Chapter 9: Arrays, Time, and Functional Debugging

Jonathan Valvano and Ramesh Yerraballi


In this chapter, we will illustrate a formal method for testing. Because embedded systems are deployed in safety-critical systems, we need to be rigorous in our methods that evaluate if the deployed system performs its tasks as required. Embedded systems not only need to arrive at the correct answer, they need to arrive at it at the correct time. We will introduce a simple hardware counter that we can use to measure time. The testing method we will develop in this chapter will be to create a data logger to store when and what our system is doing.


Learning Objectives:

  • Understand the concept of minimally intrusive debugging
  • Learn how the SysTick counter works
  • Learn about arrays
  • Learn how to use indexing to access arrays
  • Understand precision, length and origin
  • Learn how to create a debugging dump
  • Understand that a formal method to verify program correctness
  • Learn how to use the dump to collect debugging information in real time


        Video 9.0. Introduction to Arrays and Functional Debugging

9.1. Debugging Theory

Every programmer is faced with the need to debug and verify the correctness of his or her software. A debugging instrument is hardware or software used for the purpose of debugging. In this class, we will study hardware-level probes like the logic analyzer, oscilloscope, and Joint Test Action Group (JTAG standardized as the IEEE 1149.1) interface; software-level tools like simulators, monitors, and profilers; and manual tools like inspection and print statements. Nonintrusiveness is the characteristic or quality of a debugger that allows the software/hardware system to operate normally as if the debugger did not exist. Intrusiveness is used as a measure of the degree of perturbation caused in system performance by the debugging instrument itself. For example, a print statement added to your source code is very intrusive because it significantly affects the real-time interaction of the hardware and software. It is important to quantify the intrusiveness of an instrument. Let t be the average time it takes to run the software code comprising the debugging instrument. This time t is how much less time the system has, in order to perform its regular duties. Let Δt be the average time between executions of the instrument. A quantitative measure of intrusiveness is t/Δt, which is the fraction of the time consumed by the process of debugging itself. A debugging instrument is classified as minimally intrusive if it has a negligible effect on the system being debugged. In other words, if t/Δt is so small that the debugging activities have a finite but inconsequential effect on the system behavior, we classify it as minimally intrusive. In a real microcomputer system, breakpoints and single-stepping are intrusive, because the real hardware continues to change while the software has stopped. When a program interacts with real-time events, the performance can be significantly altered when using intrusive debugging tools. On the other hand, we will learn later in this chapter that dumps, dumps with filter, and monitors (e.g., which output strategic information on LEDs or an OLED display) are much less intrusive. A logic analyzer that passively monitors the activity of the software is completely nonintrusive. Interestingly, breakpoints and single-stepping on a mixed hardware/software simulator are often nonintrusive, because the simulated hardware and the simulated software are affected together.

For example, the heartbeat code GPIO_PORTF_DATA_R ^= 0x02; requires only 6 bus cycles to execute. If the heartbeat runs every 1ms, and the bus clock is 80 MHz, then is equal to 6/80000. Normally, if this ratio is less than 1/1000 we classify it minimally intrusive.

: What does it mean for a debugging instrument to be minimally intrusive? Give both a general answer and a specific criterion.

Research in the area of program monitoring and debugging mirrors the rapid pace of developments in other areas of computer architecture and software systems. Because of the complexity explosion in computer systems, effective debugging tools are essential. The critical aspect of debugging an embedded system is the ability to see what the software is doing, where it is executing, and when it did it, without the debugger itself modifying system behavior. Terms such as program testing, diagnostics, performance debugging, functional debugging, tracing, profiling, instrumentation, visualization, optimization, verification, performance measurement, and execution measurement have specialized meanings, but they are also used interchangeably, and they often describe overlapping functions. For example, the terms profiling, tracing, performance measurement, or execution measurement may be used to describe the process of examining a program from a time perspective. But, tracing is also a term that may be used to describe the process of monitoring a program state or history for functional errors, or to describe the process of stepping through a program with a debugger. Usage of these terms among researchers and users vary.

Furthermore, the meaning and scope of the term debugging itself is not clear. In this class the goal of debugging is to maintain and improve software, and the role of a debugger is to support this endeavor. The debugging process is defined as testing, stabilizing, localizing, and correcting errors. Although testing, stabilizing, and localizing errors are important and essential to debugging, they are auxiliary processes: the primary goal of debugging is to remedy faults or to correct errors in a program. Stabilization is process of fixing the inputs so that the system can be run over and over again yielding repeatable outputs.

