SAM3SD8C Atmel Corporation, SAM3SD8C Datasheet - Page 455

• Manufacturer Part Number: SAM3SD8C
• Manufacturer: Atmel Corporation

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SAM3S8/SD8
SAM3S8/SD8
Note:
• If a new value for CSS field corresponds to Main Clock or Slow Clock,
5. Selection of Programmable Clocks
If at some stage one of the following parameters, CSS or PRES is modified, the MCKRDY
bit will go low to indicate that the Master Clock and the Processor Clock are not ready yet.
The user must wait for MCKRDY bit to be set again before using the Master and Processor
Clocks.
Code Example:
The Master Clock is main clock divided by 2.
The Processor Clock is the Master Clock.
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
– Program the CSS field in PMC_MCKR.
– Wait for the MCKRDY bit to be set in PMC_SR.
– Program the CSS field in PMC_MCKR.
– Wait for the MCKRDY bit to be set in the PMC_SR.
– Program the PRES field in PMC_MCKR.
– Wait for the MCKRDY bit to be set in PMC_SR.
write_register(PMC_MCKR,0x00000001)
wait (MCKRDY=1)
write_register(PMC_MCKR,0x00000011)
wait (MCKRDY=1)
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 25.1.14.2 “Clock Switching Waveforms” on page
11090A–ATARM–10-Feb-12
11090A–ATARM–10-Feb-12
458.