Datasheet

230
SAM4S Series [DATASHEET]
11100F–ATARM–29-Jan-14
12.10.1.4 SysTick Calibration Value Register
Name: SYST_CALIB
Access: Read/Write
Reset: 0x000030D4
The SysTick SYST_CSR indicates the SysTick calibration properties.
• NOREF: No Reference Clock
It 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.
• SKEW: TENMS Value Verification
It 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.
• TENMS: Ten Milliseconds
The reload value for 10 ms (100 Hz) timing is subject to system clock skew errors. If the value reads as zero, the calibration value
is not known.
The TENMS field default value is 0x000030D4 (12500 decimal).
In order to achieve a 1 ms timebase on SystTick, the TENMS field must be programmed to a value corresponding to the proces-
sor clock frequency (in kHz) divided by 8.
For example, for devices running the processor clock at 48 MHz, the TENMS field value must be 0x0001770 (48000 kHz/8)
.
31 30 29 28 27 26 25 24
NOREF SKEW –
23 22 21 20 19 18 17 16
TENMS
15 14 13 12 11 10 9 8
TENMS
76543210
TENMS