOTO    programa         ; jump to main (at bottom)

; ══ MIDDLE — subroutines (if any) ═════════════════════
; ...

; ══ BOTTOM — main program ════════════════════════════
programa:
        BCF     TRISB, 0
        BSF     LATB, 0
loop:
        GOTO    loop               ; jump → destination loop: (line above)

        END

In MPLAB v8 you can also edit them via Configure → Configuration Bits, but in the UNEXPO course you usually declare them in .ASM so the report and .HEX match.

Each config bit in detail

FOSC — clock source (oscillator)

Selects where the execution clock comes from.

Typical value	Meaning	In this course
HS	High-Speed — external crystal (4–20 MHz+)	Use with 20 MHz crystal
XT	Low/medium speed crystal	Less common on F4550
INTIO / RC	Internal oscillator	No external crystal
EC / ECM	External clock on a pin	Special cases

With FOSC = HS you must wire the crystal + capacitors of the minimum system. Without a crystal (or bad wiring), the PIC does not run at the expected speed — or does not start at all.

With a 20 MHz crystal in HS (UNEXPO course):

Quantity	Typical value	Formula / note
Fosc	20 MHz	Oscillator frequency (crystal)
Tcy (instruction cycle)	200 ns	Tcy = 4 / Fosc = 4 / 20 MHz
MIPS (order of magnitude)	~5	1 / Tcy in million instructions/s

That Fosc is set by the configuration word (FOSC = HS) and the physical crystal — they must match. In Proteus and on the board, the PIC clock must reflect 20 MHz if you use this template.

WDT — Watchdog Timer

Value	Effect
OFF	Watchdog disabled — recommended while learning
ON	If you do not reset the WDT with CLRWDT in time, the PIC resets

In learning labs almost always WDT = OFF. If left ON without CLRWDT in long loops, the program “restarts by itself” and looks like a mysterious bug.

LVP — Low Voltage Programming

Value	Effect
OFF	Normal programming; RB5 free as GPIO
ON	Low-voltage programming mode — RB5 reserved for programming

In UNEXPO labs use LVP = OFF. With LVP = ON, RB5 may not behave as expected on your breadboard.

MCLRE — MCLR / reset pin

Value	Effect
ON	MCLR pin active as reset (with external pull-up)
OFF	MCLR can act as a digital pin (uncommon in this course)

With MCLRE = ON you wire 10 kΩ from MCLR to VDD and a button to GND for manual reset — part of the minimum system.

PBADEN — analog PORTA/B at startup

Value	Effect
OFF	RA0–RA3 and RB0–RB4 start as digital
ON	Those pins may start in analog mode

For GPIO and the first LED use PBADEN = OFF. If ON, reading/writing pins can fail until you configure ADCON1 — a typical lab mistake.

BOR — Brown-Out Reset

Value	Effect
OFF	No automatic reset on VDD drop
ON	Reset if supply voltage falls too low

Many course builds use BOR = OFF. With ON, 5 V supply variations can cause unexpected resets.

PWRT — Power-up Timer

Value	Effect
ON	Short delay at power-on so supply stabilizes
OFF	No extra delay

PWRT = ON is good practice with HS crystal and decoupling caps.

Summary — course template (20 MHz)

Block ready to copy in the top layer of all your .ASM files:

        CONFIG  FOSC = HS        ; External high-speed crystal
        CONFIG  WDT  = OFF       ; No watchdog (learning)
        CONFIG  LVP  = OFF       ; RB5 free; normal programming
        CONFIG  MCLRE = ON       ; MCLR reset enabled
        CONFIG  PBADEN = OFF     ; Digital ports at startup
        CONFIG  BOR  = OFF       ; No brown-out reset
        CONFIG  PWRT = ON        ; Power-up delay

Practical tip

Copy this block into all your course .ASM files until you understand each line. Changing one CONFIG without knowing why can cost hours of breadboard debugging.

Minimum system — hardware that goes with config bits

Config bits do not replace the circuit. With FOSC = HS and MCLRE = ON you need the Topic 2 (UNEXPO) wiring:

Component	Typical value	Function
PIC18F4550	—	Microcontroller
Crystal	20 MHz	Main clock (with FOSC = HS)
Crystal caps	C1, C2 to ground (see datasheet / lab sheet)	Oscillator load — see Topic 2 diagram
MCLR pull-up	10 kΩ to VDD	Stable reset (MCLRE = ON)
Reset button	MCLR to GND	Manual reset
Supply	5 V (VDD/VSS)	PIC power
Decoupling	100 nF (0.1 µF) near PIC	Filters VDD noise

Minimum system per Topic 2 class slide: crystal between OSC1/OSC2 with C1 and C2 to ground, 10 kΩ pull-up on MCLR, reset button, and +5 V on VDD/VSS. — Class Topic 2 and 3 — PIC Architecture (UNEXPO · Prof. Ing. Yoel Pire)

Link to Assembly — LED example: the .ASM configures software (TRISB, LATB); config bits and the crystal configure hardware so that software can run.

Practica · UNEXPO

Lab Practice 1 pre-lab

Research minimum-system wiring, explain PIC18F4550 ports, and draw the schematic including crystal, MCLR, and capacitors before breadboarding. Note on the diagram that FOSC = HS and MCLRE = ON.

