9.1.1 Using the DS90C241 and DS90C124

The DS90C241/DS90C124 serializer or deserializer (SERDES) pair sends 24 bits of parallel LVCMOS data over a serial LVDS link up to 840 Mbps. Serialization of the input data is accomplished using an on-board PLL at the serializer which embeds clock with the data. The deserializer extracts the clock/control information from the incoming data stream and deserializes the data. The deserializer monitors the incoming clockl information to determine lock status and indicates lock by asserting the LOCK output high.

9.1.2 Display Application

The DS90C241/DS90C124 chipset is intended for interface between a host (graphics processor) and a display. It supports an 18-bit color depth (RGB666) and up to 800 × 480 display formats. In a RGB666 configuration 18 color bits (R[5:0], G[5:0], B[5:0]), Pixel Clock (PCLK) and three control bits (VS, HS, and DE) along with three spare bits are supported across the serial link with PCLK rates from 5 MHz to 35 MHz.

9.2.1 Design Requirements

For the typical design application, use the following as input parameters:

The SER/DES supports only AC-coupled interconnects through an integrated DC-balanced decoding scheme. External AC coupling capacitors must be placed in series in the FPD-Link III signal path as illustrated in Figure 22 and Figure 23.

9.2.2 Detailed Design Procedure

Circuit board layout and stack-up for the LVDS serializer and deserializer devices must be designed to provide low-noise power to the device. Good layout practice also separates high frequency or high-level inputs and outputs to minimize unwanted stray noise, feedback and interference. Power system performance may be greatly improved by using thin dielectrics (2 to 4 mil) for power and ground sandwiches. This arrangement uses the plane capacitance for the PCB power system and has low-inductance, which has proven effectiveness especially at high frequencies, and makes the value and placement of external bypass capacitors less critical. External bypass capacitors must include both RF ceramic and tantalum electrolytic types. RF capacitors may use values in the range of 0.01 µF to 10 µF. Tantalum capacitors may be in the 2.2-µF to 10-µF range. The voltage rating of the tantalum capacitors must be at least 5 times the power supply voltage being used.

MLCC surface mount capacitors are recommended due to their smaller parasitic properties. When using multiple capacitors per supply pin, place the smaller value closer to the pin. A large bulk capacitor is recommended at the point of power entry. This is typically in the 50 µF to 100 µF range and smooth low frequency switching noise. TI recommends connecting power and ground pins directly to the power and ground planes with bypass capacitors connected to the plane with through on both ends of the capacitor. Connecting power or ground pins to an external bypass capacitor will increase the inductance of the path. A small body size X7R chip capacitor, such as 0603 or 0805, is recommended for external bypass. A small body sized capacitor has less inductance. The user must pay attention to the resonance frequency of these external bypass capacitors, usually in the range from 20 MHz to 30 MHz. To provide effective bypassing, multiple capacitors are often used to achieve low impedance between the supply rails over the frequency of interest. At high frequency, it is also a common practice to use two vias from power and ground pins to the planes, reducing the impedance at high frequency. Use at least a four layer board with a power and ground plane. Place LVCMOS signals away from the LVDS lines to prevent coupling from the LVCMOS lines to the LVDS lines. Closely coupled differential lines of 100 Ω are typically recommended for LVDS interconnect. The closely coupled lines help to ensure that coupled noise will appear as common mode and thus is rejected by the receivers. The tightly coupled lines will also radiate less.

9.2.2.2 Transmission Media

The serializer and deserializer can be used in point-to-point configuration, through a PCB trace, or through twisted pair cable. In a point-to-point configuration, the transmission media requires termination at both ends of the transmitter and receiver pair. Interconnect for LVDS typically has a differential impedance of 100 Ω. Use cables and connectors that have matched differential impedance to minimize impedance discontinuities. In most applications that involve cables, the transmission distance is determined on data rates involved, acceptable bit error rate and transmission medium.

The resulting signal quality at the receiving end of the transmission media may be assessed by monitoring the differential eye opening of the serial data stream. The Receiver Input Tolerance in Switching Characteristics – Deserializer and the Differential Threshold Voltage specifications in Electrical Characteristics define the acceptable data eye opening. A differential probe must be used to measure across the termination resistor at the DS90C124 inputs. Figure 24 illustrates the eye opening and relationship to the receiver input tolerance and differential threshold voltage specifications.

Figure 24. Receiver Input Eye Opening

9.2.3 Application Curves

Figure 25, Figure 26, and Figure 27 are scope shots with PCLK = 25 MHz into the DS90C241 with a 1010... pattern on the DIN[23:0] inputs. The scope was triggered on the input PCLK.