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MIPI I3C Provides a Unified, High-Performing Interface for Sensors

Planned for release in October, 2015, MIPI I3C is a sensor interface from the MIPI Alliance that alleviates sensor integration challenges in mobile, mobile influenced and embedded systems applications such as the Internet of Things.

BY DOUG HOFFMAN, QUALCOMM AND PAUL KIMELMAN, NXP SEMICONDUCTORS: MEMBERS, MIPI ALLIANCE SENSOR WORKING GROUP

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The proliferation of sensors in smartphones, tablets, and wearables is driving a new cycle of technology innovation and application development. Yet as more and more sensors are deployed in these mobile products, system integration is becoming increasingly difficult because it is hard to ensure optimum power and performance in the always-on components.  Engineers need a convenient interface that can serve multiple sensor and design architectures while delivering the challenging performance and efficiency characteristics required in these designs.

The MIPI Alliance designed MIPI I3C as the core technology for a variety of applications and market segments. The specification will be known as MIPI SenseWire when it is used for sensor systems in a mobile device.

Grappling with Traditional Interface Options for Sensor Applications

Manufacturers of smartphones and wearables are increasingly using sensors to provide activity recognition, pedestrian dead reckoning, and health and gaming capabilities, among other functions.  High-end smartphones incorporate 10 or more sensors, with more than 20 signals, and smaller products, such as smart watches, must include multiple sensors as well. As a result of this growth, the number of sensor signals in mobile devices has become unmanageable.  A design can require 12 to 18 pins/traces to connect all of the required sensors.

Another challenge is that sensor interfaces must enable always-on features. For example, even when a mobile device is in a pocket or purse with the screen off, a sensor can be operating to enable pedometer, activity recognition, and other applications. Because these sensors are always gathering and sharing data, the interface must support a very low power system architecture.

Further, industry support for sensor interfaces is fragmented. Integration requirements can vary from sensor to sensor, and the need to accommodate differing approaches increases product development and integration costs.

Among traditional interfaces, I2C has been the most widely used for sensors because its multidrop capabilities can support multiple sensors and its low-complexity and low-speed attributes help keep costs down. Yet I2C is not practical for quickly sending large amounts of batched data, which is necessary for many sensor applications.

The industry has preferred the Serial Peripheral Interface (SPI for) some applications—for example its higher speeds can more efficiently send batched data—but SPI is more complex than I2C, has a high pin count, requires a dedicated chip select pin for each sensor, and is not set up for multi-drop functionality.

Further, both I2C and SPI share some common drawbacks for sensor interconnections. Neither has a way for the sensor to notify the application processor or sensor hub master that it has data. This is an important sensor requirement, however, so both I2C and SPI use extra wires to allow these notifications via GPIOs.

Engineers must find ways to conveniently interface the components to the host processor while meeting increasing data throughput requirements, minimizing the number of pins needed to connect to the sensor hub or application processor and handle interrupt functions, while also minimizing energy consumption.

MIPI Alliance: Finding a Flexible Solution that Meets Broad Market Needs

In 2013 MIPI Alliance chartered a working group to address these challenges and define a common interface solution that would meet broad market needs. The organization’s goals were to re-use existing interfaces as much as possible while finding a way to accomplish the following: reduce pin count, provide in-band interrupts, reduce interface energy consumption, increase throughput, and reduce the cost of sensor implementations. The organization also wanted to reduce the operational time of the system infrastructure to reduce system-wide energy consumption.

MIPI Alliance is uniquely qualified and positioned to meet these needs. The global organization focuses exclusively on providing device interface specifications for mobile, mobile-influenced and embedded systems. The Alliance has introduced more than 45 specifications in the last decade. All of its interfaces are designed to meet the high-performance, low-power operation and low-electromagnetic interference requirements needed in small, compact designs.

To make sure the new interface addresses the broadest possible sensor ecosystem, MIPI Alliance collaborated with the MEMS Industry Group to survey members of both groups in order to identify the key performance and operational needs for the new interface. The MIPI I3C interface resulting from this work is thus a practical and appealing solution because it addresses technical requirements and standards needs expressed by a cross-section of sensor industry companies who participated in the surveys. Some sensor classes addressed by MIPI I3C are presented in Table 1; note this is not an exhaustive listing, but a sample list.

