DS90CR481_15 Datasheet, PDF (14/25 Pages) Texas Instruments – 48-Bit LVDS Channel Link SER/DES − 65 - 112 MHz

DS90CR481, DS90CR482
SNLS137D – NOVEMBER 2000 – REVISED APRIL 2013
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RSKMD - RECEIVER SKEW MARGIN WITH DESKEW
RSKMD is a chipset parameter and is applicable when the DESKEW feature of the DS90CR482 is employed. It
is the difference between the receiver’s strobe window and the ideal pulse locations. The DESKEW feature
adjusts for skew between each data channel and the clock channel. This feature is supported up to 80MHz clock
rate. RSKMD must be greater than the summation of: Transmitter’s Pulse Position variance, LVDS Source Clock
Jitter (TJCC), and ISI (if any). See Figure 14. With DESKEW, RSKMD will be a minimum of 25% of TBIT.
Deskew compensates for interconnect skew which includes PCB traces differences, connector skew and cable
skew (for a cable application). PCB trace and connector skew can be compensated for in the design of the
system. Note, cable skew is media type and length dependant. Cable length may be limited by the RSKMD
parameter prior to the interconnect skew reaching 1 TBIT in length due to ISI effects.
POWER DOWN
Both transmitter and receiver provide a power down feature. When asserted current draw through the supply pins
is minimized and the PLLs are shut down. The transmitter outputs are in TRI-STATE when in power down mode.
The receiver outputs are forced to a active LOW state when in the power down mode. (See Table 3 and
Table 4). The PD pin should be driven HIGH to enable the device once V_CC is stable.
CONFIGURATIONS
The transmitter is designed to be connected typically to a single receiver load. This is known as a point-to-point
configuration. It is also possible to drive multiple receiver loads if certain restrictions are made. Only the final
receiver at the end of the interconnect should provide termination across the pair. In this case, the driver still
sees the intended DC load of 100 Ohms. Receivers connected to the cable between the transmitter and the final
receiver must not load down the signal. To meet this system requirement, stub lengths from the line to the
receiver inputs must be kept very short.
CABLE TERMINATION
A termination resistor is required for proper operation to be obtained. The termination resistor should be equal to
the differential impedance of the media being driven. This should be in the range of 90 to 132 Ohms. 100 Ohms
is a typical value common used with standard 100 Ohm twisted pair cables. This resistor is required for control of
reflections and also to complete the current loop. It should be placed as close to the receiver inputs to minimize
the stub length from the resistor to the receiver input pins.
HOW TO CONFIGURE FOR BACKPLANE APPLICATIONS
In a backplane application with differential line impedance of 100Ω the differential line pair-to-pair skew can
controlled by trace layout. The transmitter-DS90CR481 “DS_OPT” pin may be set high. In a backplane
application with short PCB distance traces, pre-emphasis from the transmitter is typically not required. The “PRE”
pin should be left open (do not tie to ground). A resistor pad provision for a pull up resistor to Vcc can be
implemented in case pre-emphasis is needed to counteract heavy capacitive loading effects.
HOW TO CONFIGURE FOR CABLE INTERCONNECT APPLICATIONS
In applications that require the long cable drive capability. The DS90CR481/DS90CR482 chipset is improved
over prior generations of Channel Link devices and offers higher bandwidth support and longer cable drive with
the use of DC balanced data transmission, pre-emphasis. Cable drive is enhanced with a user selectable pre-
emphasis feature that provides additional output current during transitions to counteract cable loading effects.
This requires the use of one pull up resistor to Vcc; please refer to Table 1 to set the level needed. Optional DC
balancing on a cycle-to-cycle basis, is also provided to reduce ISI (Inter-Symbol Interference) for long cable
applications. With pre-emphasis and DC balancing, a low distortion eye-pattern is provided at the receiver end of
the cable. These enhancements allow cables 5+ meters in length to be driven. Depending upon clock rate and
the media being driven, the cable Deskew feature may also be employed - see discussion on DESKEW, RSKM
and RSKMD above.
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