NXP Semiconductors MWCT101 S Series, Reference Manual

Page: 418 / 1761

18.2 Introduction
For safety, a redundant watchdog system, External Watchdog Monitor (EWM), is
designed to monitor external circuits, as well as the MCU software flow. This provides a
back-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 software
within an MCU. The watchdog consists of a counter that if allowed to overflow, forces an
internal reset (asynchronous) to all on-chip peripherals and optionally assert the
RESET_B pin to reset external devices/circuits. The overflow of the watchdog counter
must not occur if the software code works well and services the watchdog to re-start the
actual counter.
The EWM differs from the internal watchdog in that it does not reset the MCU's CPU
and peripherals. The EWM provides an independent EWM_OUT_b signal that when
asserted resets or places an external circuit into a safe mode. The EWM_OUT_b signal is
asserted upon the EWM counter time-out. An optional external input EWM_in is
provided to allow additional control of the assertion of EWM_OUT_b signal.
18.2.1 Features
Features of EWM module include:
• Independent LPO_CLK clock source
• Programmable time-out period specified in terms of number of EWM LPO_CLK
clock cycles.
• Windowed refresh option
• Provides robust check that program flow is faster than expected.
• Programmable window.
• Refresh outside window leads to assertion of EWM_OUT_b.
• Robust refresh mechanism
• Write values of 0xB4 and 0x2C to EWM Refresh Register within 15 peripheral
bus clock cycles.
Introduction
MWCT101xS Series Reference Manual, Rev. 3, 07/2019
418 NXP Semiconductors