Microchip PIC16F873 Microcontroller Architecture and Application Design
The Microchip PIC16F873 is a cornerstone of embedded systems design, belonging to the renowned mid-range PIC16F87x family. This powerful 8-bit CMOS microcontroller, built around an enhanced RISC architecture, has been a popular choice for decades due to its robust feature set, ease of use, and cost-effectiveness. Its architecture and peripheral integration make it suitable for a vast array of applications, from industrial automation to consumer electronics.
Architectural Overview
At the heart of the PIC16F873 lies a Harvard architecture, which features separate buses for program and data memory. This separation allows for concurrent access, significantly enhancing throughput and performance over traditional von Neumann architectures.
The core executes a 35-word instruction set with a two-stage pipeline, enabling all instructions to execute in a single cycle, except for program branches which take two. It operates at a maximum clock frequency of 20 MHz, yielding a 200 ns instruction cycle time.
Key architectural components include:
Program Memory (Flash): 4K x 14 words of reprogrammable flash memory, allowing for up to 10,000 erase/write cycles. This facilitates rapid prototyping and firmware updates in the field.
Data Memory (RAM): 192 x 8 bytes of general-purpose RAM (SRAM) for data storage and manipulation during program execution.
EEPROM Data Memory: 128 x 8 bytes of independent data EEPROM. This non-volatile memory is crucial for storing critical data like calibration constants or device parameters that must be retained even after a power loss.
Peripheral Features: A key strength of the PIC16F873 is its rich set of integrated peripherals:
Three I/O Ports (PORTA, PORTB, PORTC): Providing up to 22 programmable I/O pins, each with individual direction control.
Five-Channel 10-Bit ADC: An analog-to-digital converter essential for interfacing with real-world sensors (e.g., temperature, pressure, potentiometers).
Two Timers (Timer0, Timer1): 8-bit and 16-bit timers/counters with prescalers for generating precise time delays or counting external events.
One Timer (Timer2): An 8-bit timer with a period register and prescaler/postscaler, primarily used for PWM generation.
Two Capture/Compare/PWM (CCP) Modules: These versatile modules can operate in Capture mode (to record the time of an event), Compare mode (to generate an output on a match), or PWM mode (to generate pulse-width modulated signals for motor control or power conversion).
Universal Synchronous Asynchronous Receiver Transmitter (USART): A serial communication module for establishing full-duplex UART (RS-232, RS-485) or synchronous (SPI-like) links with peripherals or a PC.
Serial Peripheral Interface (SPI) and Inter-Integrated Circuit (I²C): Supporting communication with a wide range of serial peripheral chips like memory, RTCs, and sensors.
Application Design Considerations
Designing with the PIC16F873 involves a structured process. After defining the application requirements, the designer must strategically allocate the limited hardware resources.
1. I/O Planning: The 22 I/O pins are multiplexed with peripheral functions. The designer must carefully configure the TRIS and ANSEL registers to define each pin as digital input/output or analog input, ensuring no conflicts between peripherals.
2. Clock Source Selection: The system clock can be derived from a precise external crystal, a resonator, or an internal RC oscillator, chosen based on the application's need for accuracy versus cost and board space.
3. Analog-to-Digital Conversion: Utilizing the ADC requires configuring the acquisition time, clock source, and voltage reference (VREF). Proper PCB layout is critical to minimize noise and ensure accurate analog readings.
4. Communication Protocol Implementation: For system connectivity, the appropriate protocol (UART, SPI, I²C) is selected and configured. The MSSP module is central to implementing I²C and SPI communications.
5. Interrupt Handling: The controller features multiple interrupt sources (TIMER1 overflow, ADC conversion complete, USART receive, etc.). Efficient firmware design requires a well-structured Interrupt Service Routine (ISR) to prioritize and respond to these events promptly.
6. Power Management: The PIC16F873 offers sleep modes and peripheral disable features, which are vital for designing battery-powered, low-energy applications.
A typical application, such as a smart temperature controller, would use the ADC to read a thermistor, the CCP module in PWM mode to drive a fan or heater, and the USART to relay data to a display or a central monitoring system, all controlled by logic implemented in the firmware.
The PIC16F873 remains a quintessential example of a highly integrated 8-bit microcontroller. Its balanced architecture, combining a simple yet efficient CPU core with a comprehensive peripheral set, empowers designers to create sophisticated embedded systems with minimal external components. While newer microcontrollers offer more memory and advanced features, the principles of design learned on the PIC16F873 are universally applicable, solidifying its role as an enduring and educational tool in the embedded world.
Keywords:
1. Harvard Architecture
2. RISC
3. Peripheral Integration
4. Embedded Systems
5. CCP Module
