DS125DF410_13 Datasheet, PDF (11/44 Pages) Texas Instruments – DS125DF410 Low Power Multi-Rate Quad Channel Retimer

DS125DF410
www.ti.com
SNLS398E – JANUARY 2012 – REVISED MAY 2013
The DS125DF410 uses the 25 MHz reference to determine the coarse tuning setting for its internal VCO. On
power-up, on CDR reset, and when the DS125DF410 loses lock and cannot re-acquire lock after four attempts,
the 25 MHz reference is used to calibrate the VCO frequency. The required VCO frequency is set by using the
rate/subrate settings (see Table 1) or by manually setting the PPM count and divide ratio. To calibrate the VCO
frequency, the DS125DF410 searches through the available VCO coarse tuning settings and counts the divided
VCO frequency using the 25 MHz reference as a clock source. The VCO coarse tuning setting which provides
the VCO frequency closest to the required frequency is stored, and this coarse tuning setting is used for
subsequent operation. This produces a fast, robust phase lock to the input signal.
Output Driver
Once the input data has been retimed by the DS125DF410 to the recovered, cleaned, clock, it is output to the
next device in the signal path using the output driver. The DS125DF410 is commonly used in applications where
lossy transmission media exist both at the input and the output of the DS125DF410. The CTLE and the DFE
compensate for lossy transmission media at the input to the DS125DF410. The output de-emphasis
compensates for the lossy transmission media at the output of the DS125DF410.
When there is a transition in the output data stream, the output differential voltage reaches its configured
maximum value within the configured rise/fall time of the output driver. Following this, the differential voltage
rapidly falls off until it reaches the configured VOD level minus the configured de-emphasis level. This
accentuates the high-frequency components of the output driver signal at the expense of the low-frequency
components. This pre-distorted signal, with its high-frequency components emphasized relative to its low-
frequency components, travels down the dispersive transmission media at the output of the DS125DF410 with
less inter-symbol interference than an undistorted signal would exhibit.
The output driver is capable of driving variable output voltages with variable amounts of analog de-emphasis.
The output voltage and de-emphasis level can be configured by writing registers over the SMBus. The
DS125DF410 cannot determine independently the appropriate output voltage or de-emphasis setting, so the user
is responsible for configuring these parameters. They can be set for each channel independently.
An idealized transmit waveform with analog de-emphasis applied is shown in Figure 2.
1.0
0.5
0.0
-0.5
-1.0
0 1 2 3 4 5 6 7 8 9 10
TIME (UI)
Figure 2. Idealized De-Emphasis Waveform
DEVICE CONFIGURATION
The DS125DF410 can be configured by the user to optimize its operation. The four channels can be optimized
independently in SMBus master or SMBus slave mode. The operational settings available for user configuration
include the following.
• CTLE boost setting
• DFE tap weight and polarity setting
• Rate and subrate setting
• Driver output voltage
• Driver output de-emphasis
• Driver output rise/fall time
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