PIC Microcontrollers with C18 Compiler

PIC Microcontroller

The microcontroller used for this project is the PIC18F4431 made by Microchip. This is an 8-bit microcontroller with motor PWM generators, a quadrature encoder interface (QEI), and other "peripherals". The microcontroller uses an external 40MHz oscillator.

The microcontroller controls two wheel motors with PWM driving external H-bridges. The H-bridges draw their power directly from the batteries, not regulated power.

Each of the wheels also has a quadrature encoder. The left wheel encoder is connected to the PIC's QEI via a schmitt trigger buffer. The right wheel encoder is wired to a U.S. Digital LS7183 chip (because the PIC only has 1 QEI) which outputs pulses on one of two pins, depending on whether the wheel is spinning backwards or forwards. The two pins are then each connected to a counter on the PIC, which keeps track of the number of pulses. The location of the wheel can be determined by subtracting the reverse-pulse counter from the forward-pulse counter.

The PIC is interfaced with the XBee modem via the RS-232 serial interface.

Programming the PIC

Setting Up

For this project, Microchip's free MPLAB IDE and free C compiler were used to program the PIC18F4431. If you are using the ICD2 programmer, be sure to follow the installation instructions carefully, or Windows may attempt to install the wrong drivers. After installing both MPLAB and C18, create a new project with the project wizard. Follow the prompts, and you should end up with an empty project if you did not add any existing files.

We must now add a linker script to your project. Make sure View>Project is checked, then right-click on Linker Scripts in the project panel and click on Add Files.... The linker scripts are in folder MCC18\lkr. Find the file that corresponds to your microcontroller (p18f4431.h (or p18f4431i.h if you are using the ICD2 programmer) for the PIC18F4431 microcontroller).

We now add source and header files in the same manner. To make a new source or header file, go to File>New, and then save the file as a .c or .h file. With the default settings, all the source and header files that you write must be in the same folder or MPLAB will give you an error message stating that it cannot find the files. You can change the environment variables by going to Project>Build Options...>Project.

Reading and Writing to Registers

The I/O ports and hardware peripherals on the PIC are controlled by SFRs, or special function registers. The values in these registers determines the behavior of the hardware. For example, Port B is an 8 channel digital I/O port that has corresponding SFRs TRISB, PORTB, and LATB (TRISB stands for Tri-State B, PORTB stands for Port B, and LATB stands for Latch B). Each of these registers is eight bits long, and each bit corresponds to a certain pin (channel) on the PIC. In this case, the least significat bit of the register affects pin RB0, and the most significant bit affects pin RB7. To write values to SFRs, we simply treat them like a global variable. Individual bits can usually be accessed by using the convention [SFR_name]bits.[bit_name], for example, PORTBbits.RB0. The names of the SFRs and bits can be found in the datasheet and the header file for the PIC (e.g. p18f4431.h).

TRISB determines whether the pins on Port B are configured as inputs or outputs. If we wanted RB0, RB1, and RB5 to be inputs and the rest to be outputs, we would write:

TRISB=0b00100011; //a "0b" prefix denotes that the following is a binary number

Of course, we could have used a hexademical or decimal number instead.

If we then wanted to set all the output ports high, we could write:

LATB=0b11111111;

LATB will not affect the pins designated as inputs.

If we wanted to read the state of pin RB0 and store the result into a variable named RB0_Status, then we would write:

RB0_Status=PORTBbits.RB0;

Notice that when we read an input, we used PORTB, and when we wrote to an output, we used LATB. LATB will give us the values that we want the output pins to be, while PORTB will give us the actual state of the pin. Writing to PORTB will usually do the same thing, but could also lead to a Read-Modify-Write problem that could distort the states of the pins.

Timers and Counters

PICs generally have several timers or counters, each with different capabilities. For the 18F4431, Timer0, Timer1, and Timer5 can be used as counters as well as timers, while Timer2 can only be used as a timer. The difference between using a Timer in timer mode or counter mode is simply the source of the pulses — a timer runs off the system clock, while a counter increments when it sees a rising/falling edge on a certain pin. Timers on the PIC can only count up. Timer2 and Timer5 can generate interrupts on a period match, so are particularly suitable for implementing a real-time operating system. Read the datasheet for a detailed explanation for the operation of these timers. In the datasheet, FOSC/4 refers to the frequency of the system oscillator divided by four.

