DS90C124_08 Datasheet, PDF (16/26 Pages) National Semiconductor (TI) – 5-35MHz DC-Balanced 24-Bit LVDS Serializer and Deserializer

Functional Description
The DS90C241 Serializer and DS90C124 Deserializer
chipset is an easy-to-use transmitter and receiver pair that
sends 24-bits of parallel LVCMOS data over a single serial
LVDS link from 120 Mbps to 840 Mbps throughput. The
DS90C241 transforms a 24-bit wide parallel LVCMOS data
into a single high speed LVDS serial data stream with em-
bedded clock and scrambles / DC Balances the data to en-
hance signal quality to support AC coupling. The DS90C124
receives the LVDS serial data stream and converts it back into
a 24-bit wide parallel data and recovered clock. The 24-bit
Serializer/Deserializer chipset is designed to transmit data up
to 10 meters over shielded twisted pair (STP) at clock speeds
from 5 MHz to 35 MHz.
The Deserializer can attain lock to a data stream without the
use of a separate reference clock source; greatly simplifying
system complexity and overall cost. The Deserializer syn-
chronizes to the Serializer regardless of data pattern, deliv-
ering true automatic “plug and lock” performance. It will lock
to the incoming serial stream without the need of special
training patterns or sync characters. The Deserializer recov-
ers the clock and data by extracting the embedded clock
information and validating data integrity from the incoming
data stream and then deserializes the data. The Deserializer
monitors the incoming clock information, determines lock sta-
tus, and asserts the LOCK output high when lock occurs.
Each has a power down control to enable efficient operation
in various applications.
INITIALIZATION AND LOCKING MECHANISM
Initialization of the DS90C241 and DS90C124 must be es-
tablished before each device sends or receives data. Initial-
ization refers to synchronizing the Serializer’s and
Deserializer’s PLL’s together. After the Serializers locks to the
input clock source, the Deserializer synchronizes to the Seri-
alizers as the second and final initialization step.
Step 1: When V_CC is applied to both Serializer and/or Dese-
rializer, the respective outputs are held in TRI-STATE and
internal circuitry is disabled by on-chip power-on circuitry.
When V_CC reaches V_CC OK (2.2V) the PLL in Serializer begins
locking to a clock input. For the Serializer, the local clock is
the transmit clock, TCLK. The Serializer outputs are held in
TRI-STATE while the PLL locks to the TCLK. After locking to
TCLK, the Serializer block is now ready to send data patterns.
The Deserializer output will remain in TRI-STATE while its
PLL locks to the embedded clock information in serial data
stream. Also, the Deserializer LOCK output will remain low
until its PLL locks to incoming data and sync-pattern on the
RIN± pins.
Step 2: The Deserializer PLL acquires lock to a data stream
without requiring the Serializer to send special patterns. The
Serializer that is generating the stream to the Deserializer will
automatically send random (non-repetitive) data patterns dur-
ing this step of the Initialization State. The Deserializer will
lock onto embedded clock within the specified amount of time.
An embedded clock and data recovery (CDR) circuit locks to
the incoming bit stream to recover the high-speed receive bit
clock and re-time incoming data. The CDR circuit expects a
coded input bit stream. In order for the Deserializer to lock to
a random data stream from the Serializer, it performs a series
of operations to identify the rising clock edge and validates
data integrity, then locks to it. Because this locking procedure
is independent on the data pattern, total random locking du-
ration may vary. At the point when the Deserializer’s CDR
locks to the embedded clock, the LOCK pin goes high and
valid RCLK/data appears on the outputs. Note that the LOCK
signal is synchronous to valid data appearing on the outputs.
The Deserializer’s LOCK pin is a convenient way to ensure
data integrity is achieved on receiver side.
DATA TRANSFER
After Serializer lock is established, the inputs DIN0–DIN23
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 TRFB pin. TRFB high
selects the rising edge for clocking data and low selects the
falling edge. The Serializer outputs (DOUT±) are intended to
drive point-to-point connections as shown in Figure 17.
CLK1, CLK0, DCA, DCB are four overhead bits transmitted
along the single LVDS serial data stream. The CLK1 bit is
always high and the CLK0 bit is always low. The CLK1 and
CLK0 bits function as the embedded clock bits in the serial
stream. DCB functions as the DC Balance control bit. It does
not require any pre-coding of data on transmit side. The DC
Balance bit is used to minimize the short and long-term DC
bias on the signal lines. This bit operates by selectively send-
ing the data either unmodified or inverted. The DCA bit is used
to validate data integrity in the embedded data stream. Both
DCA and DCB coding schemes are integrated and automat-
ically performed within Serializer and Deserializer.
Serialized data and clock/control bits (24+4 bits) are trans-
mitted from the serial data output (DOUT±) at 28 times the
TCLK frequency. For example, if TCLK is 35 MHz, the serial
rate is 35 x 28 = 980 Mega bits per second. Since only 24 bits
are from input data, the serial “payload” rate is 24 times the
TCLK frequency. For instance, if TCLK = 35 MHz, the payload
data rate is 35 x 24 = 840 Mbps. TCLK is provided by the data
source and must be in the range of 5 MHz to 35 MHz nominal.
The Serializer outputs (DOUT±) can drive a point-to-point
connection. The outputs transmit data when the enable pin
(DEN) is high, TPWDNB is high. The DEN pin may be used
to TRI-STATE the outputs when driven low.
When the Deserializer channel attains lock to the input from
a Serializer, it drives its LOCK pin high and synchronously
delivers valid data and recovered clock on the output. The
Deserializer locks onto the embedded clock, uses it to gen-
erate multiple internal data strobes, and then drives the re-
covered clock to the RCLK pin. The recovered clock (RCLK
output pin) is synchronous to the data on the ROUT[23:0]
pins. While LOCK is high, data on ROUT[23:0] is valid. Oth-
erwise, ROUT[23:0] is invalid. The polarity of the RCLK edge
is controlled by the RRFB input. ROUT(0-23), LOCK and
RCLK outputs will each drive a maximum of 8 pF load with a
35 MHz clock. REN controls TRI-STATE for ROUTn and the
RCLK pin on the Deserializer.
RESYNCHRONIZATION
If the Deserializer loses lock, it will automatically try to re-es-
tablish lock. For example, if the embedded clock edge is not
detected one time in succession, the PLL loses lock and the
LOCK pin is driven low. The Deserializer then enters the op-
erating mode where it tries to lock to a random data stream.
It looks for the embedded clock edge, identifies it and then
proceeds through the locking process. The logic state of the
LOCK signal indicates whether the data on ROUT is valid;
when it is high, the data is valid. The system must monitor the
LOCK pin to determine whether data on the ROUT is valid.
POWERDOWN
The Powerdown state is a low power sleep mode that the Se-
rializer and Deserializer may use to reduce power when no
data is being transferred. The TPWDNB and RPWDNB are
www.national.com
16