NVIC advanced features in ARM Architecture - Mini Project: Build & Apply

NVIC Advanced Features

📖 Scenario: You are working with an ARM Cortex-M microcontroller and want to understand how to configure the Nested Vectored Interrupt Controller (NVIC) to manage interrupts effectively in a real embedded system.

🎯 Goal: Build a step-by-step understanding of NVIC advanced features by creating a simple configuration setup that includes interrupt priority grouping, enabling interrupts, setting priorities, and using software-triggered interrupts.

📋 What You'll Learn

Create a variable representing the NVIC priority grouping configuration

Define an interrupt enable mask for specific interrupts

Set priority levels for interrupts using NVIC priority registers

Trigger an interrupt using software to test NVIC configuration

💡 Why This Matters

🌍 Real World

NVIC configuration is essential in embedded systems to manage multiple interrupts efficiently, ensuring critical tasks get processor time first.

💼 Career

Understanding NVIC advanced features is important for embedded software engineers working on ARM Cortex-M microcontrollers to write reliable and responsive firmware.

1

Set NVIC Priority Grouping

Create a variable called NVIC_PRIORITY_GROUP and set it to the value 3 to represent the priority grouping configuration in the NVIC.

ARM Architecture

# Create a variable called NVIC_PRIORITY_GROUP and set it to 3
# Your code here

Hint

The priority grouping value controls how many bits are used for pre-emption priority and subpriority.

2

Enable Specific Interrupts

Create a variable called NVIC_ENABLE_MASK and set it to 0x00000005 to enable interrupts 0 and 2 in the NVIC.

ARM Architecture

NVIC_PRIORITY_GROUP = 3
# Create a variable called NVIC_ENABLE_MASK and set it to 0x00000005
# Your code here

Hint

The mask 0x00000005 enables interrupt 0 (bit 0) and interrupt 2 (bit 2).

3

Set Interrupt Priorities

Create a dictionary called NVIC_PRIORITIES with keys 0 and 2 representing interrupt numbers, and set their priority values to 1 and 2 respectively.

ARM Architecture

NVIC_PRIORITY_GROUP = 3
NVIC_ENABLE_MASK = 0x00000005
# Create a dictionary called NVIC_PRIORITIES with keys 0 and 2 and values 1 and 2
# Your code here

Hint

Interrupt 0 has higher priority (1) than interrupt 2 (2).

4

Trigger Software Interrupt

Create a variable called NVIC_SOFTWARE_TRIGGER and set it to 2 to simulate triggering interrupt number 2 by software.

ARM Architecture

NVIC_PRIORITY_GROUP = 3
NVIC_ENABLE_MASK = 0x00000005
NVIC_PRIORITIES = {0: 1, 2: 2}
# Create a variable called NVIC_SOFTWARE_TRIGGER and set it to 2
# Your code here

Hint

This simulates a software-generated interrupt for testing purposes.

Practice

1. What is the main purpose of the NVIC's priority grouping feature?

easy

A. To disable all interrupts at once

B. To balance between pre-emption priority and subpriority levels

C. To increase the clock speed of the processor

D. To reset the microcontroller

Solution

1. Step 1: Understand NVIC priority grouping

   Priority grouping allows dividing interrupt priority into pre-emption priority and subpriority.

2. Step 2: Identify the purpose of balancing priorities

   This balance controls which interrupts can pre-empt others and which are handled in order if priorities are equal.

3. Final Answer:

   To balance between pre-emption priority and subpriority levels -> Option B

4. Quick Check:

   Priority grouping = Balancing priorities [OK]

Hint: Priority grouping splits interrupt priority levels [OK]

Common Mistakes:

• Confusing priority grouping with disabling interrupts
• Thinking it changes processor speed
• Assuming it resets the device

2. Which of the following is the correct way to enable an interrupt in NVIC?

easy

A. NVIC_DisableIRQ(IRQn);

B. NVIC_SetPriority(IRQn, priority);

C. NVIC_EnableIRQ(IRQn);

D. NVIC_ClearPendingIRQ(IRQn);

Solution

1. Step 1: Identify function to enable interrupts

   The function NVIC_EnableIRQ(IRQn) is used to enable a specific interrupt.