Scalers

Since a microcontroller can count from 0 to 65535 (sixteen bits) quite quickly, prescalers and postscalers can be used to slow down the timer or counter. A 1:8 prescaler will emit one pulse for every eight pulses it receives. In essence, the scaler will decrease the frequency of the timer or counter by some factor.

Reading and Writing 16-bit registers

The registers that hold the value of the 16-bit timers are split up into two 8-bit registers (e.g. TMR0H and TMR0L). TMR0H is not the actual counter register, but a buffer. Each time TMR0L is read, TMR0H is updated with the contents of the actual high byte of the timer register. Therefore, you must read TMR0L before you read TMR0H. This method prevents you from reading an erroneous vale due to an overflow or rollover in TMR0H while you are reading TMR0L.

When writing to a 16-bit register, you must write the high byte first, and both bytes will be loaded into the register simultaneously when you write the low byte.

(A full explanation can be found in Section 11.4 of the datahseet.)

PWM

The PIC18F4431 has four Power Control PWM modules, as well as two CCP (Capture/Compare/PWM modules). This section will discuss the Power Control PWM modules (see section 17 in the datasheet). These PWM modules have the same frequency, but each can have its own duty cycle. The core of the PWM module is a 12-bit timer, whose count is kept by the eight bits of SFR PTMRL and the four LSBs of PTMRH. This timer will increment once every four clock cycles if the prescale is 1:1 (prescaling is determined by bits 3-2 of PTCON0). Another 12-bit SFR, PTPER (which is split into PTPERH and PTPERL), determines the period of the PWM. Each time PTMER reaches the same value as PTPER, it will reset to zero and start counting again if you used the default free-running count mode. (You can also make it start counting downwards; if you use the up-down counting mode; see the datasheet for more information on this.) PTMR and PTPER is shared all four PWM modules.

Each of the four PWM modules has a 14-bit register PDC (PDC0L and PDC0H for PWM module 0) which specifies the duty cycles for that module. The output of the PWM module is set high when PTMR is less than the most significant 12 bits of PDC, and the output is set low when PTMR is greater than the most significant 12 bits of PDC but less than PTPER. When PTMR is equal to PTPER, PTMR is reset to zero and the PWM module's output is set high again. For example, if the most significant 12 bits of PDC is equal to 50 and PTPER is equal to 80, the PWM module's output will be high while PTMR is less than 50, and will be set low when PTMR is equal to 50. Once PTMR reaches 80, it will be reset to zero and the output is set high again. When the prescaler is 1:1, then the 2 least significant bits of PDC come into play. These two bits give you another two bits of resolution by allowing you to specify which of the four clock cycles you want the output to go low on (remember, PTMR only increments every four clock cycles). Using the same numbers as in the previous example, the last two bits basically let you set PDC to be 50.0, 50.25, 50.5, or 50.75. If the prescaler is not 1:1, then these two bits are ignored.

Code

main.c

In your main source file, which we will call main.c. At the top, you should include the line

#include <p18f4431.h>

This header file contains the definitions and names of the Special Function Registers (e.g. PORTA, TRISA, etc). By default, MPLAB will search the MCC18/h folder and the folder your main source file is in for .h header files.

Microchip's C18 compiler also comes with a library of functions. The documentation for the library can be found in the MCC18/Doc folder, if you choose to install them when installing C18. Otherwise, they can be found on Microchip's website. Since our program will be using the serial port, we include the serial port functions with

#include <usart.h>

For this project, we will also include a file called main.h. This file will contain macros and miscellaneous functions.

If we want to implement real-time operations (for example, having some take performed exactly once every second), then the speed of our oscillator is important. The clock speed is often used to calculate periods and frequencies, so in main.h, we define the frequency of the oscillator clock as FOSC. It takes four instruction cycles to execute one instruction, so the clock frequency divided by four is also a useful number. We define this number as FOSC4 in main.h. If you change your clock frequency, be sure to remember to edit this section.

Tips

• If MPLAB has trouble connecting with the ICD2, open the Device Manager (Control panel>System>Hardware>Device Manager) and disable then re-enable the ICD2. Also be sure that you followed in installation instructions in the manual when you installed the ICD2.

Documents

PIC18F4431 Datasheet PIC18F4431 Errata PIC18F4431 Configuration Settings