Although, a wide variety of program monitoring and debugging tools are available today, in practice it is found that an overwhelming majority of users either still prefer or rely mainly on “rough and ready” manual methods for locating and correcting program errors. These methods include desk-checking, dumps, and print statements, with print statements being one of the most popular manual methods. Manual methods are useful because they are readily available, and they are relatively simple to use. But, the usefulness of manual methods is limited: they tend to be highly intrusive, and they do not provide adequate control over repeatability, event selection, or event isolation. A real-time system, where software execution timing is critical, usually cannot be debugged with simple print statements, because the print statement itself will require too much time to execute.

Black-box testing is simply observing the inputs and outputs without looking inside. Black-box testing has an important place in debugging a module for its functionality. On the other hand, white-box testing allows you to control and observe the internal workings of a system. A common mistake made by new engineers is to just perform black box testing. Effective debugging uses both. One must always start with black-box testing by subjecting a hardware or software module to appropriate test-cases. Once we document the failed test-cases, we can use them to aid us in effectively performing the task of white-box testing.

A print statement is a common example of a debugging instrument. Using the editor, one adds print statements to the code that either verify proper operation or illustrate the programming errors. If we test a system, then remove the instruments, the system may actually stop working, because of the importance of timing in embedded systems. If we leave debugging instruments in the final product, we can use the instruments to test systems on the production line, or test systems returned for repair. On the other hand, sometimes we wish to provide a mechanism to reliably and efficiently remove all instruments when the debugging is done. Consider the following mechanisms as you develop your own unique debugging style.

Place all instruments in a unique column, so you can easily distinguish instruments from regular programs.

Define all debugging instruments as functions that all have a specific pattern in their names. In this way, the find/replace mechanism of the editor can be used to find all the calls to the instruments.

Define the instruments so that they test a run time global flag. When this flag is turned off, the instruments perform no function. Notice that this method leaves a permanent copy of the debugging code in the final system, causing it to suffer a runtime overhead, but the debugging code can be activated dynamically without recompiling. Many commercial software applications utilize this method because it simplifies “on-site” customer support.

Use conditional compilation (or conditional assembly) to turn on and off the instruments when the software is compiled. When the assembler or compiler supports this feature, it can provide both performance and effectiveness.


The emergence of concurrent languages and the increasing use of embedded real-time systems place further demands on debuggers. The complexities introduced by the interaction of multiple events or time dependent processes are much more difficult to debug than errors associated with sequential programs. The behavior of non-real-time sequential programs is reproducible: for a given set of inputs their outputs remain the same. In the case of concurrent or real-time programs this does not hold true. Control over repeatability, event selection, and event isolation is even more important for concurrent or real-time environments.

: Consider the difference between a runtime flag that activates a debugging command versus an assembly/compile-time flag. In both cases it is easy to activate/deactivate the debugging statements. For each method, list one factor for which that method is superior to the other.

: What is the advantage of leaving debugging instruments in a final delivered product?   

Observation: There are two important components of debugging: having control over events and being able to see what is happening. Remember: control and observability!

Common Error: The most common debugging mistake new programmers make is to simply observe the overall inputs and outputs system without looking inside the device. Then they go to their professor and say, “My program gives incorrect output. Do you know why?”


9.2. SysTick Timer

Before we describe the process of instumentation, we will discus a feature that exists on all CortexÔ-M microcontrollers, a timer, called SysTick. Therefore, the use of SysTick in designing your system will assure you that your system will easily port to other Cortex-M microcontrollers. SysTick is a simple counter that we can use to create time delays and generate periodic interrupts. Table 9.1 shows the register definitions for SysTick. The basis of SysTick is a 24-bit down counter that runs at the bus clock frequency. There are four steps involved in the initialization of the SysTick timer. First, we clear the ENABLE bit to turn off SysTick during initialization. Second, we set the RELOAD register. Third, we write to the NVIC_ST_CURRENT_R value to clear the counter. Lastly, we write the desired mode to the control register, NVIC_ST_CTRL_R. The mode involves the CLK_SRC and the INTEN bits. We set the CLK_SRC bit specifying the core clock will be used. We will set CLK_SRC=1, so the counter runs off the system clock. Later, we will set INTEN to enable interrupts, but in this first example we clear INTEN so interrupts will not be requested. We need to set the ENABLE bit so the counter will run. Once the initialization is complete, the timer starts to count down, i.e., CURRENT is decremented once every clock tick.
When the CURRENT value counts down from 1 to 0, the COUNT flag is set. On the next clock, the CURRENT is loaded with the RELOAD value. In this way, the SysTick counter (CURRENT) is continuously decrementing. If the RELOAD value is n, then the SysTick counter operates at modulo n+1 (…n, n-1, n-2 … 1, 0, n, n-1, …). In other words, it rolls over every n+1 counts. In this chapter we set RELOAD to 0x00FFFFFF, so the CURRENT value is a simple indicator of what count is now. Noting what the count was at some point and then what it is now, allows us to calculate the time that has elapsed






















24-bit RELOAD value




24-bit CURRENT value of SysTick counter


Table 9.1. SysTick registers.


