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UM10360_0
© NXP B.V. 2009. All rights reserved.
User manual
Rev. 00.06 — 5 June 2009
753 of 808
NXP Semiconductors
UM10360
Chapter 34: Appendix: Cortex-M3 User Guide
4.4 System timer, SysTick
The processor has a 24-bit system timer, SysTick, that counts down from the reload value
to zero, reloads (wraps to) the value in the LOAD register on the next clock edge, then
counts down on subsequent clocks.
Note: refer to the separate chapter in the LPC17xx User Manual
for
device specific information on the System Timer.
Remark:
when the processor is halted for debugging the counter does not decrement.
The system timer registers are:
[1]
SysTick calibration value. This value is specific to LPC17xx devices.
4.4.1 SysTick Control and Status Register
The SysTick CTRL register enables the SysTick features. See the register summary in
for its attributes. The bit assignments are shown in
When ENABLE is set to 1, the counter loads the RELOAD value from the LOAD register
and then counts down. On reaching 0, it sets the COUNTFLAG to 1 and optionally asserts
the SysTick depending on the value of TICKINT. It then loads the RELOAD value again,
and begins counting.
Table 646. System timer registers summary
Address
Name
Type
Required
privilege
Reset
value
Description
0xE000E010
CTRL
RW
Privileged
0x00000004
0xE000E014
LOAD
RW
Privileged
0x00000000
0xE000E018
VAL
RW
Privileged
0x00000000
0xE000E01C
CALIB
RO
Privileged
0x000F423F
Table 647. SysTick CTRL register bit assignments
Bits
Name
Function
[31:17]
-
Reserved.
[16]
COUNTFLAG
Returns 1 if timer counted to 0 since last time this was read.
[15:3]
-
Reserved.
[2]
CLKSOURCE
Indicates the clock source:
0 = external clock
1 = processor clock.
[1]
TICKINT
Enables SysTick exception request:
0 = counting down to zero does not assert the SysTick exception
request
1 = counting down to zero to asserts the SysTick exception request.
Software can use COUNTFLAG to determine if SysTick has ever
counted to zero.
[0]
ENABLE
Enables the counter:
0 = counter disabled
1 = counter enabled.