at91sam9263 ATMEL Corporation, at91sam9263 Datasheet - Page 356

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

356
AT91SAM9263 Preliminary
5. Selection of Master Clock and Processor Clock
• If a new value for CSS field corresponds to PLL Clock,
The MULB field is the PLL B multiplier factor. This parameter can be programmed between
0 and 2047. If MULB is set to 0, PLL B will be turned off, otherwise the PLL B output fre-
quency is PLL B input frequency multiplied by (MULB + 1).
The PLLBCOUNT field specifies the number of slow clock cycles before LOCKB bit is set in
the PMC_SR register after CKGR_PLLBR register has been written.
Once the PMC_PLLB register has been written, the user must wait for the LOCKB bit to be
set in the PMC_SR register. This can be done either by polling the status register or by wait-
ing the interrupt line to be raised if the associated interrupt to LOCKB has been enabled in
the PMC_IER register. All parameters in CKGR_PLLBR can be programmed in a single
write operation. If at some stage one of the following parameters, MULB, DIVB is modified,
LOCKB bit will go low to indicate that PLL B is not ready yet. When PLL B is locked, LOCKB
will be set again. The user is constrained to wait for LOCKB bit to be set before using the
PLL A output clock.
The USBDIV field is used to control the additional divider by 1, 2 or 4, which generates the
USB clock(s).
Code Example:
If PLL B and divider B are enabled, the PLL B input clock is the main clock. PLL B output
clock is PLL B input clock multiplied by 5. Once CKGR_PLLBR has been written, LOCKB bit
will be set after eight slow clock cycles.
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.
The MDIV field is used to control the Master Clock prescaler. It is possible to choose
between different values (0, 1, 2). The Master Clock output is Processor Clock divided by 1,
2 or 4, depending on the value programmed in MDIV. By default, MDIV is set to 0, which
indicates that the Processor Clock is equal to the Master 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:
– Program the PRES field in the PMC_MCKR register.
write_register(CKGR_PLLBR,0x00040805)
6249D–ATARM–20-Dec-07