NXP Semiconductors LPC11U3x User Manual

UM10462 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2016. All rights reserved.
User manual Rev. 5.5 — 21 December 2016 382 of 523
NXP Semiconductors
UM10462
Chapter 18: LPC11U3x/2x/1x System tick timer
18.5.4 System Timer Calibration value register (SYST_CALIB - 0xE000 E01C)
The value of the SYST_CALIB register is driven by the value of the SYSTCKCAL register
in the system configuration block (see Table 37
).
18.6 Functional description
The SysTick timer is a 24-bit timer that counts down to zero and generates an interrupt.
The intent is to provide a fixed 10 millisecond time interval between interrupts. The
SysTick timer is clocked from the CPU clock (the system clock, see Figure 4
) or from the
reference clock, which is fixed to half the frequency of the CPU clock. In order to generate
recurring interrupts at a specific interval, the SYST_RVR register must be initialized with
the correct value for the desired interval. A default value is provided in the SYST_CALIB
register and may be changed by software. The default value gives a 10 millisecond
interrupt rate if the CPU clock is set to 50 MHz.
18.7 Example timer calculations
To use the system tick timer, do the following:
1. Program the SYST_RVR register with the reload value RELOAD to obtain the desired
time interval.
2. Clear the SYST_CVR register by writing to it. This ensures that the timer will count
from the SYST_RVR value rather than an arbitrary value when the timer is enabled.
3. Program the SYST_SCR register with the value 0x7 which enables the SysTick timer
and the SysTick timer interrupt.
The following example illustrates selecting the SysTick timer reload value to obtain a
10 ms time interval with the LPC11U3x/2x/1x system clock set to 50 MHz.
Example (system clock = 50 MHz)
The system tick clock = system clock = 50 MHz. Bit CLKSOURCE in the SYST_CSR
register set to 1 (system clock).
RELOAD = (system tick clock frequency  10 ms) 1 = (50 MHz  10 ms) 1 = 5000001
= 499999 = 0x0007A11F.
Table 349. System Timer Calibration value register (SYST_CALIB - 0xE000 E01C) bit
description
Bit Symbol Value Description Reset
value
23:0 TENMS See Table 455
.0x4
29:24 - Reserved, user software should not write ones to
reserved bits. The value read from a reserved bit is not
defined.
NA
30 SKEW See Table 455.0
31 NOREF See Table 455
.0

NXP Semiconductors LPC11U3x Specifications

General

Brand	NXP Semiconductors
Model	LPC11U3x
Category	Microcontrollers
Language	English