Understanding MIPI A-PHY High-Speed SerDes Interface

The development of Advanced Driver Assistance Systems (ADAS), digital cockpits, infotainment systems, and autonomous driving systems has led to an increasing use of cameras, sensors, and displays in vehicles. To effectively control electromagnetic interference in these systems, the MIPI Alliance developed the A-PHY standard.

MIPI A-PHY allows for asymmetric data connections for automotive applications that require high data throughput. At the same time, it helps control electromagnetic interference issues within a certain range.

As resolution, frame rate, and bit depth continue to improve, the multi-gigabit throughput required for connecting cameras, sensors, and displays in vehicles presents unique challenges for design. This requires the development of new electronic/electrical (E/E) architectures that adopt safety-critical, long-distance, high-speed interfaces.

To support these new developments, the MIPI Alliance developed A-PHY (also recognized as IEEE 2977-2021 standard), which is a long-distance, high-speed, asymmetric serial interface for connecting cameras, sensors, and displays through MIPI CSI-2, MIPI DSI-2, DisplayPort, and other industry-standard protocols. A-PHY eliminates the need for proprietary physical layers (PHY) and bridges, simplifies the in-vehicle network, and reduces costs, weight, and development time.

Future automotive architectures will include A-PHY interfaces to complement symmetric interfaces like in-vehicle Ethernet. Unlike other interfaces, MIPI A-PHY is designed to address the unique challenges of electromagnetic interference in automotive environments.

Table 1: Improvements in Downlink Performance of MIPI A-PHY v2.0 Compared to Previous Versions (Image Source: MIPI Alliance)

A-PHY enables asymmetric data connections in point-to-point, daisy chain, and other topologies, allowing for the transmission of high-speed unidirectional data, integrated bidirectional control data, and optional power through a single cable, thus saving on cabling, costs, and weight. It offers transmission distances of up to 15 meters (including four inline connectors), low latency (for example, 6µs at 16 Gbps), supports various cable types (coaxial cables, shielded differential pairs, and star quad cables), and multiple cable power options. A-PHY is optimized for sensor integration, with a simplified design and lower baud rate suitable for most process nodes.

A-PHY v1.1.1 supports data rates of up to 16 Gbps per channel, achieving 32 Gbps through a single cable (i.e., using a star quad cable with dual downlinks). The upcoming A-PHY v2.0, planned for release in Q3 2024, will increase data rate support to 32 Gbps per channel, achieving 64 Gbps through a single cable. A-PHY maintains an extremely low bit error rate (<10^-19) throughout the vehicle’s lifespan, providing exceptional noise immunity and reliability. To optimize the interface, A-PHY can simultaneously transmit multiple high-layer protocols using a Protocol Adaptation Layer (PAL), which supports native coupling with MIPI CSI-2 (for cameras) and MIPI DSI-2/DisplayPort/Embedded DisplayPort (for displays). Additionally, other PALs support I2C, GPIO, Ethernet, SPI, audio, and MIPI I3C interfaces (Figure 1).

Figure 1: Functional Description of A-PHY v2.0 Channels

(Image Source: MIPI Alliance)

In addition to reduced signal-to-noise ratio, the electromagnetic environment in motor vehicles must also be considered. Key factors include narrowband interference, transient interference, alien crosstalk, and additive white Gaussian noise in the vehicle environment. The illustration in Figure 2 shows the time and frequency domain graphs at the receiving pad when transmitting through the channel at a symbol rate of 4 GBaud and a peak-to-peak voltage of 500 mVpp. The channel exhibits approximately 20 dB attenuation at its 2 GHz Nyquist frequency. The deep blue represents the transmitted signal, while the orange represents the signal after channel loss.

Figure 2: TX and Other Signals at the Receiver Connector

(Image Source: MIPI Alliance)

Figure 3 shows the performance of these signals at the receiver slicer after adding gain from the Analog Front End (AFE) and Feed-Forward Equalizer (FFE). Even with relatively complex equalization and filtering techniques, only about 15 dB of noise gain is allowed, and the effects of narrowband interference (NBI) remain visible relative to the expected reconstructed transmission level. In the frequency domain, this is because most of the NBI power is concentrated at frequencies where the equalizer’s noise amplification effect is higher. In contrast, transient interference (ToL) noise occurs at lower frequencies, where the noise gain is much lower.

Figure 3: Signals on the Receiver Slicer (after AFE + FFE amplification) (Image Source: MIPI Alliance)

This brief analysis indicates that high-speed automotive links are affected by electromagnetic interference, necessitating noise suppression and error correction mechanisms to counteract the negative effects of automotive electromagnetic interference, ensuring the safe and stable operation of multi-gigabit data links.

To mitigate the effects of cable aging, the MIPI A-PHY interface is designed to provide at least 40mV of near-end crosstalk (NBI) peak noise immunity at the receiving pad. To achieve this, A-PHY employs the following technologies:

■ A dynamic pulse amplitude modulation scheme for all speed grades from non-return-to-zero 8b/10b to Pulse Amplitude Modulation 16 (PAM16). This modulation scheme dynamically adjusts the amplitude of pulses based on the signal transmission needs, thereby enhancing the signal’s resistance to interference and transmission efficiency.

■ A real-time canceller that provides over 36dB of real-time near-end crosstalk (NBI) cancellation capability. This canceller can monitor and eliminate near-end crosstalk in the signal in real-time, ensuring signal clarity and accuracy.

■ A dynamic modulation local retransmission mechanism that supports a bit error rate of less than 10^-19, equivalent to an average interval time of over 10,000 years. This means that over an extremely long period (with an average interval time exceeding 10,000 years), the packet transmission error rate is extremely low and nearly negligible. This mechanism ensures that packets can reach the receiver accurately and reliably during transmission by dynamically adjusting retransmission parameters.

In contrast, other interface technologies assume a maximum near-end crosstalk (NBI) of about 6mV and rely solely on forward error correction (FEC) mechanisms, which are often insufficient to ensure the safe and reliable operation of the interface.

MIPI A-PHY is an open industry standard with strong ecosystem support covering development, testing, and interoperability, and has a clear commercialization path. Unlike other interfaces, A-PHY has high noise immunity, enabling particularly reliable high-speed data transmission through current-generation copper cables throughout the vehicle’s lifespan. Additionally, a reference consistency testing suite has been developed, and a consistency program is being introduced to verify whether A-PHY devices comply with specifications.

Valens Semiconductor (NYSE: VLN) is a leader in high-performance connectivity, dedicated to helping customers transform the digital experience for users worldwide. Valens’ chipsets are integrated into the devices of many well-known customers, supporting advanced audio-video installations, next-generation video conferencing, and driving the development of advanced driver assistance systems and autonomous driving technologies. Valens continuously pushes the boundaries of connectivity technology, establishing standards in all areas of its operations, and its technology forms the basis of industry-leading standards such as HDBaseT® and MIPI A-PHY.