at91sam9263 ATMEL Corporation, at91sam9263 Datasheet - Page 357

• Manufacturer Part Number: at91sam9263
• Description: At91 Arm Thumb Microcontrollers
• Manufacturer: ATMEL Corporation

6249D–ATARM–20-Dec-07
Note:
• If a new value for CSS field corresponds to Main Clock or Slow Clock,
6. 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 16.
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 the PMC_SCER and PMC_SCDR
registers. Depending on the system used,
abled. The PMC_SCSR provides a clear indication as to which Programmable clock is
enabled. By default all Programmable clocks are disabled.
PMC_PCKx registers 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 1 which means
that master clock is equal to slow clock.
– Wait for the MCKRDY bit to be set in the PMC_SR register.
– Program the CSS field in the PMC_MCKR register.
– Wait for the MCKRDY bit to be set in the PMC_SR register.
– Program the CSS field in the PMC_MCKR register.
– Wait for the MCKRDY bit to be set in the PMC_SR register.
– Program the PRES field in the PMC_MCKR register.
– Wait for the MCKRDY bit to be set in the PMC_SR register.
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 (CKGR_PLLAR or CKGR_PLLBR), the MCKRDY flag will go low while PLL is
unlocked. Once PLL is locked again, LOCK (LOCKA or LOCKB) goes high and MCKRDY is set.
While PLLA is unlocked, the Master Clock selection is automatically changed to Slow Clock. While
PLLB is unlocked, the Master Clock selection is automatically changed to Main Clock. For further
information, see
Section
27.8.2.
“Clock Switching Waveforms” on page
AT91SAM9263 Preliminary
4
Programmable clocks can be enabled or dis-
360.
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