Condition monitoring & predictive maintenance
IMPROVING INDUSTRIAL FUNCTIONAL SAFETY COMPLIANCE WITH HIGH- PERFORMANCE SUPERVISORY CIRCUITS: PROGRAMME SEQUENCE MONITORING USING WATCHDOG TIMERS
By Bryan Angelo Borres, senior product applications engineer, and Christopher Macatangay, senior product applications engineer, Analog Devices
Watchdog timers are key components in ensuring digital controller devices, such as microcontrollers (MCUs), function in a safe manner. The basic functional safety (FS) standard IEC 61508-2 Annex A Table A.10 recommends several diagnostic techniques and measures to control hardware failures in the programme sequences of digital devices. Such techniques include a watchdog with a separate time base with or without a time window, and a combination of temporal and logical monitoring of programme sequences. While each of these has corresponding maximum claimable diagnostic coverage, all these techniques employ watchdog timers. For this reason, this article will show how to implement these diagnostic functions using watchdog timers.
W
ith the prevalence of microcontrollers (MCUs) as processing units in safety- related systems (SRS) comes the need for diagnostic measures that will ensure safe operation. IEC 61508-2 specifies self-test supported by hardware (one channel) as one of the recommended diagnostic techniques for processing units. This measure uses special hardware that increases speed and extends the scope of failure detection, for instance, a watchdog timer IC that cyclically monitors the output of a certain bit pattern from the MCU. Also, IEC 61508-2 specifies two types of watchdogs – one with a separate time base without a time window, and another with a time window. Despite this, a watchdog can also be used in other techniques, such as a combination of temporal and logical monitoring of programme sequences. Thus, this article will provide insights into the differences between such programme sequence monitoring diagnostic measures in terms of operation and diagnostic coverage when implemented with ADI’s high-performance supervisory circuits with a watchdog function.
WATCHDOG WITH A SEPARATE TIME BASE WITHOUT A TIME WINDOW Part 2 of IEC 61508 describes simple watchdogs as external timing elements with a separate time base. Such devices allow detection of programme sequence failures in a computer device, such as an MCU, within a specified interval. This is done by having a mechanism that allows the MCU to issue a signal to reset the watchdog before it reaches the timeout, which means the programme sequence is running smoothly, or let the watchdog timeout period be reached so the watchdog can
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Figure 1. Simple watchdog operation. October 2026 Instrumentation Monthly
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