Watchdog Timer: Working Principle, Types, Applications, and 555 Timer Based Watchdog Circuit - Part 2

In the previous article we understood about the basic of watchdog timer. In this part we will understand the types of those.

Types of Watchdog Timers

Watchdog timers are classified into several types based on their operation and implementation.

 

 1. Hardware Watchdog Timer

A hardware watchdog timer is an independent hardware peripheral that monitors the processor externally or internally. It is more reliable than software watchdogs and operates independently of CPU so this continues working even if firmware crashes. It is mainly used in industrial control systems, automotive electronics, medical devices, aerospace systems.

2. Software Watchdog Timer

A software watchdog is implemented using firmware and internal timers. These are easy to implement and no additional hardware required but these have lower reliability compared to hardware watchdogs. One of the major drawback of these is that if the software hangs completely, the software watchdog may also stop functioning.

Most modern microcontrollers like PIC microcontrollers, AVR microcontrollers, STM32 series, ESP32, TI C2000 microcontrollers and ARM Cortex-M devices include built-in watchdog timers. The firmware periodically resets the watchdog using dedicated instructions.

3. Window Watchdog Timer

A window watchdog requires the watchdog to be refreshed within a specific time window. But the things to remember are that refreshing too early causes a fault and refreshing too late also causes a fault. This helps detect timing-related software errors. Its major advantages are that it detects runaway code and detects abnormal execution timing and provides higher reliability.

4. External Watchdog Timer

An external watchdog IC independently monitors the processor. It works even if MCU internal watchdog fails and have higher immunity to firmware corruption. It often includes voltage supervision.

Some of the popular Watchdog Timer ICs are described below.

 1. MAX705: it features microprocessor supervisory circuit, watchdog timer, power supply monitoring with manual reset input. This is widely used in embedded systems, Industrial automation, and automotive electronics.

2. TPS3813: It features adjustable watchdog timeout, low power consumption, reset output with high reliability.

3. ADM1232: It features voltage monitoring, watchdog timer, push-button reset and microprocessor supervision.

4. STM6315: It features precision reset monitoring, watchdog functionality and Low operating current.

5. MCP1316: It features system reset monitoring, watchdog timer and low-cost implementation.

 

Watchdog timers can be implemented using any of the ways described above based on the complexity of design and costing.

 

555 Timer Based Watchdog Timer

 A simple watchdog timer can be built using the popular NE555 timer IC. The 555 timer is commonly configured in monostable mode for watchdog applications. In a 555 timer watchdog circuit the microcontroller periodically sends pulses to the trigger pin. Each pulse resets the timing cycle. And if pulses stop, arriving the timeout period expires, the output changes state and a reset signal is generated. This provides a low-cost external watchdog solution. Although the 555 based watchdog timer have advantages like it has very low cost and is easy to design. But there are few limitations also like lower accuracy, components tolerances affecting the timing. It don’t have no advance fault diagnostics and of course it takes larger space as compared to the watchdog ICs.

 

Design Considerations for Watchdog Timers

 When designing a watchdog timer circuit, the following parameters are important.

1. Timeout Duration: The timeout should be longer than normal program execution time and short enough for fast fault recovery.

2. False Trigger Prevention: Noise or glitches should not accidentally reset the watchdog so proper filtering and PCB layout are important.

3. Independent Clock Source: High-reliability watchdog systems often use separate oscillators to ensure watchdog operation even if the main clock fails.

4. Window Timing: Window watchdogs require careful timing calculations to avoid unintended resets. 

Watchdog Timer vs Brown-Out Reset

Many designers confuse watchdog timers with brown-out reset circuits. A watchdog Timer detects software failures and monitors firmware execution. But the Brown-Out Reset detects low supply voltage and prevents operation during under voltage conditions. Many supervisory ICs combine both functions.

This completes the watchdog timer series. You can click on below video to watch the video tutorial.