Without activating the phase-lock-loop (PLL), our TM4C123 LaunchPad will run at 16 MHz, meaning the SysTick counter decrements every 62.5 ns. If we activate the PLL to run the microcontroller at 80 MHz, then the SysTick counter decrements every 12.5 ns. In general, if the period of the core bus clock is t, then the COUNT flag will be set every (n+1)t. Note that, reading the NVIC_ST_CTRL_R control register will return the COUNT flag in bit 16. The act of reading this register when the COUNT flag is set will automatically clear it (post read). Also, writing any value to the NVIC_ST_CURRENT_R  register will reset the counter to zero and clear the COUNT flag. Program 9.1 initializes the SysTick. To determine the time, one simply reads the NVIC_ST_CURRENT_R register.


#define NVIC_ST_CTRL_R      (*((volatile unsigned long *)0xE000E010))

#define NVIC_ST_RELOAD_R    (*((volatile unsigned long *)0xE000E014))

#define NVIC_ST_CURRENT_R   (*((volatile unsigned long *)0xE000E018))

void SysTick_Init(void){

  NVIC_ST_CTRL_R = 0;              // 1) disable SysTick during setup

  NVIC_ST_RELOAD_R = 0x00FFFFFF;   // 2) maximum reload value

  NVIC_ST_CURRENT_R = 0;           // 3) any write to current clears it

  NVIC_ST_CTRL_R = 0x00000005;     // 4) enable SysTick with core clock


Program 9.1. Initialization of SysTick.


                     Video 9.1. Working of the Systick Timer

Program 9.2 shows how to measure the elapsed time between calls to a function. Assume the system calls the function Action() over and over. The variable Now is the time (in 12.5ns units) when the function has been called. The variable Last is the time (also in 12.5ns units) when the function was called previously. To measure elapsed time, we perform a time subtraction. Since the SysTick counts down we subtract Last-Now. Since the time is only 24 bits and the software variables are 32 bits we “and” with 0x00FFFFFF to create a 24-bit difference.


unsigned long Now;      // 24-bit time at this call (12.5ns)

unsigned long Last;     // 24-bit time at previous call (12.5ns)

unsigned long Elapsed;  // 24-bit time between calls (12.5ns)

void Action(void){      // function under test

  Now = NVIC_ST_CURRENT_R;         // what time is it now?

  Elapsed = (Last-Now)&0x00FFFFFF; // 24-bit difference

  Last = Now;                      // set up for next



Program 9.2. Use of SysTick to measure elapsed time.

The first measurement will be wrong because there is no previous execution from which to measure. The system will be accurate as long as the elapsed time is less than 0.209 second. More precisely, as long as the elapsed time is less than 224*12.5ns. This is similar to the problem of using an analog clock to measure elapsed time. For example you notice the clock says 10:00 when you go to sleep, and you notice it says 7:00 when you wake up. As long as you are sure you slept less than 12 hours, you are confident you slept for 9 hours.

Our TM4C123 microcontroller has some 32-bit and some 64-bit timers, but we will use SysTick because it is much simpler to configure. We just have to be aware that we are limited to 24 bits.

. If we activate the PLL and change the bus clock to 50 MHz (20ns), what is the longest elapsed time we could measure with Program 9.2?

9.3. Arrays


                     Video 9.2. Introduction to Arrays

In developing our instrument we will need a place to put data. One of the simplest and fastest places to store data is in RAM memory. The TM4C123 has 32 kibibytes of RAM, and we can use it to store temporary data. If the information is a constant, and we know its values at compile time, we can place the data in ROM. The TM4C123 has 256 kibibytes of flash ROM, and we can use it to store constant data.

Random access means one can read and write any element in any order. Random access is allowed for all indexable data structures. An indexed data structure has elements of the same size and can be accessed knowing the name of the structure, the size of each element, and the element number. In C, we use the syntax [] to access an indexed structure. Arrays, matrices, and tables are examples of indexed structures available in C.

Sequential access means one reads and writes the elements in order. Pointers are usually employed in these types of data structures. Strings, linked-lists, stacks, queues, and trees are examples of sequential structures. The first in first out circular queue (FIFO) is useful for data flow problems.

