Addressing MIPI M-PHY connectivity challenges for more efficient testing
As the industry moves to adopt the MIPI Alliance's M-PHY standard, designers are encountering 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 minimize 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 10kΩ 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 minimize 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 100Ω 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 10kΩ 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 100Ω differential termination as possible.
A floating 100Ω termination is the ideal, but difficult to realize in practice due to parasitic elements in a design. Therefore, several alternative approaches have been proposed to achieve a nominal 100Ω differential termination along with sufficient return loss and high common mode impedance that minimizes DC and AC current draw on the transmitter. Table 1 lists the ideal termination and four other practical alternatives.
Since the ideal floating 100Ω cannot be achieved with real components, engineers testing M-PHY HS signaling must decide on one of the practical alternatives. Each alternative has pluses and minuses. Note that the floating 100Ω termination has a common-mode impedance that varies widely over M-PHY’s frequencies of interest due to parasitic elements. Common mode current flow and common mode input impedance are two areas where the alternatives differ significantly. All the alternate termination schemes have the differential properties of the ideal 100Ω, but exhibit other undesirable characteristics like 25Ω common-mode input impedance at high frequency.
Connecting the M-TX LINE directly to two channels of a scope is equivalent to the dual single-ended approach (#2). This alternative has the disadvantage of drawing common-mode current from the transmitter, which may cause problems for the transmitter. In alternative #3, adding DC blocking capacitors to the dual single-ended termination addresses the DC common-mode current, but the capacitors must be chosen carefully to ensure the termination meets the M-PHY return loss specification. Also, using blocking caps prevents DC measurements on the TX required by the CTS.
The two final alternatives mimic the ideal 100Ω resistor by drawing no DC common-mode current, but have low common-mode impedance (25Ω). This approach, however, allows for the common-mode impedance to be controlled better than the floating 100 Ω termination. If the capacitor (alternative 4, Cterm) used with the termination is large enough, the AC common mode impedance will be nominally constant at 25Ω. 100nF is needed for this to start at 1MHz.
The DC voltage termination (alternative 5, Vterm) capability includes a DC source inside the probe or scope, and provides an adjustable DC termination reference level. Setting Vterm to the TX’s common-mode voltage results in no CM current and eliminates the need for DC-blocks or bias-tees. Unlike a DC block, however, the signal’s DC voltage is still present at the probe or scope input. Using Vterm minimizes the DC loading. By setting the termination voltage equal to the DC bias voltage of the input signal, the probe DC loading is nulled out.
The termination of the M-PHY bus at GEAR 3 speeds and higher is further specified by the return loss for the reference load ZREF_RT according to the figure and values shown in Figure 2. High return loss limits unwanted reflections from the LINE that can impact the quality of the measurements.
Figure 2. Termination of the M-PHY bus is further specified by the return loss for the reference load ZREF_RT as shown here.
One of the issues that can prevent a probe or fixture from meeting the return loss requirement is capacitive loading across the 100Ω termination. The return loss specification sets some practical limits on the amount of series and parallel loading allowed across the ideal 100Ω termination, as shown in Table 2 for GEAR 3.
These constraints can limit the choices for line termination with a probe or scope. Figure 3 shows two termination approaches and input return loss. The first one is a probe with 50Ω differential inputs (blue trace). The second is a high impedance differential probe attached to a 100Ω termination fixture (red trace). In this example, the probe shown in blue meets the M-PHY specification by remaining below the limit line. The high impedance fixture and probe does not meet the spec as it exceeds the limit at 2.7 GHz and above.