Fault handlers (HardFault, MemManage, BusFault, UsageFault) in ARM Architecture - Mini Project: Build & Apply

Fault Handlers (HardFault, MemManage, BusFault, UsageFault)

📖 Scenario: You are working on an embedded system using an ARM Cortex-M processor. To make your system more reliable, you want to understand and set up basic fault handlers that catch different types of errors during program execution.

🎯 Goal: Build a simple fault handler setup that defines the four main ARM Cortex-M fault handlers: HardFault_Handler, MemManage_Handler, BusFault_Handler, and UsageFault_Handler. Each handler will have a basic function definition to catch faults.

📋 What You'll Learn

Create empty function definitions for the four fault handlers with exact names.

Add a global variable fault_count initialized to 0 to count faults.

Inside each fault handler, increment fault_count by 1.

Add a final comment explaining that these handlers help detect system errors.

💡 Why This Matters

🌍 Real World

Fault handlers are critical in embedded systems to catch and respond to serious errors like memory violations or illegal instructions, helping developers debug and improve system stability.

💼 Career

Embedded systems engineers and firmware developers must understand and implement fault handlers to build reliable and safe devices.

1

Create fault handler functions

Create four empty functions with these exact names: HardFault_Handler, MemManage_Handler, BusFault_Handler, and UsageFault_Handler. Each function should have no parameters and an empty body.

ARM Architecture

# Your code here to create the four empty fault handler functions

Hint

Define each function with void return type, no parameters, and empty braces.

2

Add a global fault counter

Add a global variable called fault_count of type volatile int and initialize it to 0. This variable will keep track of how many faults have occurred.

ARM Architecture

volatile int fault_count = 0;

void HardFault_Handler(void) {}
void MemManage_Handler(void) {}
void BusFault_Handler(void) {}
void UsageFault_Handler(void) {}
# Your code here to add the fault_count variable

Hint

Use volatile int fault_count = 0; outside any function.

3

Increment fault_count inside each handler

Inside each fault handler function (HardFault_Handler, MemManage_Handler, BusFault_Handler, and UsageFault_Handler), add a line that increments fault_count by 1.

ARM Architecture

volatile int fault_count = 0;

void HardFault_Handler(void) {
    // Your code here
}
void MemManage_Handler(void) {
    // Your code here
}
void BusFault_Handler(void) {
    // Your code here
}
void UsageFault_Handler(void) {
    // Your code here
}

Hint

Inside each function, write fault_count++; to increase the count.

4

Add a final comment explaining fault handlers

Add a comment at the end of the code that says: // These fault handlers help detect and count system errors during execution.

ARM Architecture

volatile int fault_count = 0;

void HardFault_Handler(void) {
    fault_count++;
}
void MemManage_Handler(void) {
    fault_count++;
}
void BusFault_Handler(void) {
    fault_count++;
}
void UsageFault_Handler(void) {
    fault_count++;
}
// Your comment here

Hint

Add the comment exactly as shown at the end of the code.

Practice

1. Which fault handler in ARM architecture is triggered when an illegal memory access occurs?

easy

A. HardFault Handler

B. UsageFault Handler

C. BusFault Handler

D. MemManage Fault Handler

Solution

1. Step 1: Understand the role of MemManage Fault

   The MemManage Fault Handler is designed to catch illegal memory accesses such as accessing protected or invalid memory regions.

2. Step 2: Differentiate from other fault handlers

   UsageFault handles instruction errors, BusFault handles bus errors, and HardFault is a catch-all for severe faults. Illegal memory access specifically triggers MemManage Fault.

3. Final Answer:

   MemManage Fault Handler -> Option D

4. Quick Check:

   Illegal memory access = MemManage Fault Handler [OK]

Hint: Illegal memory access triggers MemManage Fault [OK]

Common Mistakes:

• Confusing BusFault with MemManage Fault
• Thinking UsageFault handles memory errors
• Assuming HardFault handles all faults directly

2. Which of the following is the correct way to declare a HardFault handler function in ARM Cortex-M assembly?

easy

A. void HardFault_Handler(void) __attribute__((interrupt));

B. void HardFault_Handler(void) __attribute__((weak));

C. void HardFault_Handler(void) __attribute__((naked));

D. void HardFault_Handler(void) __attribute__((inline));

Solution

1. Step 1: Understand attribute usage for fault handlers

   HardFault handlers often use the __attribute__((naked)) to write custom assembly without compiler-generated prologue/epilogue.

