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PRELIMINARY
XRT79L71
188
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
Using the 44.736MHz/34.368MHz Synthesized Clock signal (from the SFM Synthesizer Block) as an
External Clock Signal
As mentioned earlier, if the SFM Synthesizer block has been configured to operate in the SFM (Single-
Frequency) Mode, then it will (1) accept a 12.288MHz clock signal (via the DS3CLK/SFMCLK input pin), and it
will use this 12.288MHz clock signal to (2) synthesize either a 44.736MHz or 34.368MHz clock signal
(depending upon whether the XRT79L71 has been configured to operate in the DS3 or E3 Mode) and route
this signal to the Clock and Data Recovery block.
In some applications there may be a desire to externally use the 44.736MHz or 34.368MHz clock signal that is
synthesized by the SFM Synthesizer block elsewhere in the user's board design. If this is the case, then the
XRT79L71 can support this requirement.
The XRT79L71 includes a pin (or ball) that is called CLKOUT (Ball K16). This output pin can be configured to
output the 44.736MHz/34.368MHz clock signal that is synthesized by the SFM Synthesizer block.
The user can invoke this feature by setting Bit 6 (SFM Clock Out Enable), within the LIU Channel Control
Register, to "1" as depicted below.
Once the user sets this bit-field to "1", then the output driver (associated with the CLKOUT pin) will become
active, and either a 44.736MHz or 34.368MHz clock signal (depending upon whether the XRT79L71 has been
configured to operate in the DS3 or E3 Mode) will be output via this pin.
NOTE: If this CLKOUT feature is invoked, the user must be aware that this clock signal is ultimately derived from the
12.288MHz clock signal (being applied to the DS3CLK/SFMCLK input pin) and is NOT the Recovered clock signal
from the Clock and Data Recovery. block
4.3.1.5.2
Operating the SFM Synthesizer Block in the Multiplexer Mode
To configure the SFM Synthesizer block to operate in the SFM Mode, provide any one of the following signals
to the following input pins.
A 44.736MHz clock signal to the DS3CLK/SFMCLK input pin (Ball P16), or/and
A 34.368MHz clock signal to the E3CLK input pin (Ball M16)
The SFM Synthesizer block will then multiplex and route the appropriate clock signal (e.g., the 44.736MHz or
the 34.368MHz) to the Clock and Data Recovery, depending upon whether the XRT79L71 has been configured
to operate in the DS3 or the E3 Mode.
LIU Channel Control Register (Address = 0x1306)
BIT 7
IT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
Unused
SFM
Clock Out
Enable
SFM
Enable
LIU Remote
Loop-back
Mode
LIU Local
Loop-back
Mode
Unused
R/O
R/W
R/O
0
1
0
LIU Channel Control Register (Address = 0x1306)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
Unused
SFM
Clock Out
Enable
SFM
Enable
LIU Remote
Loop-back
Mode
LIU Local
Loop-back
Mode
Unused
R/O
R/W
R/O
0
1
0