Micrel, Inc.
SY87725L Evaluation Board
July 2008
6
M9999-071108-B
or (408) 955-1690
Remote Loopback Recovered Clock
Verifies correct operation of the receive CDR. A RefClk of
155.52MHz or 77.76MHz (selected by REFFREQSEL)
must be supplied for the CDR to function. When CD is
HIGH, the recovered clock coming out of SOUT will be
synchronous and at the same data rate as the data
coming into SIN. When CD is LOW, the clock coming out
of SOUT will be synchronous with the RefClk source.
Figure 2. Switch Settings for Remote Loopback
Recovered Clock
Switch Settings
Function
RCV_FSEL0/1 = 11
Sets receive CDR frequency to
2.48832Gbps (For other
frequencies, refer to Receive
Frequency Selection Table on
page 7 of SY87725L
Datasheet.)
TESTb = 1
Disables factory test mode
(enables normal operation)
CD = 1
Enables clock and data
recovery
REFFREQSEL = 1
Selects RefClk of 155.52MHz
(Set to “0” for 77.76MHz
RefClk)
XMT_CTRL0/1 = 10
Selects the remote loopback
mode so SDOUT = recovered
clock
Table 3. Required Switch Settings for
Remote Loopback Recovered Clock Flow
