Chapter 23External Watchdog Monitor (EWM)23.1 IntroductionNOTEFor the chip-specific implementation details of this module'sinstances, see the chip configuration information.For safety, a redundant watchdog system, External Watchdog Monitor (EWM), isdesigned to monitor external circuits, as well as the MCU software flow. This provides aback-up mechanism to the internal watchdog that resets the MCU's CPU and peripherals.The watchdog is generally used to monitor the flow and execution of embedded softwarewithin an MCU. The watchdog consists of a counter that if allowed to overflow, forces aninternal reset (asynchronous) to all on-chip peripherals and optionally assert the RESETpin to reset external devices/circuits. The overflow of the watchdog counter must notoccur if the software code works well and services the watchdog to re-start the actualcounter.The EWM differs from the internal watchdog in that it does not reset the MCU's CPUand peripherals. The EWM provides an independent EWM_out signal that when assertedresets or places an external circuit into a safe mode. The EWM_out signal is assertedupon the EWM counter time-out. An optional external input EWM_in is provided toallow additional control of the assertion of EWM_out signal.23.1.1 FeaturesFeatures of EWM module include:• Independent LPO_CLK clock source• Programmable time-out period specified in terms of number of EWM LPO_CLKclock cycles.K22F Sub-Family Reference Manual, Rev. 4, 08/2016NXP Semiconductors 509
