Atmel SAM4S Series, Предварительный информационный лист

Страница: 479 / 1163

479
11100B–ATARM–31-Jul-12
SAM4S Series [Preliminary]
27.1.5.1 4/8/12 MHz Fast RC Oscillator
After reset, the 4/8/12 MHz Fast RC Oscillator is enabled with the 4 MHz frequency selected and
it is selected as the source of MAINCK. MAINCK is the default clock selected to start up the
system.
The Fast RC Oscillator 8 and 12 MHz frequencies are calibrated in production. Note that is not
the case for the 4 MHz frequency.
Please refer to the “DC Characteristics” section of the product datasheet.
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 MHz, 8 MHz or
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 Regis-
ter (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, PLLACK or PLLBCK.
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.
27.1.5.2 4/8/12 MHz Fast RC Oscillator Clock Frequency Adjustment
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 4 MHz oscillator frequency, SEL4 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 CAL8/12 in order to adjust the 8/12 MHz frequency oscillator.
It is possible to adjust the oscillator frequency while operating from this clock. For example,
when running on 4 MHz 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 is
set, the MAINF field returns the frequency of the main clock and 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.