Atmel ARM-Based Evaluation Kit for SAM4S16C, 32-Bit ARM® Cortex® Microcontroller ATSAM4S-WPIR-RD ATSAM4S-WPIR-RD Data Sheet

SAM4S Series [DATASHEET]

Atmel-11100G-ATARM-SAM4S-Datasheet_27-May-14

502

The software can disable or enable the 4/8/12 MHz fast RC oscillator with the MOSCRCEN bit in the Clock 

Generator Main Oscillator Register (CKGR_MOR).
The user can also select the output frequency of the fast RC oscillator, either 4/8/12 MHz are available. It can be 

done through MOSCRCF bits in CKGR_MOR. When changing this frequency selection, the MOSCRCS bit in the 

Power Management Controller Status Register (PMC_SR) is automatically cleared and MAINCK is stopped until 

the oscillator is stabilized. Once the oscillator is stabilized, MAINCK restarts and MOSCRCS is set.
When disabling the main clock by clearing the MOSCRCEN bit in CKGR_MOR, the MOSCRCS bit in the Power 

Management Controller Status Register (PMC_SR) is automatically cleared, indicating the main clock is off.
Setting the MOSCRCS bit in the Power Management Controller Interrupt Enable Register (PMC_IER) can trigger 

an interrupt to the processor.
It is recommended to disable the main clock as soon as the processor no longer uses it and runs out of SLCK.
The CAL4, CAL8 and CAL12 values in the PMC Oscillator Calibration Register (PMC_OCR) are the default values 

set by Atmel during production. These values are stored in a specific Flash memory area different from the main 

memory plane. These values cannot be modified by the user and cannot be erased by a Flash erase command or 

by the ERASE pin. Values written by the user's application in PMC_OCR are reset after each power up or 

peripheral reset.

It is possible for the user to adjust the main RC oscillator frequency through PMC_OCR. By default, SEL4/8/12 are 

low, so the RC oscillator will be driven with Flash calibration bits which are programmed during chip production.
The user can adjust the trimming of the 4/8/12 MHz fast RC oscillator through this register in order to obtain more 

accurate frequency (to compensate derating factors such as temperature and voltage). 
In order to calibrate the oscillator lower frequency, SEL

4

 must be set to 1 and a good frequency value must be 

configured in CAL4. Likewise, SEL8/12 must be set to 1 and a trim value must be configured in CAL

8

/12 in order 

to adjust the other frequencies of the oscillator.
It is possible to adjust the oscillator frequency while operating from this clock. For example, when running on 

lowest frequency it is possible to change the CAL4 value if SEL4 is set in PMC_OCR.
It is possible to restart, at anytime, a measurement of the main frequency by means of the RCMEAS bit in Main 

Clock Frequency Register (CKGR_MCFR). Thus, when MAINFRDY flag reads 1, another read access on Main 

Clock Frequency Register (CKGR_MCFR) provides an image of the frequency of the main clock on MAINF field. 

The software can calculate the error with an expected frequency and correct the CAL4 (or CAL8/CAL12) field 

accordingly. This may be used to compensate frequency drift due to derating factors such as temperature and/or 

voltage.

After reset, the 3 to 20 MHz crystal or ceramic resonator-based oscillator is disabled and it is not selected as the 

source of MAINCK.
The user can select the 3 to 20 MHz crystal or ceramic resonator-based oscillator to be the source of MAINCK, as 

it provides a more accurate frequency. The software enables or disables the main oscillator so as to reduce power 

consumption by clearing the MOSCXTEN bit in the Main Oscillator Register (CKGR_MOR). 
When disabling the main oscillator by clearing the MOSCXTEN bit in CKGR_MOR, the MOSCXTS bit in PMC_SR 

is automatically cleared, indicating the main clock is off.
When enabling the main oscillator, the user must initiate the main oscillator counter with a value corresponding to 

the start-up time of the oscillator. This start-up time depends on the crystal frequency connected to the oscillator.
When the MOSCXTEN bit and the MOSCXTST are written in CKGR_MOR to enable the main oscillator, the XIN 

and XOUT pins are automatically switched into oscillator mode and MOSCXTS bit in the Power Management