PIC16F873 Voltmeter with LCD Display

Project Overview

This is an embedded systems project written in PIC assembly language for the PIC16F873 microcontroller. The current implementation functions as a digital voltmeter with an LCD display, with the ultimate goal of evolving into a hygrometer with memory storage capabilities.

Author: David Robert (david@ombrepixel.com)
Microcontroller: PIC16F873
Language: Assembly (MPASM)

Hardware Configuration

Microcontroller Settings

Processor: PIC16F873
Clock Frequency: High-Speed Oscillator (HS_OSC) - 4 MHz to 20 MHz range
Power-up Timer: Enabled (PWRTE_ON)
Watchdog Timer: Disabled (WDT_OFF)
Brown-out Detect: Disabled (BODEN_OFF)
Low Voltage Programming: Disabled (LVP_OFF)
Code Protection: Disabled (CP_OFF)

Pin Configuration

PORT A (Analog Input)

RA0-RA5: Configured as analog inputs (bits 0-5 as inputs)
RA2: ADC input channel (configured in ADCON0)
Uses internal voltage reference

PORT B (LCD Interface)

RB4-RB7: LCD data lines (4-bit mode)
RB2 (bLCD_RS): LCD Register Select
RB3 (bLCD_E): LCD Enable signal
All configured as outputs

PORT C

All pins configured as inputs (TRISC = 0xFF)

LCD Display

Interface Type: 4-bit mode
Configuration: 2 lines, 5×7 dot matrix characters
Connection: PORT B (upper nibble for data, RB2/RB3 for control)

Analog-to-Digital Converter (ADC)

Resolution: 10-bit (0-1023)
Channel: RA2 (Channel 2)
Clock Source: Internal RC with predivider ÷8
Voltage Reference: VDD (5V typical)
Acquisition Time: ~80µs (4× 20µs delays)

Software Architecture

Main Program Flow

1. System Initialization
   ├── Configure I/O ports
   ├── Set up interrupts (prepared but not used)
   ├── Initialize ADC
   └── Clear RAM

2. LCD Initialization
   └── Configure 4-bit mode, 2-line display

3. Main Loop (infinite)
   ├── Acquire analog signal (ADC conversion)
   ├── Convert raw ADC value to voltage
   ├── Display voltage on LCD
   ├── Wait (temporal delay)
   └── Repeat

Key Modules

1. pic_voltmeter.asm (Main Program)

The core application logic containing:

System Configuration
- Register definitions (OPTION_REG, INTCON, PIE1, PIE2)
- Port direction setup
- Interrupt handlers (framework present but unused)
Data Acquisition (acquire)
- Configures ADC for channel 2 (RA2) with predivider ÷8
- Waits for acquisition time (80µs)
- Starts conversion and polls GO bit
- Reads 10-bit result from ADRESH:ADRESL into auxreg1:auxreg2
Voltage Calculation (voltage)
- Converts raw 10-bit ADC value to voltage representation
- Uses bit-shifting and arithmetic operations
- Formula approximates: Voltage = (ADC_Value × 5V) / 1024
- Result stored in auxreg1:auxreg2 (scaled for display)
Display Formatting (LCD_ShowAcq)
- Positions cursor at line 1
- Displays "Ascii : " label
- Converts voltage to ASCII decimal (XXX.XX format)
- Appends "V" unit suffix
Timing (tempo)
- Triple-nested loop delay routine
- Provides ~262ms delay between readings

2. lcd.inc (LCD Driver)

LCD control and communication routines:

LCD_Init_Display: Initializes LCD in 4-bit mode
- Sends initialization sequence
- Configures 2-line, 5×7 character display
- Enables display, disables cursor
LCD_SendCMD: Sends commands to LCD (RS=0)
- Splits byte into two nibbles
- Toggles enable pin for each nibble
LCD_SendCHAR: Sends characters to LCD (RS=1)
- Similar to SendCMD but with RS=1 for data
LCD_Clear: Clears display content
LCD_ShowLogo: Displays "Hello!" welcome message

3. ascii.inc (Conversion Utilities)

Number formatting and conversion routines:

HEX16_2_ASCII: Converts 16-bit binary to 5-digit ASCII decimal
- Input: auxreg1:auxreg2 (16-bit value)
- Output: aBCD0:aBCD1:aBCD2:aBCD3:aBCD4 (ASCII characters)
- Implements binary-to-BCD conversion via bit testing
- Suppresses leading zeros
BCD_normalize: Handles decimal carry during conversion
Timing Functions:
- __Delay_5uS: 5 microsecond delay
- __Delay_20uS: 20 microsecond delay
- __Delay_50uS: 50 microsecond delay
- __Delay_Nx: Variable delay (N × 50µs)

4. macro.inc (Assembly Macros)

Utility macros for cleaner code:

Comparison Macros: JE, JNE, JL, JG, JLE, JGE, JZ, JNZ
Data Movement: MOVLF, MOVFF
LCD Macros: CHR_LCD (send character), INS_LCD (send instruction)
Bank Selection: F_BANK

Memory Organization

Bank 0 (0x20-0x6F)

User variables:

test, table[10]: General purpose
cmpt1, cmpt2, cmpt3: Counter variables for delays
asciibit: Binary-to-ASCII conversion helper
TEMP0, TEMP1: Voltage calculation temporaries
auxreg1, auxreg2: ADC result and intermediate calculations

Common RAM (0x70-0x7F)

Accessible from all banks:

w_temp, status_temp, FSR_temp, PCLATH_temp: Interrupt context saving
aBCD0-aBCD4: ASCII decimal representation (5 digits)
dlyLSD, dlyMSD: Delay loop counters
LCD_Char: LCD character buffer

Banks 1-3

Reserved for future expansion (currently unused).

