ARM Cortex-M4 - Systick Current Value Register; Systick Calibration Value Register; SYST_CALIB Register Bit Assignments

Cortex-M4 Peripherals
ARM DUI 0553A Copyright © 2010 ARM. All rights reserved. 4-35
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4.4.3 SysTick Current Value Register
The SYST_CVR register contains the current value of the SysTick counter. See the register
summary in Table 4-32 on page 4-33 for its attributes. The bit assignments are:
4.4.4 SysTick Calibration Value Register
The SYST_CALIB register indicates the SysTick calibration properties. See the register
summary in Table 4-32 on page 4-33 for its attributes. The reset value of this register is
implementation-defined. See the documentation supplied by your device vendor for more
information about the meaning of the SYST_CALIB field values. The bit assignments are:
If calibration information is not known, calculate the calibration value required from the
frequency of the processor clock or external clock.
31
0
CURRENTReserved
2324
Table 4-35 SYST_CVR register bit assignments
Bits Name Function
[31:24] - Reserved.
[23:0] CURRENT Reads return the current value of the SysTick counter.
A write of any value clears the field to 0, and also clears the SYST_CSR COUNTFLAG bit to 0.
31
0
TENMSReserved
2324
30
SKEW
NOREF
29
Table 4-36 SYST_CALIB register bit assignments
Bits Name Function
[31] NOREF Indicates whether the device provides a reference clock to the processor:
0 = reference clock provided
1 = no reference clock provided.
If your device does not provide a reference clock, the SYST_CSR.CLKSOURCE bit reads-as-one
and ignores writes.
[30] SKEW Indicates whether the TENMS value is exact:
0 = TENMS value is exact
1 = TENMS value is inexact, or not given.
An inexact TENMS value can affect the suitability of SysTick as a software real time clock.
[29:24] - Reserved.
[23:0] TENMS Reload value for 10ms (100Hz) timing, subject to system clock skew errors. If the value reads as
zero, the calibration value is not known.