Debugging C on a PIC

The ICD can be used just as a programmer, to transfer compiled code to the PIC and start it running. The ICD can also be used as an interactive debugger, to monitor code execution, insert breakpoints, and let you probe variable values.

To use it in debugger mode you can insert the statement #DEVICE ICD=TRUE at the top of your program, or, just recompile using the Debug menu item. Either way, some debugging and communication code is added, which you will want to remove later to save space and time.

The debugger is wonderful, especially in its ability to let you set breakpoints and probe variable values. However for an embedded computer working with real-time hardware, a lot of problems can't be investigated except in actual operation at full speed. So, besides the debugger, here are some other ways to debug a running program on a PIC:

Illuminate LEDs in your program, to monitor program flow or variable values.

For fast signals, drive an output pin high/low, and monitor with an oscilloscope.

Set up RS232 communication, so your program can use printf statements to write to a 2-line LCD device, or to a serial port window on your laptop. PIC C has no scanf function, so it can't read numbers that you type in a text window. It does has functions for reading characters and strings.

• tip - don't try to debug a whole program at once. Get each part working individually. Write small test programs if necessary to help you get each part working.

• warning - a variable defined outside the scope of main() or any function is global, and is used in all of them. If you define a commonly used variable such as i as a global, your functions may inadvertently overwrite it. Very mysterious behavior results (e.g. loops never finish.)

• warning - because many variables will be int8s, you need to be especially aware of how type conversion is handled. For example int16value = int8value100 + int8value200 will give a result of 44.

• help - read the PIC C book. Search the CCS forum.

Using the debugger The CCS C Windows IDE is a software package that includes a source code editor, a C compiler, and an interface with a programming device (such as the ICD-U40). Together, these three components make up an Integrated Development Environment, or IDE. The general purpose of the IDE is to allow the user to create and edit a source code in C, then compile and program that code into a PIC microcontroller. Install and Setup Installing the CCS C Windows IDE is quite straightforward. Simply run the setup file from the C Compiler installation disk, and follow the on screen instructions. At a certain point in the installation, the installer will request a directory to be used for program storage. The default will be C:\Program Files\PICC\Projects , however you may wish to change this depending on where you would like your projects to be stored. Project Basics The basic process taken when developing and programming in Windows IDE follows a few steps. First, a source code is created using C programming language. Next, using the Compile button (in the Compile Menu) a number of different files are created, including a .hex file which is used directly to program the MCU. Finally, the .hex file is loaded onto the MCU using the Program Chip button (also in the Compile Menu). Although the programming done in the Windows IDE is done by directly editing only the source file, many other files will be generated upon compiling the source file. All of these files will be saved into the same directory as the source file, and will have the same name (with different file extensions). A list of file extensions and their significance is given below.

Extension	Significance
.c	Source code file
.cof	Binary machine code file
.err	Listing and description of any errors
.esym	IDE file containing comment information and definitions from header file
.hex	Programmable output file compatible with all programmers
.lst	Line by line listing of source code along with generated assembly code
.pjt	Main project file
.sta	Memory usage summary (statistics)
.sym	List of symbols used and their corresponding registers
.tre	Shows every function used in the source code, and the memory used by each

Debugging As the name suggests, the ICD-U40 is an "In Circuit Debugger", which allows the Windows IDE to connect with the MCU and control the running of the programmed MCU via the debugger window. This tool is extremely useful, as it allows the user to follow the execution of code on a step-by-step basis in order to work out any errors.

Starting the Debugger To use the debugger, first verify that the hardware (ICD-U40 and PIC MCU) is connected properly. In order to allow the IDE to enter into debug mode, you must have the following in your code:

  #device ICD=TRUE

Without the above section of code, the debugger will display a warning instructing you to include it. You can chose to enter debug mode once the warning is displayed, but to avoid it all together, use the above chunk of code. Once that is set, the Enable Debugger button in the Debug Menu will automatically compile your source code, load the code onto the MCU, and enter into debug mode. This will open a debug window where all of the commands for debugging are displayed.

Debugger Tools The main toolbar in the debug window is at the top, and is shown below, both while stopped and while running.

These are the basic tools of the debugger. The Run and Stop buttons will start and halt the execution of the program, and the Reset button reloads the code, as it did when the debugger was initially enabled. The Step Over and Step Into buttons are very useful in debugging, as they allow the user to step through their program and watch how it progresses. The Step Into button will execute one line of source code every time it is pressed. The Step Over button works in much the same way, only when a function call is encountered, it will step through the entire function rather than the function's individual steps. The Run to Cursor button will run the program until it reaches the point where your cursor is in the source code window. Clicking the Snapshot button will open a window where certain details about the current state of the system can be selected to be saved in a text file, or sent to a printer.

Watches

Another quite useful capability of the Windows IDE debugger is setting and checking watches. In the debug window there is a tab titled Watches that will look like the picture to the right. A watch functions to provide a value for a certain variable as it changes throughout the course of running the program. If you are stepping through a program, the watch tab will display a value for each variable after each step. If you are running the program, the watch tab will display a value for each variable once the program has been halted.

To set a watch, click on the + symbol and select the variable to be watched from the drop-down menu labelled "Variables in scope at cursor location", or type in the name of the variable (in correct C syntax).

Windows IDE Details Apart from specific uses of the Windows IDE, there are a lot of little details that make working with it easier. This section details a few.

Cursor-Over Reading In debug mode, if the cursor is held over a variable in the source code window, the value of that variable will be displayed. This is akin to reading a watch.

Open Files at Cursor When outside files are referenced in a source code (header files, etc.) the cursor can be placed on any part of the file name, and right clicking will give the option to "open file at cursor". This will open the selected file in a new tab in the Windows IDE.

Datasheets Along with installing the IDE itself, the installation disk included a library of PIC microcontroller datasheets. Right clicking in the source code window will give the option to "view datasheet". Clicking this will open a window where the specific PIC microcontroller can be specified, and the datasheet can be opened. This window can also be reached by clicking the Datasheet button in the View menu. The .pdf file will open in a new window.

Valid Fuses and Interrupts Also in the View menu are two buttons: Valid Fuses and Valid Interrupts. These buttons will open new windows that show all of the possible interrupts and fuses for the specific PIC microcontroller.

References PIC MCU C Compiler Reference Manual, Custom Computer Services, Inc., 2006.