Operational Details

Voltage Measurement Process

ADC Configuration
- Selects analog input channel 2 (RA2)
- Configures clock source with predivider ÷8
- Sets right-justified result format

Conversion Sequence

Acquisition Time → Start Conversion → Poll Status → Read Result
     (80µs)              (GO bit)         (GO=0)      (10 bits)

Voltage Calculation Algorithm The voltage subroutine performs fixed-point arithmetic:

V = ADC_Value / 1024 × 5V

Implementation (optimized shifts and adds):
- Right shift 2 bits (÷4)
- Subtract from original
- Right shift 5 bits (÷32)
- Add back original
- Right shift 1 bit (÷2)

Net effect: V ≈ ADC × 0.0048828 (approximately ADC × 5/1024)

Display Format
- Converts to ASCII decimal: "XXX.XX V"
- Leading zeros suppressed as spaces
- Fixed 2 decimal places
- Example: " 3.25 V" or " 4.98 V"

LCD Display Layout

Line 1: "Ascii :  XX.XX V"
Line 2: (unused)

Timing Characteristics

ADC Conversion Time: ~20µs (at Fosc/8 prescaler)
Display Update Rate: ~262ms (controlled by tempo routine)
Effective Sampling Rate: ~3.8 samples/second

Interrupt Framework

The code includes a comprehensive interrupt handling framework (currently unused):

Prepared Interrupt Sources

Timer 0 (TMR0): General purpose timer
RB0/INT: External interrupt
RB4-RB7: Port change interrupt
ADC: Conversion complete
USART: Serial communication (RX/TX)
SSP: Synchronous serial port
CCP1/CCP2: Capture/Compare/PWM modules
TMR1/TMR2: Additional timers
EEPROM: Write complete
Bus Collision: I²C collision detection

All interrupt handlers are present as stubs but return immediately. This provides a foundation for future enhancements.

Future Development Goals

According to the project header comments, planned enhancements include:

Hygrometer Functionality
- Replace voltage measurement with humidity sensing
- Integrate appropriate humidity sensor (capacitive/resistive)
Data Logging
- Utilize internal EEPROM (present macros: REEPROM, WEEPROM)
- Store min/max readings or time-series data
- Implement timestamp functionality
Enhanced User Interface
- Use second LCD line for additional information
- Display min/max values, trends
- Add menu system for configuration
Real-Time Features
- Enable timer interrupts for precise sampling
- Implement interrupt-driven ADC conversion
- Add alarm thresholds

Building and Programming

Required Tools

Assembler: MPASM (Microchip PIC Assembler)
IDE: MPLAB X IDE or compatible
Programmer: PICkit, ICD, or similar device

Project Files

pic_voltmeter.asm - Main program
pic_voltmeter.mcp - MPLAB Classic project
pic_voltmeter.mcw - MPLAB workspace
lcd.inc - LCD driver library
macro.inc - Macro definitions
ascii.inc - Conversion utilities
p16F873.inc - PIC16F873 register definitions (Microchip standard)

Build Process

# Using MPASM command line
mpasm pic_voltmeter.asm /p16F873

# Or use MPLAB X IDE:
# 1. Open pic_voltmeter.mcp
# 2. Build → Build Main Project
# 3. Program device with generated .hex file

Technical Specifications

Parameter	Value
Microcontroller	PIC16F873
Flash Memory	4K words (8KB)
RAM	192 bytes (4 banks × 48 bytes + 16 common)
EEPROM	128 bytes
ADC Resolution	10-bit (1024 levels)
Input Voltage Range	0V to VDD (typically 5V)
Voltage Measurement Resolution	~4.88 mV/step (5V / 1024)
Display Update Rate	~3.8 Hz
Operating Frequency	4-20 MHz (HS oscillator)

Code Style and Conventions

v

Language: Comments (original documentation)
Naming: Mixed case for variables, uppercase for macros
Indentation: Tab-based alignment
Bank Management: Explicit bank switching using BANK0/BANK1/BANK2/BANK3 macros
Page Management: Inter-page jump macros (GOTOX, CALLX) for programs >2K

Troubleshooting

Common Issues

No LCD Display
- Check PORT B connections (RB2=RS, RB3=E, RB4-7=Data)
- Verify LCD contrast adjustment
- Confirm 5V power supply
Incorrect Voltage Readings
- Verify RA2 input within 0-5V range
- Check ADC reference voltage (VDD)
- Calibrate using known voltage source
Erratic Behavior
- Ensure stable power supply
- Add decoupling capacitors (0.1µF near VDD/VSS)
- Check for proper grounding
Build Errors
- Verify all .inc files are in project directory
- Ensure p16F873.inc is in MPASM include path
- Check for MPASM version compatibility

License and Credits

This project was created by David ROBERT on February 29, 2004, as an educational/hobbyist embedded systems project. The code demonstrates fundamental PIC microcontroller programming techniques including:

ADC signal acquisition
LCD interfacing (4-bit mode)
Binary-to-ASCII conversion
Fixed-point arithmetic
Assembly language optimization

Contact

Author: David ROBERT
Project Date: February 29, 2004
Last Updated: October 8, 2025 (Documentation)

Name		Name	Last commit message	Last commit date
README.md		README.md
ascii.inc		ascii.inc
lcd.inc		lcd.inc
macro.inc		macro.inc
pic_voltmeter.asm		pic_voltmeter.asm
pic_voltmeter.mcp		pic_voltmeter.mcp

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