2. Step 2: Differentiate from other NVIC functions

   NVIC_DisableIRQ disables interrupts, SetPriority sets priority, ClearPendingIRQ clears pending flags.

3. Final Answer:

   NVIC_EnableIRQ(IRQn); -> Option C

4. Quick Check:

   Enable interrupt = NVIC_EnableIRQ [OK]

Hint: Enable interrupts with NVIC_EnableIRQ function [OK]

Common Mistakes:

• Using NVIC_DisableIRQ to enable interrupts
• Confusing priority setting with enabling
• Clearing pending interrupts instead of enabling

3. Consider the following code snippet:

NVIC_SetPriorityGrouping(3);
NVIC_SetPriority(IRQn, NVIC_EncodePriority(3, 1, 2));

What does this code do?

medium

A. Sets priority grouping to 3 and assigns pre-emption priority 1 and subpriority 2 to IRQn

B. Disables IRQn interrupt

C. Enables IRQn interrupt with default priority

D. Resets priority grouping to default

Solution

1. Step 1: Analyze NVIC_SetPriorityGrouping(3)

   This sets the priority grouping to 3, defining how many bits are for pre-emption and subpriority.

2. Step 2: Decode NVIC_SetPriority call

   NVIC_EncodePriority(3, 1, 2) encodes pre-emption priority 1 and subpriority 2 according to grouping 3, then assigns it to IRQn.

3. Final Answer:

   Sets priority grouping to 3 and assigns pre-emption priority 1 and subpriority 2 to IRQn -> Option A

4. Quick Check:

   Priority grouping 3 + encoded priority = Sets priority grouping to 3 and assigns pre-emption priority 1 and subpriority 2 to IRQn [OK]

Hint: Priority grouping affects encoding of priorities [OK]

Common Mistakes:

• Assuming it disables or enables interrupts
• Ignoring priority encoding details
• Confusing priority grouping with resetting

4. A developer wrote this code to disable an interrupt but it does not work:

NVIC_EnableIRQ(IRQn);

What is the error?

medium

A. The function NVIC_EnableIRQ enables, not disables, interrupts

B. IRQn is not a valid interrupt number

C. NVIC_DisableIRQ should be called before NVIC_EnableIRQ

D. Priority must be set before disabling interrupts

Solution

1. Step 1: Understand function purpose

   NVIC_EnableIRQ enables the interrupt, so using it to disable is incorrect.

2. Step 2: Identify correct function to disable

   NVIC_DisableIRQ should be used to disable interrupts, not NVIC_EnableIRQ.

3. Final Answer:

   The function NVIC_EnableIRQ enables, not disables, interrupts -> Option A

4. Quick Check:

   EnableIRQ enables, not disables [OK]

Hint: EnableIRQ enables; use DisableIRQ to disable [OK]

Common Mistakes:

• Confusing enable and disable functions
• Assuming priority affects enabling/disabling
• Thinking order of calls disables interrupts

5. You want to ensure that a critical interrupt always pre-empts others regardless of subpriority. Which NVIC configuration achieves this?

hard

A. Use NVIC_DisableIRQ on all other interrupts

B. Set all interrupts to the same priority level

C. Disable subpriority by setting it to maximum value

D. Set priority grouping to allocate all bits to pre-emption priority and zero to subpriority

Solution

1. Step 1: Understand priority grouping effect

   Setting priority grouping to allocate all bits to pre-emption priority means subpriority is zero, so pre-emption controls interrupt order fully.

2. Step 2: Confirm this ensures critical interrupt pre-empts others

   With no subpriority bits, the critical interrupt's pre-emption priority dominates, allowing it to always pre-empt.

3. Final Answer:

   Set priority grouping to allocate all bits to pre-emption priority and zero to subpriority -> Option D

4. Quick Check:

   All bits to pre-emption priority = critical pre-emption [OK]

Hint: Maximize pre-emption bits to ensure critical interrupt priority [OK]

Common Mistakes:

• Thinking disabling subpriority means setting it high
• Assuming same priority levels allow pre-emption
• Disabling other interrupts is not practical