2. Step 2: Compare other attributes

   __attribute__((interrupt)) is not standard for ARM Cortex-M, __attribute__((weak)) allows override but is not for declaration style, __attribute__((inline)) is unrelated.

3. Final Answer:

   void HardFault_Handler(void) __attribute__((naked)); -> Option C

4. Quick Check:

   HardFault handler uses naked attribute [OK]

Hint: HardFault handlers use naked attribute for assembly code [OK]

Common Mistakes:

• Using interrupt attribute which is not standard here
• Confusing weak with naked attribute
• Assuming inline is correct for handlers

3. Given the following ARM Cortex-M code snippet, which fault handler will be triggered if an undefined instruction is executed?

__attribute__((naked)) void UsageFault_Handler(void) {
  while(1) {}
}

void main(void) {
  __asm volatile ("udf #0");
}

medium

A. UsageFault

B. BusFault

C. MemManage Fault

D. HardFault

Solution

1. Step 1: Identify the fault caused by undefined instruction

   The assembly instruction "udf #0" is an undefined instruction that triggers UsageFault in ARM Cortex-M processors.

2. Step 2: Match the fault handler to the fault type

   UsageFault_Handler is designed to catch undefined instructions and other usage errors, so it will be triggered here.

3. Final Answer:

   UsageFault -> Option A

4. Quick Check:

   Undefined instruction = UsageFault [OK]

Hint: Undefined instructions trigger UsageFault [OK]

Common Mistakes:

• Thinking undefined instruction triggers HardFault
• Confusing BusFault with UsageFault
• Assuming MemManage handles instruction faults

4. In an ARM Cortex-M system, a BusFault handler is not executing as expected after a bus error. Which of the following is the most likely cause?

medium

A. BusFault is not enabled in the System Control Block (SCB)

B. MemManage Fault handler is overriding BusFault

C. UsageFault handler is masking BusFault

D. HardFault handler is disabled

Solution

1. Step 1: Check fault enabling in SCB

   BusFault must be enabled in the System Control Block (SCB) to be active. If not enabled, BusFault exceptions won't trigger the handler.

2. Step 2: Understand fault priority and masking

   MemManage and UsageFault handlers do not override or mask BusFault. HardFault cannot be disabled as it is a core fault handler.

3. Final Answer:

   BusFault is not enabled in the System Control Block (SCB) -> Option A

4. Quick Check:

   BusFault handler requires SCB enable [OK]

Hint: Enable BusFault in SCB to activate its handler [OK]

Common Mistakes:

• Assuming HardFault can be disabled
• Confusing fault masking between handlers
• Ignoring SCB configuration for faults

5. You are designing a fault handler system for an embedded ARM Cortex-M device. You want to catch all memory protection faults and bus errors but handle usage faults separately. Which configuration should you apply?

hard

A. Disable MemManage; enable BusFault and UsageFault handlers only

B. Enable MemManage and BusFault handlers; keep UsageFault enabled but with separate handler

C. Enable only HardFault handler to catch all faults

D. Enable UsageFault and MemManage; disable BusFault handler

Solution

1. Step 1: Identify handlers for memory protection and bus errors

   MemManage Fault handler catches memory protection faults, BusFault handler catches bus errors.

2. Step 2: Keep UsageFault separate

   UsageFault handler should remain enabled and separate to handle usage errors like undefined instructions.

3. Step 3: Choose configuration matching requirements

   Enabling MemManage and BusFault handlers while keeping UsageFault enabled separately matches the design goal.

4. Final Answer:

   Enable MemManage and BusFault handlers; keep UsageFault enabled but with separate handler -> Option B

5. Quick Check:

   MemManage + BusFault enabled; UsageFault separate [OK]

Hint: Enable MemManage and BusFault; keep UsageFault separate [OK]

Common Mistakes:

• Disabling MemManage when memory faults needed
• Relying only on HardFault for all errors
• Disabling BusFault when bus errors must be caught