AT32UC3A0512_1 Datasheet, PDF (58/826 Pages) ATMEL Corporation – AVR32 32-Bit Microcontroller

AT32UC3A
13.5.5
Synchronous clocks
The slow clock (default), Oscillator 0, or PLL0 provide the source for the main clock, which is the
common root for the synchronous clocks for the CPU/HSB, PBA, and PBB modules. The main
clock is divided by an 8-bit prescaler, and each of these four synchronous clocks can run from
any tapping of this prescaler, or the undivided main clock, as long as f_CPU f_PBA,B,. The synchro-
nous clock source can be changed on-the fly, responding to varying load in the application. The
clock domains can be shut down in sleep mode, as described in ”Sleep modes” on page 60.
Additionally, the clocks for each module in the four domains can be individually masked, to avoid
power consumption in inactive modules.
Sleep
instruction
Sleep
Controller
Slow clock
Osc0 clock
PLL0 clock
MCSEL
Prescaler
0
Main clock
1
CPUDIV
CPUSEL
Mask
CPUMASK
CPU clocks
HSB clocks
PBAclocks
PBB clocks
13.5.5.1
13.5.5.2
Figure 13-4. Synchronous clock generation
Selecting PLL or oscillator for the main clock
The common main clock can be connected to the slow clock, Oscillator 0, or PLL0. By default,
the main clock will be connected to the slow clock. The user can connect the main clock to Oscil-
lator 0 or PLL0 by writing the MCSEL bitfield in the Main Clock Control Register (MCCTRL). This
must only be done after that unit has been enabled, otherwise a deadlock will occur. Care
should also be taken that the new frequency of the synchronous clocks does not exceed the
maximum frequency for each clock domain.
Selecting synchronous clock division ratio
The main clock feeds an 8-bit prescaler, which can be used to generate the synchronous clocks.
By default, the synchronous clocks run on the undivided main clock. The user can select a pres-
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