NXP Semiconductors LPC1311 User Manual

UM10375 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2011. All rights reserved.
User manual Rev. 3 — 14 June 2011 298 of 368
NXP Semiconductors
UM10375
Chapter 17: LPC13xx System tick timer
17.7 Example timer calculations
To use the system tick timer, do the following:
1. Program the LOAD register with the reload value RELOAD to obtain the desired time
interval.
2. Clear the VAL register by writing to it. This ensures that the timer will count from the
LOAD value rather than an arbitrary value when the timer is enabled.
The following examples illustrate selecting SysTick timer reload values for different
system configurations. All of the examples calculate an interrupt interval of
10 milliseconds, as the SysTick timer is intended to be used, and there are no rounding
errors.
System clock = 72 MHz
Program the CTRL register with the value 0x7 which selects the system clock as the clock
source and enables the SysTick timer and the SysTick timer interrupt.
RELOAD = (system clock frequency  10 ms) 1 = (72 MHz  10 ms) 1 = 720000 1 =
719999 = 0x000AFC7F
System tick timer clock = 24 MHz
Program the CTRL register with the value 0x3 which selects the clock from the system tick
clock divider (use DIV = 3) as the clock source and enables the SysTick timer and the
SysTick timer interrupt.
RELOAD = (system tick timer clock frequency  10 ms) 1 = (24 MHz  10 ms) 1 =
240000 1 = 239999 = 0x0003A97F
System clock = 12 MHz
Program the CTRL register with the value 0x7 which selects the system clock as the clock
source and enables the SysTick timer and the SysTick timer interrupt.
In this case the system clock is derived from the IRC clock.
RELOAD = (system clock frequency  10 ms) 1 = (12 MHz  10 ms) 1 = 120000 1 =
119999 = 0x0001D4BF

NXP Semiconductors LPC1311 Specifications

General

Brand	NXP Semiconductors
Model	LPC1311
Category	Controller
Language	English