at91cap7e ATMEL Corporation, at91cap7e Datasheet - Page 217

• Manufacturer Part Number: at91cap7e
• Description: Customizable Microcontroller
• Manufacturer: ATMEL Corporation

8549A–CAP–10/08
Note:
5. Selection of Master Clock and Processor Clock
• If a new value for CSS field corresponds to PLL Clock,
• If a new value for CSS field corresponds to Main Clock or Slow Clock,
The Master Clock and the Processor Clock are configurable via the PMC_MCKR register.
The CSS field is used to select the Master Clock divider source. By default, the selected
clock source is slow clock.
The PRES field is used to control the Master Clock prescaler. The user can choose between
different values (1, 2, 4, 8, 16, 32, 64). Master Clock output is prescaler input divided by
PRES parameter. By default, PRES parameter is set to 1 which means that master clock is
equal to slow clock.
Once the PMC_MCKR register has been written, the user must wait for the MCKRDY bit to
be set in the PMC_SR register. This can be done either by polling the status register or by
waiting for the interrupt line to be raised if the associated interrupt to MCKRDY has been
enabled in the PMC_IER register.
The PMC_MCKR register must not be programmed in a single write operation. The pre-
ferred programming sequence for the PMC_MCKR register is as follows:
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.
– Program the PRES 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 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 24.2.9.2
.
“Clock Switching Waveforms” on page 221
AT91CAP7E
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