UM10462
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User manual
Rev. 5.5 — 21 December 2016
372 of 523
NXP Semiconductors
UM10462
Chapter 17: LPC11U3x/2x/1x Windowed Watchdog Timer (WWDT)
17.7 Using the WWDT lock features
The WWDT supports several lock features which can be enabled to ensure that the
WWDT is running at all times:
•
Accidental overwrite of the WWDT clock source
•
Changing the WWDT clock source
•
Changing the WWDT reload value
17.7.1 Accidental overwrite of the WWDT clock
If bit 31 of the WWDT CLKSEL register (
) is set, writes to bit 0 of the CLKSEL
register, the clock source select bit, will be ignored and the clock source will not change.
17.7.2 Changing the WWDT clock source
If bit 5 in the WWDT MOD register is set, the current clock source as selected in the
CLKSEL register is locked and can not be changed either by software or by hardware
when Sleep, Deep-sleep or Power-down modes are entered. Therefore, the user must
ensure that the appropriate WWDT clock source for each power mode is selected
before
setting bit 5 in the MOD register:
•
Active or Sleep modes: Both the IRC or the watchdog oscillator are allowed.
•
Deep-sleep mode: Both the IRC and the watchdog oscillator are allowed. However,
using the IRC during Deep-sleep mode will increase the power consumption. To
minimize power consumption, use the watchdog oscillator as clock source.
•
Power-down mode: Only the watchdog oscillator is allowed as clock source for the
WWDT. Therefore, before setting bit 5 and locking the clock source, the WWDT clock
source must be set to the watchdog oscillator. Otherwise, the part may not be able to
enter Power-down mode.
•
Deep power-down mode: No clock locking mechanisms are in effect as neither the
WWDT nor any of the clocks are running. However, an additional lock bit in the PMU
can be set to prevent the part from even entering Deep power-down mode (see
).
The clock source lock mechanism can only be disabled by a reset of any type.
17.7.3 Changing the WWDT reload value
If bit 4 is set in the WWDT MOD register, the watchdog time-out value (TC) can be
changed only after the counter is below the value of WDWARNINT and WDWINDOW.
The reload overwrite lock mechanism can only be disabled by a reset of any type.