The MIPI Alliance has been the plumber for the mobile industry and now hopes to migrate that success into the automotive and IoT spaces.
You might not know much about the MIPI Alliance if you aren’t designing mobile phones, but you will soon. Other application areas are taking interest in what this group has accomplished.
The alliance was founded in 2003 to create standards for hardware and software interfaces in mobile devices. Successful examples include a camera serial interface (CSI) and a display serial interface (DSI), each of which enables a connection to the application processor.
“We have evolved our standards over the years and what we are finding is that our specs are low power, low EMI — or at least have consideration for EMI —and also high bandwidth,” says Peter Lefkin, managing director of the MIPI Alliance. “There are other industries that are looking at those requirements, as well, especially the success of the camera and display interfaces. The success in mobile is being transferred to automotive where you have multiple cameras and multiple displays. In addition, IoT and wearables are looking at what we have created, so we are finding that our specs are not just in the phone anymore. We are also working on sensor interfaces and RF Front-End (RFFE) that enables antenna tuning plus other specs around the mobile architecture.”
The concept is similar to bus standards that were created 20 years ago. At that time, VSIA created an interface intended to allow generic blocks of IP to be connected to a processor. That interface was commercialized by Sonics, which later donated it to OCP-IP which in turn was adopted by Accellera. Its aim was to provide the segregation between components.
While it achieved some success, the MIPI standards are much more focused and as such have had more success. Lefkin is confident that their standards are probably in use by all phones today.
But a standard needs support by a whole ecosystem, and that includes IP providers. “Synopsys and Cadence are two of the largest IP developers (with the exception of ARM) and we are essentially interested in the plumbing around the application processor (AP),” says Sachin Dhingra, senior product marketing manager in the IP group of Cadence. “We developed that IP – the physical layers and the controller that goes with it. If you are building a device that has a camera or a sensor or even an AP, we are providing those interfaces off-the-shelf.”
“We are both contributing members of the MIPI Alliance,” says Hezi Saar, staff product marketing manager for MIPI Phys and controllers at Synopsys. “The camera interface, CSI, was specified more than ten years ago and it aimed to standardize the connection between the AP and the image sensor. This means that you can select a different AP and image sensors and can support a variety of pixel rates and speeds.”
MIPI successfully standardized CSI in the mobile market where you have the front and back cameras and today, they are using the MIPI CSI-2 interface. “Now we are starting to see this kind of interface go beyond mobile into IoT and automotive,” adds Saar. “High volume, low cost and low power, plus the same mobile friendly features, were the attributes that those markets really liked. CSI-2 has support for high- and low-resolution cameras. Now we also have CSI-3, which can support a network-based camera using MIPI M-PHY and Uni-pro which are more like standard SerDes connectivity that allows you to connect a network-based camera to a network-based AP.”
The physical layer and controller are both on-chip so they are part of the plumbing. It is a point-to-point connection. “We are looking to create the most optimal point-to-point interface that will allow you to send data at whatever speed you want,” says Saar. “We need to do that with minimal overhead and be able to switch to a lower power mode when it is not bursting. It was also designed not to have extra unnecessary features. So it is a scalable interface that has a clock lane, and then as many data lines as required where each data line can go from 80Mbit/s up to 4.5Gbit/s.”
It can be configured with a lower number of lanes running faster or you can go wider and slower, depending on the application needs. “The most popular configuration is MIPI CSI-2 camera interface that connects using a physical layer called D-PHY,” explains Saar. “D-PHY specifically has a clock forwarding scheme, so the clock is not actually embedded in the data and provides a simplistic implementation with scalable number of data lanes from one to four. The latest introduction to CSI-2 was the new physical layer, which is a unique implementation using just three wires with the clock being embedded and has different use cases. For example, if you want less switching you can accommodate that and thus lower your EMI.”
Lefkin adds that there are a number of different use cases that are coming, such as using the camera interface for radar and lidar. “Some of those requirements are now coming to MIPI, so it not necessarily just the mobile industry controlling the specifications anymore.”
New markets, such as wearable and the IoT require many new sensor types. “We have MEMS, accelerometers and smart sensors and multiple kinds of simple sensors with simple processing on them,” says Cadence’s Dhingra. “Traditionally they were connected with I2C as a standard interface that has been around forever. Then there was Spi for sensors that needed higher throughput, such as a touch screen or fingerprint scanner. They are not as high-end as a camera sensor, but still have high requirements. I2C is simple but it lacks throughput, and you need a lot of different pins added on top if you want interrupts and other capabilities.”
In many applications, there is an application processor plus a sensor hub, which may have 10 or more sensors on it. Each of those has a dedicated I2C bus plus other pins, which adds up to a minimum of 20 pins. “What we did in the new spec is to take out the extra pins,” explains Dhingra. “It is now a two-pin interface, one data and the other clock, and these run across all of the sensors using the same two-pin bus with multi-drop. You can just hook up the sensors to it. It is backward compatible with I2C, so you can also use your old sensors. The other aspect is that it has built-in interrupts and command codes, so you no longer need additional pins for that. It has a better I/O design, which saves power compared to I2C. And you have higher throughput because you can go up to 40Mb/s, while I2C default is 400kb/s —so two orders of magnitude improvement.”
Many IoT discussions these days involve at least a mention of security, but those rarely cross over into the MIPI world.
“So far, security has not been a topic down at the level of the MIPI discussions and sits above this,” says Saar. “There have been some recent discussions, although not active in an assigned working group. Today, they are having informal discussions, but people may be rethinking that. We may start looking at it more from the hardware perspective rather than leaving it to software. It is not there today, but it is percolating up. There are so many organizations looking at it and we will not directly be in that space.”
Some of the recent car hacks involve adding a new node to an existing network. Saar explains that MIPI specs do contain hot-join capabilities. “This enables you to plug in better sensors and so you can hot-plug something.”
Lefkin agrees that more people are getting interested in this at the device level. With Ethernet today there is not much security built in, so you have to add security protocols layers.
Saar adds that “at this point in time, we are dealing with the transfer of data, and doing that in the most power efficient manner possible. We are not yet getting into the authentication of the devices and we have to rely on the upper layers to do that. “
Dhingra provides an example of how a system can be made secure. “JEDEC is using the MIPI M-PHY for their physical layer and MIPI Uni-pro for their reliable link. On top of that they defined UFS which is a mobile storage interface, and in that is a protocol layer that runs on top of Uni-pro. So they rely on a good spec and reliability transport interface to send and receive the data efficiently, and then define the security, authentication and encryption aspects that allows you to store secure data into the storage. So MIPI enabled other partners to develop this kind of connectivity.”
Some of the new application areas do have requirements beyond those in the mobile space. “As our focus is expanding and we start to look at automotive and IoT we need to tap people in different segments of the company,” says Lefkin. “Security is one of those.”
Since this discussion took place at the Design Automation Conference, MIPI has had a number of releases including early access to the I3C interface specification. According to MIPI, I3C is groundbreaking because it incorporates and advances the available I2C, SPI and UART interfaces into a consolidated specification, and at the same time provides backwards compatibility with most types of I2C devices. The new approach makes it easier for both device manufacturers and software developers to add more sensors to devices, and to combine multiple sensors from different vendors in products, while reducing component and implementation costs. The specification also improves performance and power efficiency and provides sensor management capabilities not previously available.
For those who would like to find out more about MIPI and their interfaces, the first developers conference (MIPI DevCon) will be held at the Computer History Museum in Mountain View, Calif., on Sept. 14-15. More information is available here.
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