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Address M-PHY issues to boost test efficiency

Posted: 14 Oct 2014 ?? ?Print Version ?Bookmark and Share

Keywords:MIPI? M-PHY? oscilloscope? Physical Layer? Conformance Test Suite?

As the industry moves to adopt the MIPI Alliance's M-PHY standard, designers are facing some significant challenges related to oscilloscope measurements and, more specifically, probing. These challenges include strict requirements such as bus termination and input return loss, as well as the need to minimise common mode loading on the device under test (DUT) and signal fidelity requirements such as wide bandwidth, low noise, and high sensitivity.

The intent of this article is to provide information that will increase your chances of accurate and repeatable test results to ensure compliance with the standard. We will first review the requirements of the M-PHY standard relevant to oscilloscope probing, discuss the tests required in the M-PHY Physical Layer Conformance Test Suite (CTS), and provide practical examples of M-PHY probing with currently available oscilloscopes and probes.

M-PHY bus operation/modes
The MIPI M-PHY standard supports different speed modes, high-speed (HS) and lower speed PWM and SYS modes, each with different data rates or GEARs. In HS mode, the official specification is released for GEAR 1 (~1.5 Gb/s), GEAR 2 (~2.9 Gb/s), and GEAR 3 (~5.8 Gb/s).

To increase the likelihood of MIPI M-PHY designs from different manufacturers working when used together, the MIPI Alliance recommends that designs be tested against the M-PHY Physical Layer Conformance Test Suite (CTS). With the development of the GEAR 4 (~11.6 Gb/s) specification proceeding, testing conformance is becoming more of a challenge.

Unterminated and terminated modes
The voltage measurements specified in the M-PHY standard assume that the bus has a known reference load (RREF in figure 1) connected between the positive (TXDP) and negative (TXDN) outputs of the M-PHY transmitter (M-TX). The reference load varies if the LINE is either not terminated (NT) or resistively terminated (RT). When the LINE is operating in NT mode, RREF_NT is specified as a minimum impedance of 10k次 between the TXDP and TXDN pins. The NT mode is most often used for low-speed communications, since driving a high speed signal into a high impedance transmission line can be challenging.

Figure 1: The voltage measurements specified in the M-PHY standard assume that the bus has a known reference load (RREF in the image) connected between the positive (TXDP) and negative (TXDN) outputs of M-TX.

High speed and terminated mode
When the M-PHY LINE is operating in high speed mode (HS-MODE), it will most likely be terminated to minimise reflections and mimic a receiver connection with a known transmission line impedance. For terminated mode of the LINE, the M-PHY standard defines RREF_RT as a floating 100次 impedance across the TXDP and TXDN pins.

While the M-PHY specification defines both RT and NT states, conformance testing for HS-MODE is typically only defined for RT. Further, the M-PHY CTS only specifies high speed tests in RT mode, stating that high speed data measurements such as jitter on non-terminated signals are typically not practical or even possible.

When needed, non-terminated signals can be measured using either a high impedance active or differential probe. These oscilloscope probes can meet or exceed the specification with 10k次 or higher impedance for signal frequencies that are less than a few MHz. Measuring the LINE in high-speed terminated mode with an oscilloscope requires that either the scope or its probe be as close to the desired 100次 differential termination as possible.

A floating 100次 termination is the ideal, but difficult to realise in practice due to parasitic elements in a design. Therefore, several alternative approaches have been proposed to achieve a nominal 100次 differential termination along with sufficient return loss and high common mode impedance that minimises DC and AC current draw on the transmitter. Table 1 lists the ideal termination and four other practical alternatives.

Table 1: This table lists the ideal termination for M-PHY HS and four other practical alternatives.

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