UM11029
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© NXP Semiconductors N.V. 2017. All rights reserved.
User manual
Rev. 1.0 — 16 June 2017
424 of 515
NXP Semiconductors
UM11029
Chapter 23: LPC84x Self-wake-up timer (WKT)
23.4 Pin description
The WKT can use a clock input on the external pin PIO0_28 for clocking the wake-up
timer in sleep, deep-sleep, power-down, and deep power-down modes. Select the
external clock source by setting bit SET_EXTCLK in the CTRL register (see
23.5 General description
The self-wake-up timer is a 32-bit, loadable down counter. Writing any non-zero value to
this timer automatically enables the counter and launches a count-down sequence. When
the counter is being used as a wake up timer, this write can occur just prior to entering a
reduced power mode.
When a starting count value is loaded, the self-wake-up timer automatically turns on,
counts from the pre-loaded value down to zero, generates an interrupt and/or a wake up
request, and then turns itself off until re-launched by a subsequent software write.
23.5.1 WKT clock sources
The self-wake-up timer can be clocked from two alternative clock sources:
Turn off the low-power oscillator in deep power-down mode if not used for wake-up timing.
Fig 56. WKT clocking
WKT
SYSCON
PMU
system clock
SYSAHBCLKCTRL
(WKT clock enable)
FRO
PDRUNCFG
(enable FRO and FRO output
FRO_PD, FROOUT_PD)
LPOSC
DPDCTRL
(low-power oscillator enable
LPOSCEN)
DPDCTRL
(WKTCLKIN pad control
WAKECLKPAD_DISABLE)
WKT registers
12 MHz
32-bit counter
COUNT
CLKSEL
SET_EXTCLK
WKTCLKIN
CTRL
div
750 kHz
10 kHz
