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UM10503
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© NXP B.V. 2015. All rights reserved.
User manual
Rev. 2.1 — 10 December 2015
1085 of 1441
36.1 How to read this chapter
The Alarm timer is available on all LPC43xx/LPC43Sxx parts.
36.2 Basic configuration
The Alarm timer is configured as follows:
•
See
for clocking and power control. The 32 kHz output of the 32 kHz
oscillator must be enabled in the CREG0 register in the CREG block (see
).
•
The Alarm timer interrupt is connected to slot # 4 in the Event router.
Remark:
Only write to the Alarm timer registers while the 32 kHz oscillator is running.
Repeated writes to the Alarm timer registers without the 32 kHz clock can stall the CPU.
To confirm that the 32 kHz clock is running, read the Alarm timer counter register
(DOWNCOUNTER, see
), which counts down from a preset value using the
1024 Hz signal derived from the 32 kHz oscillator.
36.3 General description
The alarm timer is a 16-bit timer and counts down from a preset value. The counter
triggers a status bit when it reaches 0x00 and asserts an interrupt if enabled.
The alarm timer operates in the RTC power domain. It consists of a 16-bit counter
(DOWNCOUNTER) running at a 1024 Hz clock. The 1024 Hz clock is derived from the
32 kHz crystal clock. The alarm timer is inactive when this clock is not active.
The alarm timer counts down from an initial value PRESET. When it reaches 0x0 and the
interrupt is enabled (via SET_EN), bit STATUS is triggered. The counter continues
counting down starting from PRESET. The countdown period is 1 input clock
cycles.
STATUS is propagated to the interrupt output. The interrupt is connected to the Event
router and can be used to wake up the device from a low power mode.
UM10503
Chapter 36: LPC43xx/LPC43Sxx Alarm timer
Rev. 2.1 — 10 December 2015
User manual
Table 864. Alarm timer clocking and power control
Base clock
Branch clock Operating
frequency
Clock to alarm timer register interface
BASE_M4_CLK
CLK_M4_BUS up to 204 MHz
32 kHz crystal oscillator output for the
counter/timer clock
-
-
1024 Hz (fixed
frequency)