An array is made of elements of equal precision and allows random access. The precision is the size of each element. Typically, precision is expressed in bits or bytes. The length is the number of elements. The origin is the index of the first element. A data structure with the first element existing at index zero is called zero-origin indexing. In C, zero-origin index is the default. For example, Data[0] is the first element of the array Data.

Just like any variable, arrays must be declared before they can be accessed. The number of elements in an array is determined by its declaration. Appending a constant expression in square brackets to a name in a declaration identifies the name as the name of an array with the number of elements indicated. Multi-dimensional arrays require multiple sets of brackets. These examples illustrate valid declarations of arrays. Because there is no const modifier, the arrays will be defined in RAM. The C compiler will add code that runs before your main, which will initialize all RAM-based variables to zero.

short Data[5];      // allocate space for 5 16-bit integers

long scores[20];    // allocate space for 20 32-bit integers

int Width[6];       // 6 signed, precision depends on compiler

char Image[5][10];  // allocate space for 50 8-bit integers

short Points[5][5][5];// allocate space for 125 16-bit ints


If you would like the compiler to initialize to something other than zero, you can explicitly define the initial values. Again, this initialization will occur before your main is executed. Even though they have nonzero initial values, these arrays are in RAM, and thus can be modified at run time.

short Data[5]={1,-1,2,-2,6};         

long scores[20]={-2,-1,0,1,1,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2}

int Width[6]={-10,-20,10,100,2000,40};    

short Image[5][10]={







Arrays in Memory

Figure 9.1. The Data array has 5 16-bit elements, and the scores array has 20 32-bit elements.


If the array contains constants, and we know the values at compile time, we can place the data in ROM using the const modifier. For ROM-based data, we must define the value explicitly in the software. These arrays are in ROM, and thus cannot be modified at run time.

const short Data2[5]={1,2,3,4,5};         

const long scores2[20]={-2,-1,0,1,1,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2}

const int Width2[6]={-10,-20,10,100,2000,40};    

const short Image2[5][10]={







In a formal way, const actually means “constant”. More specifically it means the executing software cannot change the value at run time. This means in C programs not implemented on an embedded system, const data could be stored in RAM, initialized at startup. In this case, the const modifier simply means the running program cannot change the values. However, in the class we are writing C code that will be deployed into a microcontroller as part of an embedded system, so const data will be stored in ROM.

When an array element is referenced, the subscript expression designates the desired element by its position in the data. The first element occupies position zero, the second position one, and so on. It follows that the last element is subscripted by [N-1] where N is the number of elements in the array. The statement

scores[9] = 0;

for instance, sets the tenth element of the array to zero. The array subscript can be any expression that results in an integer. The following for-loop clears 20 elements of the array data to zero.

for(j=0;j<20;j++) scores[j] = 0;

If the array has two dimensions, then two subscripts are specified when referencing. As programmers we may any assign logical meaning to the first and second subscripts. For example we could consider the first subscript as the row and the second as the column. Then, the statement

TheValue = Image[3][5];

copies the information from the 4th row 6th column into the variable TheValue. If the array has three dimensions, then three subscripts are specified when referencing. Again we may assign any logical meaning to the various subscripts. For example we could consider the first subscript as the x coordinate, the second subscript as the y coordinate and the third subscript as the z coordinate. Then, the statement


sets the values at position (2,3,4) to 100. Array subscripts are treated as signed integers. It is the programmer's responsibility to see that only positive values are produced, since a negative subscript would refer to some point in memory preceding the array. One must be particularly careful about assuming what existing either in front of or behind our arrays in memory.


Example 9.1. Assume there are 50 students in the class, and their grades on the first exam are stored in an array. Write a function that calculates and returns class average. Write a second function that finds and returns the highest grade on the exam.

Solution: Assuming the grades vary from 0 to 100, the data could be stored in char, short or long format. To be compatible with the ARM architecture, we will create a 32-bit array. In this example, we will not worry about how grades are entered, but assume there is data in this array.


long Grades[50];


To calculate average, we sum the values and divide by the number of students. The return statement will return the 32-bit calculation of sum/50. This will never create an error, because the sum of 50 numbers ranging from 0 to 100 will always be less than 4 trillion (232) and the average of these numbers will never go below 0 or above 100.


long Average(void){long sum,i;

  sum = 0;

  for(i=0; i<50; i++){

    sum = sum+Grades[i];


  return (sum/50);


Program 9.3. A function that calculates the average of data stored in an array.



