Atmel SAM4S Series, Preliminary Data Sheet

Page: 491 / 1163

491
11100B–ATARM–31-Jul-12
SAM4S Series [Preliminary]
Note: IF PLLx clock was selected as the Master Clock and the user decides to modify it by writing in
CKGR_PLLR, the MCKRDY flag will go low while PLL is unlocked. Once PLL is locked again,
LOCK goes high and MCKRDY is set.
While PLL is unlocked, the Master Clock selection is automatically changed to Slow Clock. For fur-
ther information, see
Section 27.2.14.2 “Clock Switching Waveforms” on page 493 .
Code Example:
write_register(PMC_MCKR,0x00000001)
wait (MCKRDY=1)
write_register(PMC_MCKR,0x00000011)
wait (MCKRDY=1)
The Master Clock is main clock divided by 2.
The Processor Clock is the Master Clock.
5. Selection of Programmable Clocks
Programmable clocks are controlled via registers, PMC_SCER, PMC_SCDR and
PMC_SCSR.
Programmable clocks can be enabled and/or disabled via PMC_SCER and PMC_SCDR. 3
Programmable clocks can be enabled or disabled. The PMC_SCSR provides a clear indica-
tion as to which Programmable clock is enabled. By default all Programmable clocks are
disabled.
Programmable Clock Registers (PMC_PCKx) are used to configure Programmable clocks.
The CSS field is used to select the Programmable clock divider source. Four clock options
are available: main clock, slow clock, PLLACK, PLLBCK. By default, the clock source
selected is slow clock.
The PRES field is used to control the Programmable clock prescaler. It is possible to choose
between different values (1, 2, 4, 8, 16, 32, 64). Programmable clock output is prescaler
input divided by PRES parameter. By default, the PRES parameter is set to 0 which means
that master clock is equal to slow clock.
Once PMC_PCKx has been programmed, The corresponding Programmable clock must be
enabled and the user is constrained to wait for the PCKRDYx bit to be set in PMC_SR. This
can be done either by polling the status register or by waiting the interrupt line to be raised, if
the associated interrupt to PCKRDYx has been enabled in the PMC_IER register. All param-
eters in PMC_PCKx can be programmed in a single write operation.
If the CSS and PRES parameters are to be modified, the corresponding Programmable
clock must be disabled first. The parameters can then be modified. Once this has been
done, the user must re-enable the Programmable clock and wait for the PCKRDYx bit to be
set.
6. Enabling Peripheral Clocks
Once all of the previous steps have been completed, the peripheral clocks can be enabled
and/or disabled via registers PMC_PCER0, PMC_PCER, PMC_PCDR0 and PMC_PCDR.