TLK2521

1 to 2.5 Gbps TRANSCEIVER

SLLS574B − JULY 2003 − REVISED JANUARY 2004

4

POST OFFICE BOX 655303

GTX_CLK

TXDn

tsu

th

Figure 1. Transmit Timing Waveform

transmission latency

The data transmission latency of the TLK2521 is defined as the delay from the initial 18-bit word load to the serial

transmission of bit 0. The transmit latency is fixed once the link is established. However, due to silicon process

variations and implementation variables such as supply voltage and temperature, the exact delay varies slightly.

Figure 2 illustrates the timing relationship between the transmit data bus, GTX_CLK, and serial transmit pins.

Detailed latency information can be found in the transmitter/receiver characteristics table.

16-Bit Word to Transmit

Transmitted 20-Bit Word

DOUTTXP,

DOUTTXN

TXD(0−17)

GTX_CLK

td(Tx latency)

Figure 2. Transmitter Latency

start/stop framing logic

All true serial interfaces require a method of encoding to insure minimum transition density so that the receiving

PLL has a minimal number of transitions in which to stay locked onto the data stream. The signal coding also

provides a mechanism for the receiver to identify the byte boundary for correct deserialization. The TLK2521

wraps a start bit (1) and a stop bit (0) around the 18-bit data payload as shown in Figure 3. This is transparent

to the user as the TLK2521 internally adds the framing bits to the data such that the user reads and writes actual

18-bit data.

start/stop framing logic (continued)

...

Start

Bit

Stop

Bit

TD0

TD1

TD16 TD17

Start

Bit

Stop

Bit

Figure 3. Serial Output Data Stream with Start and Stop Bit