Which mobile protocol is ideal for your next-generation design?
PCIe has long been a backbone interface in the PC world. With the advent of M-PCIe—a specification for utilizing the PCIe architecture over the MIPI M-PHY physical layer—mobile device designers can now take advantage of an expansive ecosystem tied to PCIe. This ecosystem complements advances in mobile processor and memory technologies to deliver the portability, battery life, scalability, interoperability, and processing power to support both business and consumer applications.
The MIPI Alliance’s UniPro specification defines a layered protocol for interconnecting devices and components within mobile device systems. By simplifying and unifying the interconnection of peripherals, the UniPro specification can decrease time to market and lower design costs while maintaining a high level of performance.
Another emerging protocol is USB SSIC, which uses the MIPI M-PHY and is based on the USB 3.0 protocol. Defined by the USB Implementers Forum, SSIC defines a chip-to-chip USB-based internal interconnect for mobile devices. In addition, there is the LLI protocol, also developed by the MIPI Alliance primarily for sharing DRAM memory between the application processor and the modem/baseband processor, resulting in significant cost reduction of mobile devices.
Let’s take a closer look at the advantages and disadvantages of each mobile protocol.
M-PCIe: bringing PC-class performance to the mobile platform
The chip design world has certainly embraced PCIe—the bus standard that supports the performance scaling and throughput that define desktop systems. PCIe has emerged in almost every market segment, from networking to gaming, and includes application-specific “add-on” protocols like SR-IOV and MR-IOV for servers and NVM Express and SATA Express for storage. Until now, the only domain that PCIe hadn’t entered was mobile.
With the advent of M-PCIe, mobile designers now have access to the performance capabilities they want to include in their designs, along with the low power consumption that smartphones, tablets, and the like need in order to be viable, competitive products. Leveraging the layered architecture of PCIe, which allows an easy way to replace the traditional physical layer (PHY) with M-PHY, designers can enjoy both worlds—a best-in-class, low-power, scalable PHY (M-PHY) with the broad ecosystem support of applications, software support, and testability.
Changing the PHY in order to create M-PCIe represents a big step in the right direction, but there are many other parts of the specification that also must be optimized for mobile devices. The initialization and discovery process, for example, can be costly and redundant in mobile devices, and which impacts the productivity of the new interface. In addition, although the M-PCIe spec (an engineering change notice (ECN) to PCIe 3.0) deals with most of the PCIe/M-PHY integration aspects, mobile designers are still left with some design-specific glue logic between the protocol and the new PHY that creates another barrier for adoption.
As early adopters implement M-PCIe in their designs, their success with the protocol will certainly influence when M-PCIe reaches a point of critical mass.
UniPro: tailored for the new mobile era
The MIPI Alliance developed UniPro with the ambition that it would become the unified channel for any mobile hardware interface. While addressing the dueling requirements for rich design and low power, UniPro can also lower design costs. You basically have one IP block on top of which you can build a variety of applications, from display to camera to storage features. However, UniPro supports a smaller bandwidth spectrum than does M-PCIe.
From a pure design standpoint, UniPro is straightforward to use, because it was developed to accommodate mobile device constraints. However, because it has not yet built up an expansive infrastructure, the challenge lies in integrating the protocol into the SoC, creating the proper software, and finding the deep experience needed to write code on top of it and test the system. So far, adoption of UniPro has been slow but steady, driven mainly by the JEDEC Universal Flash Storage (UFS) protocol and the recent CSI-3 specification, which addresses next-generation camera requirements (3D, HD, etc.).
SSIC: high bandwidth, low power, and expansive ecosystem
SSIC enables USB 3.0, another ubiquitous PC interface, to connect chips within a mobile device using the low-power and high-bandwidth M-PHY interface. This enables designers to take advantage of the existing—and large—USB hardware and software ecosystem in their mobile environment, not to mention the performance enhancements of the SuperSpeed USB protocol layer.
With 4G and beyond long-term evolution (LTE) devices and mobile networks, the need for ever greater performance continues unabated. A standard such as SSIC can enable mobile designers to meet these performance demands while taking advantage of their existing USB 3.0 software stack investments.
LLI: new level of efficiency in a niche domain
LLI was designed with a very practical, business-oriented goal: Reduce the cost of mobile phone devices by enabling an ultra-efficient interface that allows two devices on the separate chips to seamlessly share a common memory. The LLI specification, like all MIPI new-generation protocols, utilizes the M-PHY and offers a simpler and lighter connectivity option, compared to the three previous interfaces discussed here. The LLI protocol is memory mapped, another significant quality that minimizes the need for software drivers.
Despite the technical advantages, it seems that LLI is being adopted in limited designs, utilizing the main benefit it provides in sharing DRAM. But so far, it hasn’t gained a central role in the new mobile design world.