To make this more general, we will pass in the array and the size. 

long Average(long class[],long size){

long sum,i;

  sum = 0;

  for(i=0; i<size; i++){

    sum = sum+class[i];  // add up all values


  return (sum/size);



The array parameter is actually a pointer to the array. So to use this new function we call

  MyClassAverage = Average(Grades, 50);

Similarly to find the largest we will search the array. We initialized the result parameter (largest) to the smallest possible answer, and then every time we find one larger, we replace the result parameter. 

long Max(long class[],long size){

long largest,i;

  largest = 0;  // smallest possible value

  for(i=0; i<size; i++){

    if(class[i] > largest){

      largest = class[i];  // new maximum



  return (largest);



To use this new function we call

  HighestsScore = Max(Grades, 50);




Example 9.2. Design an exponential function, y = 10x, with a 32-bit output.

Solution: Since the output is less than 4,294,967,295, the input must be between 0 and 9. One simple solution is to employ a constant word array, as shown in Figure 9.3. Each element is 32 bits. In assembly, we define a word constant using DCD, making sure in exists in ROM.

In C, the syntax for accessing all array types is independent of precision. See Program 9.4. The compiler automatically performs the correct address correction. We will assume the input is less than or equal to 9. If x is the index and Base is the base address of the array, then the address of the element at x is Base+4*x. In assembly, we can access the array using indexed addressing. We will assume the input (which is in Register R0) is less than or equal to 9. We show the asembly equivalent of the C code below (and examples that follow) just to pique your curiosity, you may ignore it if you choose.





















Figure 9.2. A word array with 10 elements. Addresses illustrate the array is stored in ROM as 4 bytes each.




Powers DCD  1, 10, 100, 1000, 10000

       DCD  100000, 1000000, 10000000

       DCD  100000000, 1000000000

; Input: R0=x    Output: R0=10^x

power  LSL R0, R0, #2   ; x = x*4

       LDR R1, =Powers  ; R1 = &Powers

       LDR R0, [R0, R1] ; y=Powers[x]

       BX  LR                     

const unsigned long Powers[10]





unsigned long power(unsigned long x){

  return Powers[x];  


Program 9.4. Array implementation of a nonlinear function.


In general, let n be the precision of a zero-origin indexed array in bytes. If I is the index and Base is the beginning address of the array, then the address of the element at I is



The origin of an array is the index of the first element. The origin of a zero-origin indexed array is zero. In general, if o is the origin of the array, then the address of the element at I is


9.4. Strings

A string is simply an array of ASCII characters. In C, strings are automatically null-terminated by the compiler:

const char Hello[] = "Hello world\n\r";

const char Name[] = "Yerraballi";


Hello    Name

When defining constant strings or arrays we must specify their value because the data will be loaded into ROM and cannot be changed at run time. In the previous constant arrays we specified the size; however for constant arrays the size can be left off and the compiler will calculate the size automatically. Notice also the string can contain special characters, some of which are listed in Table 9.1. The Hello string has 13 characters followed by a null (0) termination.



Escape Sequence

alert (beep)






carriage return


double quote


form feed


horizontal tab




null character


single quote


vertical tab


question mark


Table 9.1. Escape sequences.


Note, that in C, ASCII strings are stored with null-termination. So, the C compiler automatically adds the zero at the end, but in assembly, the zero must be explicitly defined.



Example 9.3. Write software to output an ASCII string an output device.

Solution: Because the length of the string may be too long to place all the ASCII characters into the registers at the same time, call by reference parameter passing will be used. With call by reference, a pointer to the string will be passed. The function OutString, shown in Program 9.5, will output the string data to the display. We will assume the function OutChar is given to us, which outputs a single ASCII character. In C, we process one character (*pt gives us the current character) of the string in each iteration of the while loop, which ends when the null value is reached. Each iteration advances the pointer by incrementing it (pt++). In the assembly version, R4 is used as a pointer to the string; one is added to the pointer each time through the loop because each element in the string is one byte. Since this function calls a subfunction it must save its original return address (LR). The POP PC operation will perform the function return.


;Input: R0 points to string


     PUSH {R4, LR}

     MOV  R4, R0                     

loop LDRB R0, [R4]

     ADD  R4, #1  ;next

     CMP  R0, #0  ;done?                    

     BEQ  done    ;0 termination                      

     BL   OutChar ;print character

     B    loop  

done POP  {R4, PC}      

// displays a string

void OutString(char *pt){


    OutChar(*pt); // output

    pt++;         // next




Program 9.5. A variable length string contains ASCII data.


Observation: Most C compilers have standard libraries. If you include “string.h” you will have access to many convenient string operations.


