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Peripheral Clocking
6.3.6 I/O Domains
The I/O domains refer to the frequencies of the peripherals that communicate through device pins. In
many cases, there are frequency requirements for a peripheral pin interface that are set by an outside
standard and must be met. It is not necessarily possible to obtain these frequencies from the on-chip clock
generation circuitry, so the frequencies must be obtained from external sources and are asynchronous to
the CPU frequency by definition.
The peripherals can be divided into the following groups, depending upon their clock requirements, as
shown in
Table 6-8. Peripherals
Peripherals
Contained
Peripheral Group
Peripheral Group Definition
within Group
Source of Peripheral Clock
RTC
Operates off of a dedicated 32 kHz
RTC
—
crystal oscillator.
Fixed-Frequency Peripherals
As the name suggests,
Timer64P0/P1
—
fixed-frequency peripherals have a
I2C0
—
fixed-frequency. They are fed the
AUXCLK directly from the oscillator
input.
Synchronous Peripherals
Synchronous peripherals have their MMC/SD0
PLL0_SYSCLK2
frequencies derived from the ARM
UART0
PLL0_SYSCLK2
clock frequency. The peripheral
GPIO
PLL0_SYSCLK4
system clock frequency changes
accordingly, if the PLL0 frequency
changes. Most synchronous
peripherals have internal dividers
so they can generate their required
clock frequencies.
Asynchronous Peripherals
Asynchronous peripherals are not
UART1/2
ASYNC3
required to operate at a fixed ratio
Timer64P2/P3
ASYNC3
of the ARM clock.
EMIFA
DIV_4P5 or PLL0_SYSCLK3
DDR2/mDDR
PLL1_SYSCLK1 or
PLL1 Direct Output
Synchronous/Asynchronous
Synchronous/asynchronous
McASP0
ASYNC3 or
Peripherals
peripherals can be run with either
Peripheral Serial Clock
internally generated synchronous
SPI0
PLL0_SYSCLK2 or
clocks, or externally generated
Peripheral Serial Clock
asynchronous clocks.
SPI1
ASYNC3 or
Peripheral Serial Clock
EMAC
PLL0_SYSCLK4 or
RMII_MHZ_50_CLK
USB2.0
USB_REFCLKIN or AUXCLK
69
SPRUGX5A
–
May 2011
Device Clocking
Copyright
©
2011, Texas Instruments Incorporated
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