DS90CR218 National Semiconductor, DS90CR218 Datasheet - Page 10

• Manufacturer Part Number: DS90CR218
• Description: +3.3V Rising Edge Data Strobe LVDS 21-Bit Channel Link Receiver - 75 MHz
• Manufacturer: National Semiconductor

www.national.com
Applications Information
DECOUPLING CAPACITORS: Bypassing capacitors are
needed to reduce the impact of switching noise which could
limit performance. For a conservative approach three
parallel-connected decoupling capacitors (Multi-Layered Ce-
ramic type in surface mount form factor) between each V
and the ground plane(s) are recommended. The three ca-
pacitor values are 0.1 µF, 0.01 µF and 0.001 µF. An example
is shown in Figure 11 . The designer should employ wide
traces for power and ground and ensure each capacitor has
its own via to the ground plane. If board space is limiting the
number of bypass capacitors, the PLL V
the most filtering/bypassing. Next would be the LVDS V
pins and finally the logic V
CLOCK JITTER: The CHANNEL LINK devices employ a
PLL to generate and recover the clock transmitted across the
LVDS interface. The width of each bit in the serialized LVDS
data stream is one-seventh the clock period. For example, a
75 MHz clock has a period of 13.33 ns which results in a
data bit width of 1.90 ns. Differential skew ( t within one
differential pair), interconnect skew ( t of one differential pair
to another) and clock jitter will all reduce the available win-
FIGURE 11. CHANNEL LINK
Decoupling Configuration
CC
pins.
FIGURE 10. LVDS Serialized Link Termination
CC
(Continued)
should receive
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CC
CC
10
dow for sampling the LVDS serial data streams. Care must
be taken to ensure that the clock input to the transmitter be
a clean low noise signal. Individual bypassing of each V
ground will minimize the noise passed on to the PLL, thus
creating a low jitter LVDS clock. These measures provide
more margin for channel-to-channel skew and interconnect
skew as a part of the overall jitter/skew budget.
COMMON-MODE vs. DIFFERENTIAL MODE NOISE MAR-
GIN: The typical signal swing for LVDS is 300 mV centered
at +1.2V. The CHANNEL LINK receiver supports a 100 mV
threshold therefore providing approximately 200 mV of dif-
ferential noise margin. Common-mode protection is of more
importance to the system’s operation due to the differential
data transmission. LVDS supports an input voltage range of
Ground to +2.4V. This allows for a
center point due to ground potential differences and
common-mode noise.
TRANSMITTER INPUT CLOCK: The transmitter input clock
must always be present when the device is enabled (PWR
DWN = HIGH). If the clock is stopped, the PWR DWN pin
must be used to disable the PLL. The PWR DWN pin must
be held low until after the input clock signal has been reap-
plied. This will ensure a proper device reset and PLL lock to
occur.
POWER SEQUENCING AND POWERDOWN MODE: Out-
puts of the CHANNEL LINK transmitter remain in TRI-STATE
until the power supply reaches 2V. Clock and data outputs
will begin to toggle 10 ms after V
Powerdown pin is above 1.5V. Either device may be placed
into a powerdown mode at any time by asserting the Pow-
erdown pin (active low). Total power dissipation for each
device will decrease to 5 µW (typical).
The transmitter input clock may be applied prior to powering
up and enabling the transmitter. The transmitter input clock
may also be applied after power up; however, the use of the
PWR DWN pin is required as described in the Transmitter
Input Clock section. Do not power up and enable (PWR
DWN = HIGH) the transmitter without a valid clock signal
applied to the TxCLK IN pin.
CC
has reached 3V and the
±
1.0V shifting of the
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CC
to