DS92LV1021A Datasheet, PDF (2/12 Pages) National Semiconductor (TI) – 16-40 MHz 10 Bit Bus LVDS Serializer

Block Diagrams (Continued)
Application
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Functional Description
The DS92LV1021A is an upgrade to the DS92LV1021. The
DS92LV1021A no longer has a power-up sequence require-
ment. Like the DS92LV1021, the DS92LV1021A is a 10-bit
Serializer designed to transmit data over a differential back-
plane at clock speeds from 16 to 40MHz. It may also be used
to drive data over Unshielded Twisted Pair (UTP) cable.
The DS92LV1021A can be used with any of National’s 10-bit
BLVDS Deserializers (DS92LV1212A for example) and has
three active states of operation: Initialization, Data Transfer,
and Resynchronization; and two passive states: Powerdown
and TRI-STATE.
The following sections describe each active and passive
state.
Initialization
Before data can be transferred, the Serializer must be initial-
ized. Initialization refers to synchronization of the Serializer’s
PLL to a local clock.
When V_CC is applied to the Serializer, the outputs are held in
TRI-STATE and internal circuitry is disabled by on-chip
power-on circuitry. When V_CC reaches V_CC OK (2.5V) the
Serializer’s PLL begins locking to the local clock. The local
clock is the transmit clock, TCLK, provided by the source
ASIC or other device.
Once the PLL locks to the local clock, the Serializer is ready
to send data or SYNC patterns, depending on the levels of
the SYNC1 and SYNC2 inputs. The SYNC pattern is com-
posed of six ones and six zeros switching at the input clock
rate.
Control of the SYNC pins is left to the user. One recommen-
dation is a direct feedback loop from the LOCK pin. Under all
circumstances, the Serializer stops sending SYNC patterns
after both SYNC inputs return low.
Data Transfer
After initialization, the Serializer inputs DIN0–DIN9 may be
used to input data to the Serializer. Data is clocked into the
Serializer by the TCLK input. The edge of TCLK used to
strobe the data is selectable via the TCLK_R/F pin.
TCLK_R/F high selects the rising edge for clocking data and
low selects the falling edge. If either of the SYNC inputs is
high for 5*TCLK cycles, the data at DIN0-DIN9 is ignored
regardless of the clock edge.
A start bit and a stop bit, appended internally, frame the data
bits in the register. The start bit is always high and the stop
bit is always low. The start and stop bits function as the
embedded clock bits in the serial stream.
Serialized data and clock bits (10+2 bits) are transmitted
from the serial data output (DO±) at 12 times the TCLK
frequency. For example, if TCLK is 40 MHz, the serial rate is
40 x 12 = 480 Mega bits per second. Since only 10 bits are
from input data, the serial “payload” rate is ten times the
TCLK frequency. For instance, if TCLK = 40 MHz, the pay-
load data rate is 40 x 10 = 400 Mbps. TCLK is provided by
the data source and must be in the range of 16 MHz to 40
MHz nominal.
The outputs (DO±) can drive a backplane or a point-to-point
connection. The outputs transmit data when the enable pin
(DEN) is high, PWRDN is high, and SYNC1 and SYNC2 are
low. The DEN pin may be used to TRI-STATE the outputs
when driven low.
Ideal Crossing Point
The ideal crossing point is the best case start and stop point
for a normalized bit. Each ideal crossing point is found by
dividing the clock period by twelve--two clock bits plus ten
data bits. For example, a 40 MHz clock has a period of 25ns.
The 25ns divided by 12 bits is approximately 2.08ns. This
means that each bit width is approximately 2.08ns, and the
ideal crossing points occur every 2.08ns. For a graphical
representation, please see Figure 9.
Resynchronization
The Deserializer LOCK pin driven low indicates that the
Deserializer PLL is locked to the embedded clock edge. If
the Deserializer loses lock, the LOCK output will go high and
the outputs (including RCLK) will be TRI-STATE.
The LOCK pin must be monitored by the system to detect a
loss of synchronization, and the system must decide if it is
necessary to pulse the Serializer SYNC1 or SYNC2 pin to
resynchronize. There are multiple approaches possible. One
recommendation is to provide a feedback loop using the
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