Signal Descriptions
2-10
Freescale Semiconductor
2.3.2
PLL and Clock Signals
describes signals that are used to support the on-chip clock generation circuitry.
2.3.3
Mode Selection
Table 2-4. PLL and Clock Signals
Signal Name
Abbreviation
Function
I/O
External Clock In
EXTAL
Always driven by an external clock input except when used as a
connection to the external crystal if the internal oscillator circuit is
used. Clock source may be configured during reset. See
“Chip Configuration Module (CCM),”
for more details.
Note:
This signal is also PCI_CLK (33 or 66 MHz) when running from
an external oscillator with PCI enabled.
I
Crystal
XTAL
Used as a connection to the external crystal when the internal
oscillator circuit is used to drive the crystal.
O
RTC External Clock In
EXTAL32K
Crystal input clock for the real-time clock module.
I
RTC Crystal
XTAL32K
Oscillator output to EXTAL RTC crystal.
O
FlexBus Clock Out
FB_CLK
Reflects one-half of the internal bus clock (or one-fourth the
core/system clock). (f
sys/4
)
O
USB Clock In
USB_CLKIN
This pin allows the user to drive the reference clock to the USB module
as an alternate method of generating the USB reference clock during
FS/LS operation. This pin should be driven only with a 60 MHz clock.
When using the ULPI USB interface, this pin is the ULPI input clock.
I
SSI Clock In
SSI_CLKIN
This pin allows the user to drive a specific clock frequency to the SSI
module.
I
Table 2-5. Mode Selection Signals
Signal Name
Abbreviation
Function
I/O
Boot mode
BOOTMOD[1:0] Indicates the device’s boot mode and chip configuration at reset. See
Chapter 11, “Chip Configuration Module (CCM),”
for the signal
encodings.
I
Содержание MCF54455
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