When dealing with strings we must remember that they are arrays of characters with null termination. In C, we can pass a string as a parameter, but doing so creates a constant string and implements call by reference. Assuming Hello is as defined above, these three invocations are identical:



     OutString("Hello world\n\r");


 Previously we dealt with constant strings. With string variables, we do not know the length at compile time, so we must allocate space for the largest possible size the string could be. E.g., if we know the string size could vary from 0 to 19 characters, we would allocate 20 bytes.

char String1[20];

char String2[20];


In C, we cannot assign one string to another. I.e., these are illegal

    String1 = "Hello";   //********illegal************

    String2 = String1;   //********illegal************


We can make this operation occur by calling a function called strcpy, which copies one string to another. This function takes two pointers. We must however make sure the destination string has enough space to hold the string being copied.

       strcpy(String1,"Hello");  // copies "Hello" into String1

       strcpy(String2,String1);  // copies String1 into String2


Program 9.6 shows two implementations of this string copy function. R0 and R1 are pointers, and R2 contains the data as it is being copied. In this case, dest++; is implemented as an “add 1” because the data is one byte each (char). In other non-string situations, the increment pointer would be “add 2” for halfword data (short) and would be “add 4” for word data(long). Again, the C compiler does this automatically, but when writing in assembly one has to do this explicitly.

; Input: R0=&dest    R1=&source

strcpy LDRB R2,[R1]  ;source data

       STRB R2,[R0]  ;copy

       CMP  R2,#0    ;termination?

       BEQ  done

       ADD  R1,#1    ;next

       ADD  R0,#1

       B    strcpy

done   BX   LR

;faster version

strcpy LDRB R2,[R1],#1 ;source data

       STRB R2,[R0],#1 ;copy

       CMP  R2,#0      ;termination?

       BEQ  done

       B    strcpy

done   BX   LR                    

// copy string from source to dest

void strcpy(char *dest, char *source){


    *dest = *source;  // copy

    dest++;           // next



  *dest = *source;   // termination


// another version

void strcpy(char *dest, char *source){

char data;


   data = *dest++ = *source++;

  } while(data);


Program 9.6. Simple string copy functions.


9.5. Functional debugging


                     Video 9.3. Testing and Debugging - Intrusiveness

9.5.1. Stabilization

Functional debugging involves the verification of input/output parameters. Functional debugging is a static process where inputs are supplied, the system is run, and the outputs are compared against the expected results. Four methods of functional debugging are presented in this section. There are two important aspects of debugging: control and observability. The first step of debugging is to stabilize the system. In the debugging context, we stabilize the system by creating a test routine that fixes (or stabilizes) all the inputs. In this way, we can reproduce the exact inputs over and over again. Stabilization is an effective approach to debugging because we can control exactly what software is being executed. Once stabilized, if we modify the program, we are sure that the change in our outputs is a function of the modification we made in our software and not due to a change in the input parameters. When a system has a small number of possible inputs (e.g., less than a million), it makes sense to test them all. When the number of possible inputs is large we need to choose a set of inputs. There are many ways to make this choice. We can select values:

Near the extremes and in the middle

Most typical of how our clients will properly use the system

Most typical of how our clients will improperly attempt to use the system

That differ by one

You know your system will find difficult

Using a random number generator


To stabilize the system we define a fixed set of inputs to test, run the system on these inputs, and record the outputs. Debugging is a process of finding patterns in the differences between recorded behavior and expected results. The advantage of modular programming is that we can perform modular debugging. We make a list of modules that might be causing the bug. We can then create new test routines to stabilize these modules and debug them one at a time. Unfortunately, sometimes all the modules seem to work, but the combination of modules does not. In this case we study the interfaces between the modules, looking for intended and unintended (e.g., unfriendly code) interactions.

9.5.2. Single Stepping

Many debuggers allow you to set the program counter to a specific address then execute one instruction at a time. The debugger provides three stepping commands StepStepOver and StepOut commands. Step is the usual execute one assembly instruction. However, when debugging C we can also execute one line of C. StepOver will execute one assembly instruction, unless that instruction is a subroutine call, in which case the debugger will execute the entire subroutine and stop at the instruction following the subroutine call. StepOut assumes the execution has already entered a subroutine, and will finish execution of the subroutine and stop at the instruction following the subroutine call.

9.5.3. Breakpoints

A breakpoint is a mechanism to tag places in our software, which when executed will cause the software to stop. Normally, you can break on any line of your program.

One of the problems with breakpoints is that sometimes we have to observe many breakpoints before the error occurs. One way to deal with this problem is the conditional breakpoint. To illustrate the implementation of conditional breakpoints, add a global variable called Count and initialize it to 32 in the initialization ritual. Add the following conditional breakpoint to the appropriate location in your software. Using the debugger, we set a regular breakpoint at bkpt. We run the system again (you can change the 32 to match the situation that causes the error.)

