Grasp LVDS and automated measurements (Part 1)

Characterising LVDS devices requires us to make measurements under a range of power-supply voltage, temperature, and process variations. An automated test system makes and records the measurements and analyses the results. I have used those experiments in a modified way with fully automated processes done using LabVIEW for the characterisation of OpenLDI (Open LVDS Display Interface) used for controlling automotive displays. In this two-part article, I'll first provide a background on LVDS. Part 2 will explain how we automate the measurements.

LVDS basics
The LVDS standard was created to address applications in data communications, telecommunications, servers, peripherals, and computers that need high-speed data transfer. Defined in the TIA/EIA-644 standard, LVDS is a low voltage, low power, differential technology used primarily for point-to-point and multi-drop cable driving applications. The standard was developed under the Data Transmission Interface committee TR30.2. It specifies a maximum data rate of 655Mbit/s although some of today's applications exceed this data rate.

LVDS has a typical voltage swing of 350 mV with a typical offset voltage of 1.25 V above ground. Its low swing differential constant-current source configuration supports fast switching speeds and low power consumption.

LVDS can use 5 V, 3.3 V, and 2.5 V power supply voltages. Low-voltage signals have many advantages including faster bit rates, lower power, and better noise performance than higher voltages. To increase noise immunity and noise margins, LVDS uses differential data transmission.

LVDS display bridge
The LVDS physical interface is part of an LDB (LVDS Display Bridge), which is a component of an SoC. Figure 1 shows The block diagram of the SoC. The DCU (display control unit) retrieves data from memory, processes it, and transfers it to the LDB. The LDB then serialises this data and transfers it to displays where it is again obtained in parallel format using converters. My main task is to characterise the data obtained at the LBD's output.

Figure 1: An SoC containing an LVDS Display Bridge requires automation or characterise.

The LVDS physical layer
Figure 2 shows the equivalent structure of the LVDS physical layer. The driver's current source limits the output to about 3 mA and minimises current spikes. The switch box steers the current through the termination resistor. This differential driver produces a differential-mode signal, with equal and opposite currents flowing in the transmission lines. The current returns within the wire pair, so the current area is small and therefore generates the lowest amount of EMI.

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