SoC
  • Home
  • Arm
  • Arm Cortex M0/M0+
  • Arm Cortex M4
  • Arm Cortex M3
  • Contact
Reading: ARM Cortex M4 Watchdog
SUBSCRIBE
SoCSoC
Font ResizerAa
  • Home
  • Arm
  • Arm Cortex M0/M0+
  • Arm Cortex M4
Search
  • Home
  • Arm
  • Arm Cortex M0/M0+
  • Arm Cortex M4
Have an existing account? Sign In
Follow US
  • Looking for Something?
  • Privacy Policy
  • About Us
  • Sitemap
  • Contact Us
© S-O-C.ORG, All Rights Reserved.
Arm

ARM Cortex M4 Watchdog

Ryan Ryan
Last updated: October 5, 2023 9:56 am
Ryan Ryan 7 Min Read
Share
SHARE

The watchdog timer module in ARM Cortex M4 microcontrollers provides a robust mechanism to detect system hangs or crashes and reset the system automatically. The 120-word quick answer is that the Cortex M4 watchdog is a built-in timer that triggers a processor reset if the main program neglects to periodically service it. This prevents system lockups and increases reliability. The watchdog needs to be enabled, configured with a timeout period, and serviced by writing to its reset register before the timeout expires. If the reset doesn’t occur in time, the watchdog will reset the Cortex M4. This allows unattended systems to automatically recover from software glitches.

Contents
Watchdog BasicsWhy Use a WatchdogCortex M4 Watchdog OperationWatchdog InitializationServicing the WatchdogWatchdog Uses in Cortex M4 SystemsGuidelines for Effective UseSummary

Watchdog Basics

A watchdog timer (sometimes called a computer operating properly or COP timer) is an electronic timer that is used to detect and recover from computer malfunctions. They are often used in embedded systems to reset the processor in case the software crashes or gets stuck in an infinite loop. The watchdog must be periodically serviced (reset) by the main program within a timeout period. If the reset doesn’t happen in time, the watchdog timer will reset the system’s microcontroller to recover functionality.

The ARM Cortex M4 processor contains an on-chip watchdog module to serve this purpose. It is a fixed-function hardware block that operates independently of the main software. Once enabled, it will reset the Cortex M4 after a programmable timeout unless the software writes to a specific register to service it. This provides a way for the system to automatically recover from software crashes or lockups.

Why Use a Watchdog

Watchdog timers are an essential component in many embedded devices to improve reliability, especially in unattended or inaccessible applications. For example:

  • Industrial controllers – Prevent machine/process lockups
  • Remote weather stations – Recover from software crashes
  • Automotive systems – Reset glitchy ECUs
  • Medical devices – Increase safety

Without a watchdog, a system hang or crash would require manual intervention or cause complete failure. The watchdog provides automatic recovery so the system can resume normal operation after a reset.

Cortex M4 Watchdog Operation

The Cortex M4 watchdog module is clocked by its own low-power oscillator separate from the system clock. This allows it to keep running even if the main clock fails. The watchdog timer will reset the processor after a programmed timeout period unless the software services it regularly by writing to the WDOG_INTCLR register. This periodic servicing indicates the software is still executing properly.

If the servicing write does not occur within the timeout window, the watchdog will escalate and generate a reset signal to the Cortex M4. The processor and any peripherals will be reset to recover from the presumed lockup or malfunction. The watchdog then starts counting down again, waiting for the next servicing write.

Watchdog Initialization

To use the Cortex M4 watchdog, it must first be enabled and configured during system initialization:

  1. Enable the watchdog module clock
  2. Configure the watchdog clock source
  3. Set the watchdog timeout period
  4. Enable watchdog reset generation
  5. Enable the watchdog timer

The timeout period determines how often the main program must service the watchdog to prevent a reset. It should be set short enough to detect hangs quickly but long enough for the servicing writes to occur.

Servicing the Watchdog

The main program must periodically write a value of 0xA followed by 0x5 to the WDOG_INTCLR register to service the watchdog and prevent it from resetting the Cortex M4. This servicing write should be performed in the main loop or any frequently executed interrupts.

Typically the servicing routine disables interrupts temporarily to prevent any timing discrepancies. The WDOG_INTCLR write must complete before the next timeout expires. Failing to service the watchdog in time will result in a reset.

Watchdog Uses in Cortex M4 Systems

The Cortex M4 watchdog has some common uses in embedded systems:

  • Software crash recovery – Reset after illegal instructions, infinite loops, stack overflows, etc.
  • Improved reliability – Recover from intermittent glitches and faults.
  • Robust field operation – Allow remote or inaccessible devices to automatically recover.
  • Safety mechanisms – Detect and reset dangerous system states.
  • Sanity checking – Reset if multiple tasks miss deadlines.

It can also be used creatively for things like:

  • Force a boot from ROM after too many crashes
  • Reset consistently glitching peripherals
  • Contain and reset on security violations

Guidelines for Effective Use

Here are some guidelines for making the best use of the Cortex M4 watchdog:

  • Set the timeout short enough to detect hangs quickly
  • Service the watchdog at least 2x as fast as the timeout period
  • Use a dedicated servicing routine in the main loop
  • Disable interrupts during servicing to prevent timing issues
  • Watch for reset at key checkpoints to identify trouble spots
  • Evaluate needed timeout period through testing
  • Consider NMI handler for special reset handling

The watchdog is not a substitute for fixing bugs and crashes in software. It should be used as an extra layer of protection when reliability is critical.

Summary

The ARM Cortex M4 watchdog module provides an automatic reset mechanism that can recover systems from software failures. It operates independently to reset the processor after a configurable timeout if the main program neglects to service it. Using the on-chip watchdog helps improve system reliability and robustness.

Newsletter Form (#3)

More ARM insights right in your inbox

 


Share This Article
Facebook Twitter Email Copy Link Print
Previous Article Arm WFF Instruction
Next Article General Purpose Watchdog Timer Component for a Multitasking System
Leave a comment Leave a comment

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

2k Followers Like
3k Followers Follow
10.1k Followers Pin
- Sponsored-
Ad image

You Might Also Like

What Is Arm Cortex M

Arm Cortex-M is a family of 32-bit RISC ARM processor…

11 Min Read

What is the difference between JTAG and SWD?

JTAG and SWD are two common interfaces used for debugging…

7 Min Read

Does GCC work on ARM?

Yes, the GNU Compiler Collection (GCC) does work on ARM…

9 Min Read

Cortex-M0/M3/M4 support for Thumb vs Thumb-2 instructions

The Cortex-M0, Cortex-M3, and Cortex-M4 ARM processors all support both…

6 Min Read
SoCSoC
  • Looking for Something?
  • Privacy Policy
  • About Us
  • Sitemap
  • Contact Us
Welcome Back!

Sign in to your account