Unit 2: PIC I/O Ports and Timers

Introduction to PIC I/O Ports

PIC microcontrollers have versatile Input/Output (I/O) ports that allow them to interact with external devices such as sensors, switches, LEDs, and motors. Understanding the I/O port structure and how to manipulate these ports is crucial for designing embedded systems. This section delves into the structure of I/O ports, how they are programmed, and how to manipulate individual bits of an I/O port using Embedded C.

I/O Port Structure

The I/O port structure in PIC microcontrollers is relatively straightforward but highly efficient. Each I/O port is made up of several pins (typically 8 pins per port), which can be configured individually as either input or output. The number of I/O ports varies depending on the specific PIC microcontroller model.

For example, the PIC18F452 has five I/O ports: PORTA, PORTB, PORTC, PORTD, and PORTE. Each port consists of 8 pins (except PORTE, which has 3 pins in some models).

• PORTA (RA0 - RA7): General-purpose I/O, also shares analog input functionality.
• PORTB (RB0 - RB7): General-purpose I/O, RB0 has an additional interrupt function.
• PORTC (RC0 - RC7): General-purpose I/O, includes UART, SPI, and I2C communication pins.
• PORTD (RD0 - RD7): General-purpose I/O, often used for external memory interfacing.
• PORTE (RE0 - RE2): General-purpose I/O, also has analog input capabilities.

Data Direction Registers

Each I/O port is controlled by two primary registers:

1. TRIS Register (Tri-state register): This register configures the direction of the pins on the port.

   • If a bit in the TRIS register is set to 1, the corresponding pin is configured as an input.
   • If a bit in the TRIS register is set to 0, the corresponding pin is configured as an output.

   Example:

   • TRISA = 0xFF; // All pins of PORTA set as input.
   • TRISB = 0x00; // All pins of PORTB set as output.

2. PORT Register: This register holds the actual values on the I/O pins. When reading from the port, the microcontroller reads the state of the pins. When writing to the port, the microcontroller sets the state of the output pins.

   • For output pins: Writing to the PORT register affects the output voltage (high or low) on the respective pin.
   • For input pins: Reading from the PORT register retrieves the logic level present on the input pin.

   Example:

   • PORTA = 0x55; // Output binary value 01010101 to PORTA.
   • x = PORTB; // Read the value from PORTB.

I/O Port Programming

I/O port programming involves configuring the TRIS register to set the direction (input/output) and manipulating the PORT register to read or write data to the port.

Example: Basic I/O Port Programming

#include <xc.h> // Header file for PIC18 microcontroller
 
void main(void) {
    // Set PORTB as output and PORTA as input
    TRISB = 0x00;  // All pins of PORTB are set as output
    TRISA = 0xFF;  // All pins of PORTA are set as input
 
    while(1) {
        if(PORTA & 0x01) {  // Check if RA0 is high (button press)
            PORTB = 0xFF;   // Turn on all LEDs connected to PORTB
        } else {
            PORTB = 0x00;   // Turn off all LEDs connected to PORTB
        }
    }
}

In this example, the program reads the state of the first pin of PORTA (RA0). If the pin is high, all the LEDs connected to PORTB are turned on. Otherwise, they remain off.

I/O Bit Manipulation Programming

In embedded systems, sometimes it is necessary to manipulate individual bits of an I/O port rather than working with the entire port at once. This process is known as bit manipulation.

Example: Bit Manipulation

PORTB |= (1 << 2);  // Set RB2 high (1)
PORTB &= ~(1 << 2); // Set RB2 low (0)

In this example:

• PORTB |= (1 << 2); sets the third bit (RB2) of PORTB to high.
• PORTB &= ~(1 << 2); clears the third bit (RB2) of PORTB to low.

Timers and Counters in PIC Microcontrollers

Timers and counters are critical features of microcontrollers. They allow for the measurement of time intervals, generation of time delays, and event counting. In PIC microcontrollers, timers/counters can be configured to operate either as timers (counting clock cycles) or counters (counting external events).

Registers Used for Timer/Counter Operation

Each timer in a PIC microcontroller is controlled by several registers. In the case of the PIC18F series, there are multiple timers (e.g., Timer0, Timer1, Timer2, etc.), and each of them has dedicated control registers.

1. TMRx: This is the main timer register that holds the current value of the timer.
2. TCON: The timer control register is used to configure the timer's mode of operation (e.g., start/stop, enable/disable interrupts).
3. PRx: This is the period register, which determines the timer overflow interval.

Example: Timer0 Control Register (T0CON)

For Timer0, the T0CON register is used to configure the operation of the timer.

• TMR0ON: Enables/disables the Timer0.
• T08BIT: Selects between 8-bit and 16-bit mode for the timer.
• T0CS: Selects the clock source (internal or external).
• PSA: Determines whether a prescaler is used for the timer.

Delay Calculations

When using a timer to generate a delay, you must calculate the appropriate initial value for the timer register, based on the desired delay, the clock frequency, and the prescaler value.

Programming of Timers Using Embedded C

The following example demonstrates how to configure and use Timer0 to generate a delay using Embedded C.

#include <xc.h>
 
 // Header file for PIC18 microcontroller
 
void delay_ms(unsigned int ms) {
    for(unsigned int i = 0; i < ms; i++) {
        T0CON = 0x07;  // Set Timer0 with 1:256 prescaler
        TMR0 = 240;    // Load TMR0 register
        T0CONbits.TMR0ON = 1; // Start Timer0
 
        while(INTCONbits.TMR0IF == 0); // Wait for the overflow flag
        INTCONbits.TMR0IF = 0; // Clear overflow flag
        T0CONbits.TMR0ON = 0;  // Stop Timer0
    }
}
 
void main(void) {
    TRISB = 0x00; // Configure PORTB as output
 
    while(1) {
        PORTB = 0xFF; // Turn on all LEDs
        delay_ms(1000); // Delay of 1 second
        PORTB = 0x00; // Turn off all LEDs
        delay_ms(1000); // Delay of 1 second
    }
}

In this example:

• Timer0 is configured with a 1:256 prescaler.
• The timer is loaded with the initial value of 240 to generate a 1 ms delay.
• A loop toggles all pins on PORTB on and off with a 1-second delay.