Reset types on the PIC18

The PIC can restart for several reasons. Config bits control some; others come from hardware or code:

Type	Cause	Config bit link
Power-On Reset (POR)	VDD turned on	Always on power-up
Power-up Timer (PWRT)	Delay after POR	PWRT = ON/OFF
MCLR	Low pulse on MCLR pin	MCLRE = ON
Watchdog (WDT)	WDT not cleared in time	WDT = ON + missing CLRWDT
Brown-Out (BOR)	VDD below threshold	BOR = ON/OFF
Software reset	RESET instruction	From your code

Mental power-on flow:

  Apply 5 V
       ↓  POR (+ PWRT if ON)
  Config bits applied (FOSC, PBADEN…)
       ↓
  PC = 0x0000 → GOTO start (your ORG)

If the program “does not start”, check in this order: supply and decoupling → crystal and FOSC → MCLR and MCLRE → LVP/PBADEN → only then Flash code.

Config bits vs SFR registers

	Config bits	SFR registers (TRISB, INTCON…)
When applied	At power-on / after reset	On each instruction in your program
Where defined	CONFIG directives in .ASM	Runtime instructions
Change at runtime	No (unless reprogramming)	Yes — your code modifies them
Example	FOSC = HS	BCF TRISB, 0

Config bits set the stage; your assembly program works on registers once the PIC is already running.

Ejercicio · Parcial

Diagnose the problem

A classmate programs this .ASM with a correctly wired 20 MHz crystal, but RB0 never changes and Proteus simulation also misbehaves:

        CONFIG  FOSC = INTIO
        CONFIG  WDT  = ON
        CONFIG  PBADEN = ON

Likely causes:

1. FOSC = INTIO — code/hardware expect external HS crystal, but config selects internal oscillator → clock mismatch.
2. WDT = ON — no CLRWDT → continuous reset.
3. PBADEN = ON — RB0 may start analog → digital GPIO fails.

Typical course fix:

        CONFIG  FOSC = HS
        CONFIG  WDT  = OFF
        CONFIG  PBADEN = OFF

Common mistakes

• Forgetting the CONFIG block: the .HEX may use defaults different from the lab.
• Wrong FOSC: crystal wired but FOSC = INTIO (or vice versa) → no stable clock.
• WDT = ON without CLRWDT: PIC resets itself in loops.
• LVP = ON: RB5 reserved for programming — breadboard conflicts.
• PBADEN = ON: pins “dead” as digital until ADC/analog setup.
• Confusing CONFIG with code: CONFIG lines do not replace BCF TRISB, 0 or turn on LEDs by themselves.
• Crystal without capacitors: even with FOSC = HS, the oscillator may not start.

Exam-style questions

What is the configuration word?

The startup bit block that sets oscillator, WDT, MCLR, pin modes at power-on, etc. You define it with CONFIG in .ASM and it is programmed with the .HEX — not executable code.

Where are config bits stored?

In the PIC’s configuration memory, not in normal executable Flash.

What does CONFIG WDT = OFF do?

Disables the watchdog timer so the PIC does not reset on WDT timeout.

Why FOSC = HS with a 20 MHz crystal?

Selects high-speed external crystal mode, matching the course minimum system.

What problem does PBADEN = ON cause in a first GPIO program?

Several PORTA/PORTB pins may start as analog, blocking correct digital read/write.

Interactive exam

Configuration bits

Based on: PIC18LF4455-I-PT.PDF

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CONFIG WDT = OFF disables...

The watchdog timer UART ADC Timer2

FOSC = HS selects...

Internal RC High-speed crystal USB only Permanent sleep

CONFIG directives in MPASM are programmed into...

Configuration memory (not executable code) WREG User data EEPROM Proteus simulator only

CONFIG LVP = OFF in UNEXPO labs allows...

Use RB5 as normal GPIO Disable the crystal Turn off all ports Remove the need for MCLR

CONFIG PBADEN = OFF makes pins start as...

Digital on RA0-RA3 and RB0-RB4 Always analog only Inputs with fixed internal pull-up Disconnected from GPIO

CONFIG MCLRE = ON requires in the minimum system...

10 kΩ pull-up on MCLR (and optional button to GND) Power only, no reset Remove the crystal Tie MCLR permanently to GND

If WDT = ON and you never run CLRWDT, the PIC...

Resets due to watchdog timeout Runs twice as fast Turns off port B only Programs EEPROM automatically

CONFIG PWRT = ON introduces...

A short delay at power-on to stabilize VDD A second internal crystal Automatic UART mode MCLR disable

The course 20 MHz crystal goes with CONFIG...

FOSC = HS FOSC = INTIO WDT = ON LVP = ON

Config bits are applied mainly...

At power-on or after reset, before your main code On every ADDWF instruction Only when pressing MCLR during execution When writing to LATB

The configuration word in the .HEX is...

Startup bits stored separately from executable code The same code as GOTO programa A GPR variable in RAM Visible in simulation only, not on the real PIC

With FOSC = HS and a 20 MHz crystal, one instruction cycle is about...

200 ns 20 ms 1 s 4 MHz

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