           PUSH {R1, R2}   ; save R1 and R2

           LDR  R2, =Count ; R2 = Count

           LDR  R1, [R2]   ; R1 = Count

           SUBS R1, R1, #1 ; Count = Count – 1

           STR  R1, [R2]   ; store to Count

           BNE  DEBUG_skip ; if Count != 0, skip

DEBUG_bkpt NOP             ; put breakpoint here

DEBUG_skip POP {R1, R2}    ; restore R1 and R2





Notice that the breakpoint occurs only on the 32nd time the break is encountered. Any appropriate condition can be substituted. Most modern debuggers allow you to set breakpoints that will trigger on a count. However, this method allows flexibility of letting you choose the exact conditions that cause the break.

9.5.4. Instrumentation: Print Statements

The use of print statements is a popular and effective means for functional debugging. One difficulty with print statements in embedded systems is that a standard “printer” may not be available. Another problem with printing is that most embedded systems involve time-dependent interactions with its external environment. The print statement itself may be so slow, that the debugging process itself causes the system to fail. In this regard, the print statement is intrusive. Therefore, throughout this book we will utilize debugging methods that do not rely on the availability of a standard output device.


9.5.5. Instrumentation: Dump into Array without Filtering

There are three limitations of using print statements to debug. First, many embedded systems do not have a standard output device onto which we could stream debugging information. A second difficulty with print statements is that they can significantly slow down the execution speed in real-time systems. The bandwidth of the print functions often cannot keep pace with the real-time execution. For example, our system may wish to call a function 1000 times a second (or every 1 ms). If we add print statements to it that require more than 1 ms to perform, the presence of the print statements will cause the system to crash. In this situation, the print statements would be considered extremely intrusive. Another problem with print statements occurs when the system is using the same output hardware for its normal operation, as is required to perform the print function. For example, your watch may have an LCD, but that display is used to implement the watch functionality. If we output debugging information to the LCD, the debugger output and normal system output are intertwined.

To solve these limitations, we can add a debugging instrument that dumps strategic information into an array at run time. We can then observe the contents of the array at a later time. One of the advantages of dumping is that the JTAG debugger allows you to visualize memory even when the program is running. So this technique will be quite useful in systems with a JTAG debugger. Assume happy and sad are strategic 8-bit variables. The first step when instrumenting a dump is to define a buffer in RAM to save the debugging measurements.

SIZE     EQU 20



Cnt      SPACE 4

#define SIZE 20

unsigned char HappyBuf[SIZE];

unsigned char SadBuf[SIZE];

unsigned long Cnt;


The Cnt will be used to index into the buffers. Cnt must be initialized to zero, before the debugging begins. The debugging instrument, shown in Program 9.7, dumps the strategic variables into the buffers. When writing debugging instruments it is good style to preserve all registers.

Save PUSH {R0-R3,LR}

     LDR  R0,=Cnt  ;R0 = &Cnt

     LDR  R1,[R0]  ;R1 = Cnt

     CMP  R1,#SIZE

     BHS  done     ;full?

     LDR  R3,=Happy

     LDRB R3,[R3]  ;R3 is happy

     LDR  R2,=HappyBuf

     STRB R3,[R2,R1] ;save happy

     LDR  R3,=Sad

     LDRB R3,[R3]  ;R3 is sad

     LDR  R2,=SadBuf

     STRB R3,[R2,R1] ;save sad

     ADD  R1,#1

     STR  R1,[R0] ;save Cnt 

done POP  {R0-R3,PC}

void Save(void){

  if(Cnt < SIZE){

    HappyBuf[Cnt] = happy;

    SadBuf[Cnt] = sad;




Program 9.7. Instrumentation dump.

Next, you add BL Save statements at strategic places within the system. You can either use the debugger to display the results, or add software that prints the results after the program has run and stopped.

9.5.6. Instrumentation: Dump into Array with Filtering.

One problem with dumps is that they can generate a tremendous amount of information. If you suspect a certain situation is causing the error, you can add a filter to the instrument. A filter is a software/hardware condition that must be true in order to place data into the array. In this situation, if we suspect the error occurs when another variable gets large, we could add a filter that saves in the array only when the variable is above a certain value. In the example shown in Program 9.8, the instrument dumps only when sad is greater than 100.

Save PUSH {R0-R3,LR}

     LDR  R3,=Sad

     LDRB R3,[R3]  ;R3 is sad

     CMP  R3,#100

     BLS  done     ;assuming unsigned

     LDR  R0,=Cnt  ;R0 = &Cnt

     LDR  R1,[R0]  ;R1 = Cnt

     CMP  R1,#SIZE

     BHS  done     ;full?

