OM13008,598 NXP Semiconductors, OM13008,598 Datasheet - Page 273

• Manufacturer Part Number: OM13008,598
• Description: BOARD LPC XPRESSO LPC122X
• Manufacturer: NXP Semiconductors
• Series: LPCXpressor

Specifications of OM13008,598

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 568-6642; OM13008

NXP Semiconductors
15.6 Functional description
15.7 Example timer calculations
UM10441
User manual
Table 253. System Timer Calibration value register (SYST_CALIB - 0xE000 E01C) bit
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
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 <tbd> MHz.
The following example illustrates selecting the SysTick timer reload value to obtain a
10 ms time interval with the LPC122x system clock set to 30 MHz.
Example (system clock = 30 MHz)
The system tick clock = system clock = 30 MHz.
RELOAD = (system tick clock frequency × 10 ms) −1 = (30 MHz × 10 ms) −1 = 300000 −1
= 299999 = 0x000493DF.
Bit
23:0
29:24
30
31
To use the system tick timer, do the following:
1. Program the SYST_RVR register with the reload value RELOAD to obtain the desired
2. Clear the SYST_CVR register by writing to it. This ensures that the timer will count
3. Program the SYST_SCR register with the value 0x7 which enables the SysTick timer
time interval.
from the SYST_RVR value rather than an arbitrary value when the timer is enabled.
and the SysTick timer interrupt.
Symbol
TENMS
-
SKEW
NOREF
description
All information provided in this document is subject to legal disclaimers.
Value
Rev. 1.1 — 10 March 2011
Description
See
Reserved, user software should not write ones to
reserved bits. The value read from a reserved bit is not
defined.
See
See
Table
Table
Table
Chapter 15: LPC122x System Tick (SysTick) timer
395.
395.
395.
Figure
UM10441
© NXP B.V. 2011. All rights reserved.
3) or from the
Reset
value
0x1F
NA
0
0
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