Low-Level PIC Microcontroller Assembly (MPASM) Library

Project Overview

A low-level bare-metal firmware library engineered in pure MPASM Assembly Language for the Microchip PIC16F873 and PIC16F84A 8-bit Harvard architecture microcontrollers, developed for advanced embedded systems coursework at the University of Brighton and awarded a First Class 79% (A).

The project focuses on register-level execution, cycle-accurate timing loops, interrupt servicing, and bus orchestration without relying on compiler abstractions or runtime overhead.

Core Circuit: PIC16F873 Reference Design

The flagship hardware system pairs two PIC16F873 microcontrollers over a full-duplex hardware UART serial bus. The master controller samples real-time analogue/digital inputs and streams command packets to the slave controller, which dynamically computes PWM waveforms and drives high-power peripheral loads.

PIC16F873 Core Circuit Schematic Figure 1: PIC16F873 reference circuit showing the 16 MHz crystal oscillator with 20 pF load capacitors, ICSP programming pins (PGD/PGC), and a current-limited LED output on RB0.

System Specifications & Registers

Feature	Hardware Implementation	Target Registers
Microcontroller	Microchip PIC16F873 (28-pin DIP, 20 MHz Crystal)	`PORTA`, `PORTB`, `PORTC`
Serial Bus	Hardware USART (Asynchronous 9600 Baud, 8-N-1)	`RCSTA`, `TXSTA`, `SPBRG`, `RCREG`
PWM Output	Hardware CCP1/CCP2 in PWM Mode with Timer 2	`CCP1CON`, `CCPR1L`, `T2CON`, `PR2`
Display Drivers	Multiplexed Common-Cathode 7-Segment Hex Displays	Direct bit latching on `PORTB` & `PORTC`
Interrupts	RX Serial Interrupt & Timer Overflow ISR	`INTCON`, `PIE1`, `PIR1`

PWM Modulation & Timer State Machine

The pulse-width modulation subsystem relies on hardware Timer 2 (TMR2) and period register PR2 to generate clean, jitter-free square waves without loading the CPU:

PWM State Flowchart & Period Modulation Figure 2: Execution flowchart for dynamic PWM duty cycle adjustment and UART packet decoding state transitions.

Register-Level MPASM Assembly Implementation

The assembly routines leverage direct bit manipulation, bank switching (STATUS, RP0), and cycle-accurate delay loops:

MPASM Assembly UART-to-PWM Firmware Card Figure 3: Carbon-style syntax-highlighted MPASM assembly showing USART configuration, bank selection, and CCP1 PWM duty cycle loading.

Assembly Firmware Modules

16F873_RX_to_PWM.asm: Hardware UART packet reception triggering dynamic PWM duty-cycle modulation.
16F873_RX_to_LEDs_PORTC.ASM: Direct byte latching and parallel register bit driving for visual diagnostics on Port C.
PWM.asm & PWM_Simple.asm: Hardware Timer 2 (TMR2) and PR2 period registers for high-efficiency pulse modulation.
SimplePWMTest.asm: Automated bench verification loop for pulse width integrity checking.

Source Code & Academic Attribution

GitHub Repository: HarryRogers073/pic-microcontroller-code
Author: Harry Rogers (University of Brighton)
Target Silicon: Microchip PIC16F873, PIC16F84A
Compilers & IDEs: Microchip MPLAB X IDE, MPASM Assembler, MPLAB XC8
Programmer: Microchip PICkit 3 / PICkit 4 In-Circuit Debugger
Attribution & Provenance:
- MPASM Assembly Firmware (firmware_asm/): All register-level assembly routines (16F873_RX_to_PWM.asm, 16F873_RX_to_LEDs_PORTC.ASM, PWM.asm, SimplePWMTest.asm) were authored and verified by Harry Rogers.
- Embedded C Drivers (firmware_c/): Differential drive, ultrasonic ranging, and sensor routines developed for the mobile sensor buggy project by Harry Rogers & Albie Gullis. The I2C character LCD driver (lcd.c) was adapted from a tutorial by Khaled Magdy (DeepBlueEmbedded).
- Microchip register include files (p16f873.inc, <xc.h>) are copyright Microchip Technology Inc.