Information

other stages, N – 2, N – 3... and N + 1, N + 2... are enabled for the test on byte N. These
disabled stages, except the most significant stage of the counter, are loaded with a value
of 0xFF.
23.5 Backup reset generator
The backup reset generator generates the final reset which goes out to the system. It has a
backup mechanism which ensures that in case the bus clock stops and prevents the main
state machine from generating a reset exception/interrupt, the watchdog timer's time-out
is separately routed out as a reset to the system. Two successive timer time-outs without
an intervening system reset result in the backup reset generator routing out the time-out
signal as a reset to the system.
23.6 Generated resets and interrupts
The watchdog generates a reset in the following events, also referred to as exceptions:
• A watchdog time-out
• Failure to unlock the watchdog within WCT time after system reset deassertion
• No update of the control and configuration registers within the WCT window after
unlocking. At least one of the following registers must be written to within the WCT
window to avoid reset:
• WDOG_ST_CTRL_H, WDOG_ST_CTRL_L
• WDOG_TO_VAL_H, WDOG_TO_VAL_L
• WDOG_WIN_H, WDOG_WIN_L
• WDOG_PRESCALER
• A value other than the unlock sequence or the refresh sequence is written to the
unlock and/or refresh registers, respectively.
• A gap of more than 20 bus cycles exists between the writes of two values of the
unlock sequence.
• A gap of more than 20 bus cycles exists between the writes of two values of the
refresh sequence.
Backup reset generator
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
416 Freescale Semiconductor, Inc.