Writing Code with the C18 Compiler

Setting Up the Project

If you are using the ICD2 programmer, be sure to follow the installation instructions carefully, or Windows may attempt to install the wrong drivers. If you've installed the wrong drivers, follow instructions in the user guide to remove the 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, and that the Files tab is selected in the Project window. 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 by right-clicking on Source Files and Header Files, and selecting Add Files...'. 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. Documentation for the ICD2 Programmer

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.

Pre/Post-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 count once for every eight counts 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

(Beware that equations 17.1, 17.2, and 17.3 in the PIC18F4431 datasheet are wrong. See the errata for the correct equations.)

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 essentially 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.

Quadrature Encoder Interface

Some PICs, such as the PIC18F4431, have a built-in quadrature encoder interface. The PIC reads the two channels from the encoder, and will increment or decrement a counter, depending on which way the encoder is spinning. This value of this counter is read in the exact same way one would read any other counter or timer.