Table 1
Typical Sensor Classes Addressed By MIPI I3c

Unifying Fragmented Sensor Interface Technologies

MIPI I3C incorporates and unifies key positive attributes of I2C and SPI while improving the capabilities and performance of each approach with a comprehensive, scalable interface and architecture.  The specification provides a two-pin interface that is backward compatible with I2C, allowing legacy I2C devices to coexist on the same interface as new devices that support MIPI I3C’s features. Equally, MIPI I3C devices may work on legacy I2C buses. At the same time, MIPI I3C provides data throughput capabilities comparable to SPI while keeping logic complexity low, using standard I/O pads and providing an accommodating bus topology.

MIPI I3C provides important data transmission and management features. It allows in-band prioritized interrupts within the 2-wire interface, eliminating the need for a dedicated interrupt pin, thus drastically reducing the device pin count and signal paths to facilitate incorporation of more sensors in a device.  It offers multi-master support and uses dynamic addressing and standardized commands to control the bus. It also offers advanced power management capabilities to minimize energy consumption and extend battery life. The bus also allows for smaller devices with a lower data rate to be as small as possible, while allowing for integrated and batching devices to use a high data rate without a correspondingly faster clock speed.

MIPI I3C works equally well with standalone sensor/context hubs and with integrated hubs in the application processor (Figure 1). It fully supports combining both, trading off roles depending on phone state.

Figure 1
Using MIPI I3C to interface a sensor engine or a discrete sensor hub

Focus on Efficiency and Performance

Efficiency and performance are extremely important in sensor applications. Always-on sensors and/or hubs often need to accumulate (batch) data over specified intervals, even while the host processor is powered down to a low-power state. The sensor or hub must be able to batch the data and then transmit it as quickly as possible to minimize energy consumption of the host processor. MIPI I3C offers important capabilities to help designers address these needs.

As can be seen in the left of Figure 2, MIPI I3C is always more efficient than I2C. This is the case even when using the I2C-like single data rate (SDR) protocol as well as its high data rate (HDR) protocol modes.

Figure 2
Comparison of I2C and MIPI I3C energy consumption (left) and comparison of I2C and MIPI I3C raw data rates

MIPI I3C also provides substantial data throughput advantages compared to I2C, as shown in right graph in Figure 2.  On standard CMOS I/O, MIPI I3C moves away from a pure open drain bus with strong resistors, which I2C employs, and instead uses a push/pull drive to operate at clock speeds up to 12.5 MHz compared to the 400 kHz or 1 MHz that I2C offers. When MIPI I3C uses its higher performance high-data-rate (HDR) modes, it can send data at two to three times faster speeds at the same bus frequency. Additionally, due to the much higher speeds, devices connected by MIPI I3C interfaces are active for a shorter time, even when sending large amounts of data.

Figure 2
Comparison of I2C and MIPI I3C energy consumption (left) and comparison of I2C and MIPI I3C raw data rates

MIPI I3C allows the use of two different types of mastering to help reduce signaling complexity, minimize energy requirements and keep costs down.  For example, if the host processor is in sleep mode, a secondary master can collect sensor data for batching or computation and then provide it to the host when it wakes up.

Figure 3 shows a typical system with a context/sensor hub that that controls the bus when the application processor/host is powered down. However, it can also control the bus all of the time as well.  The handoff master model allows for efficiency without contention issues. Additionally, for a sensor that needs to sometimes read from another sensor (e.g. a magnetometer), it can use peer-to-peer mastering, which does not require the mechanisms of a master, such as supplying the bus clock.

Figure 3
MIPI I3C Context/sensor hub offloading, always-on sensing and wake sources

MIPI I3C is a game changer for sensor vendors and engineers who work to interconnect sensors in their designs to create new products and applications. The new interface will enable manufacturers to combine multiple sensors from different vendors in a device, streamline integration and drive cost efficiencies. It supports connection to any type of device that employs a host processor or microcontroller using standardized sensor interface to ensure high-performance, low-power operation. It will be practical not only for smartphones, tablets, and wearables, but also for IoT, toys, gaming devices, medical, industrial equipment and other use cases. There is a bright future ahead for MIPI I3C.

NXP Semiconductors
San Jose, CA
(408) 518-5500
www.nxp.com

Qualcomm
San Diego, CA
(858) 587-1121
www.qualcomm.com

MIPI Alliance
www.mipi.org