In the last 10 years, the Mobile Industry Processor Interface (MIPI) Alliance has created a set of specifications for communication between components within mobile devices, such as phones and tablets. In 2007, the UNH InterOperability Laboratory (UNH-IOL) launched its MIPI consortium, which is currently one of the only labs that tests conformance against the MIPI D-PHY specification. D-PHY is the high speed physical layer specification that is used to communicate with cameras and displays. The physical layer is the lowest layer in data transmission and defines the electrical and physical characteristics of the signal and its medium, such as wires and connectors.
In an effort to allow flexibility and innovation in various implementations, the D-PHY specification intentionally does not set requirements for pins or traces. It does however, demand that the devices support a high speed data transmission mode, a low power idle mode, differential signaling, a dedicated clock lane, at least one data lane, and very low noise output.
The two primary modes that D-PHY devices must operate in are High Speed (HS) and Low Power (LP). HS mode has a variable bitrate, which typically runs at 500 to 1000 Mbps per lane, with each lane running at less than a quarter of a volt. LP swings between 0 and 1 volts but switches bitrates much slower. Since most of the power in data transmission is used in switching, LP ends up using less energy than HS even though it runs at a higher voltage. The transition between LP and HS is very important since the receiver must be given time to wake up and synchronize with the transmitter so that data is not lost.
YMIPI devices must also support differential signaling, meaning that a single data or clock lane is comprised of two complementary lines that mirror each other. At the receiving end they are subtracted for a signal that is twice the size of each of the individual lines. If the data lane is x, with the positive line at +x and the negative line at –x, then x – (-x) will yield a clearer signal. This method of signaling is also used because the electromagnetic emissions from the individual lines end up canceling if the line traces are next to each other, as they typically are. Since the signal is reconstructed to be twice as large at the receiving end the signal is more immune to noise and can be run at higher speeds. Mobile devices have very sensitive components, like GPS and wireless communication radios. Differential signaling is preferred because the electromagnetic emissions are low and there will be less interference with radios that are next to the traces on a MIPI-powered mobile device.
In order for a device to be considered conformant to the D-PHY specification, the differential signals must be symmetric in a variety of ways. If one line is a little ahead of its complement or rises or falls at a different rate, electromagnetic noise will be put out into the mobile device. Since this can easily have a negative effect on cellular and GPS radios within the phone, it is important that components do not generate noise.
MIPI specifications are used in the majority of phones today and play a large role in a rapidly expanding industry. Specifications such as M-PHY, the successor to D-PHY, are still being produced and refined. The UNH-IOL will continue to adapt testing services as the standards evolve, giving MIPI Alliance members an opportunity to test against conformance of MIPI specifications.
Paul Willis, MIPI Technician