NXP Semiconductors freescale KV4 Series, Reference Manual

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watchdog functional test. A maximum time period of ~2 clock A cycles plus ~2
clock B cycles elapses from the time a switch is requested to the occurrence of the
actual clock switch, where clock A and B are the two input clocks to the clock mux.
• For the windowed mode, there is a two to three bus clock latency between the
watchdog counter going past the window value and the same registering in the bus
clock domain.
• For proper operation of the watchdog, the watchdog clock must be at least five times
slower than the system bus clock at all times. An exception is when the watchdog
clock is synchronous to the bus clock wherein the watchdog clock can be as fast as
the bus clock.
• WCT must be equivalent to at least three watchdog clock cycles. If not ensured, this
means that even after the close of the WCT window, you have to wait for the
synchronized system reset to deassert in the watchdog clock domain, before
expecting the configuration updates to take effect.
• The time-out value of the watchdog should be set to a minimum of four watchdog
clock cycles. This is to take into account the delay in new settings taking effect in the
watchdog clock domain.
• You must take care not only to refresh the watchdog within the watchdog timer's
actual time-out period, but also provide enough allowance for the time it takes for the
refresh sequence to be detected by the watchdog timer, on the watchdog clock.
• Updates cannot be made in the bus clock cycle immediately following the write of
the unlock sequence, but one bus clock cycle later.
• It should be ensured that the time-out value for the watchdog is always greater than
2xWCT time + 20 bus clock cycles.
• An attempted refresh operation, in between the two writes of the unlock sequence
and in the WCT time following a successful unlock, will go undetected.
• Trying to unlock the watchdog within the WCT time after an initial unlock has no
effect.
• The refresh and unlock operations and interrupt are not automatically disabled in the
watchdog functional test mode.
• After emerging from a reset due to a watchdog functional test, you are still expected
to go through the mandatory steps of unlocking and configuring the watchdog. The
watchdog continues to be in its functional test mode and therefore you should pull
the watchdog out of the functional test mode within WCT time of reset.
Chapter 25 Watchdog Timer (WDOG)
KV4x Reference Manual, Rev. 2, 02/2015
Freescale Semiconductor, Inc.
Preliminary
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