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XRT79L71
PRELIMINARY
363
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
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.
Invoke this feature by setting Bit 6 (SFM Clock Out Enable), within the "LIU Channel Control" Register, to "1"
as depicted below.
Once this bit-field is set 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 poin) and is NOT the "Recovered" clock
signal from the "Clock and Data Recovery" block.
5.3.1.5.2
Operating the "SFM Sythesizer" Block in the "Multiplexer" Mode
If the SFM Synthesizer block is configured to operate in the SFM Mode, then it is expected that any one of the
following signals are provided 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)
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
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