     LDR  R2,=SadBuf

     STRB R3,[R2,R1] ;save sad

     LDR  R3,=Happy

     LDRB R3,[R3]  ;R3 is happy

     LDR  R2,=HappyBuf

     STRB R3,[R2,R1] ;save happy

     ADD  R1,#1

     STR  R1,[R0] ;save Cnt 

done POP  {R0-R3,PC}    

void Save(void){

  if(sad > 100){

    if(Cnt < SIZE*2){

      HappyBuf[Cnt] = happy;

      SadBuf[Cnt] = sad;





Program 9.8. Instrumentation dump with filter.

9.5.7. Prove that our system is functioning as intended.


                     Video 9.4. Making the case for Functional Debugging

When embedded systems are deployed in safety critical situations we need to document how the system was tested, and provide proof it is functioning as intended. We will illustrate the process with the example from module 2. In Lab02 the goal was to create a system that toggled the light at 5 Hz. This means the red LED should be on for 0.1 sec and off for 0.1 sec. If we look at the LED with our eyes it looks like it is running correctly. If we look at the signal on the oscilloscope or logic analyzer, again it looks correct. But can we prove it? In this first debugging process we will dump the value of PF1 in one array and the time difference in a second array, see Program 9.9. When we run this program, it reveals all 49 measurements where the average time difference is 1,599,996 bus cycles, which is 0.09999975 seconds, which is close to the desired time of 0.1 second.


                     Video 9.5. Functional Debugging with Lab02 modified

// first data point is wrong, the other 49 will be correct

unsigned long Time[50];

unsigned long Data[50];

int main(void){  unsigned long i,last,now;

                  // initialize PF0 and PF4 and make them inputs

  PortF_Init();   // make PF3-1 out (PF3-1 built-in LEDs)

  SysTick_Init(); // initialize SysTick, runs at 16 MHz

  i = 0;          // array index



    Led = GPIO_PORTF_DATA_R;   // read previous

    Led = Led^0x02;            // toggle red LED

    GPIO_PORTF_DATA_R = Led;   // output


      now = NVIC_ST_CURRENT_R;

      Time[i] = (last-now)&0x00FFFFFF;  // 24-bit time difference

      Data[i] = GPIO_PORTF_DATA_R&0x02; // record PF1

      last = now;






Program 9.9. Instrumentation to record the first 49 time differences (debugging shown in bold).

However compelling this data is, it doesn’t prove it is always 0.1 sec. Let’s further specify the desire is to create a system that is accurate to ±0.01%. This means any time difference 1,600,000±160 cycles is acceptable. In Program 9.10 we will count the number of times the time difference is unacceptable. If we run this program for a month we can observe its behavior over 25 million times. Furthermore, we can leave this debugging code into the deployed system, and verify the system is running as expected for the entire life of the system.

// first data point is wrong, the others will be correct

long Errors;

#define CORRECT 1600000

#define TOLERANCE 160

int main(void){  unsigned long last,now,diff;

                  // initialize PF0 and PF4 and make them inputs

  PortF_Init();   // make PF3-1 out (PF3-1 built-in LEDs)

  SysTick_Init(); // initialize SysTick, runs at 16 MHz

  Errors = -1;    // no errors (ignore first measurement)



    Led = GPIO_PORTF_DATA_R;   // read previous

    Led = Led^0x02;            // toggle red LED

    GPIO_PORTF_DATA_R = Led;   // output

    now = NVIC_ST_CURRENT_R;

    diff = (last-now)&0x00FFFFFF;  // 24-bit time difference




    last = now;




Program 9.10. Instrumentation to count the number of mistakes (debugging shown in bold).

One interesting feature we could add to this system is to implement the Error count in ROM. The flash ROM allows the running program to change its value. The disadvantage of storing data in ROM is it takes over 1ms to cause the change. In situations where we have infrequent but important data, this 1ms overhead is not significant. The advantage of storing infrequent but important debugging information in ROM is that this data is available even if power is removed and restored. For example, if the embedded system were to be involved in a loss of life accident, the data stored in ROM could be recovered to determine if any run-time errors might have contributed to the accident. Conversely, this data stored in ROM could verify that no errors in the operation of the embedded system had occurred prior to the accident. If you wish to write to flash ROM, look at example projects called “flash” on


Reprinted with approval from Embedded Systems: Introduction to ARM Cortex-M Microcontrollers, 2014, ISBN: 978-1477508992,


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Embedded Systems - Shape the World by Jonathan Valvano and Ramesh Yerraballi is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
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