Vintage data-transfer tech gets upgrade after 35 years

The I3C bus is designed for newer sensors that feed tons of data to wearables, smart devices, and computers

Tech artifacts like old Mac computers are finding their way to museums, but some never-say-die technologies continue to serve requirements important to computing.

Inside wearables, smart devices, robots, and computers like Raspberry Pi are communications buses called I2C (Inter Integrated Circuits), which date back to 1982, and SPI (Serial Peripheral Interface), which was born in 1979.

Those buses have driven short-range communication between circuits and microcontrollers for decades. They now serve as key interfaces for sensor-related communication on smart devices, wearables, and computers.

But as devices get equipped with more powerful, bandwidth-hungry sensors like 360-degree cameras, these out-of-date buses won't be able to keep up in the long run. So standards-setting organization MIPI Alliance wants to bury I2C and replace it with the faster and modern I3C bus and also merge SPI into the new interface.

The new protocol was officially released this week and is available to hardware makers to use on boards. The upgrade is analogous to moving from USB 2.0 to the much faster USB 3.0, although it took 35 years to move from I2C to I3C.

More sensors are being packed into smart devices, robots, drones, and industrial devices, and MIPI Alliance says I3C will serve as a speedy short-distance communication channel that sips power.

The I3C will primarily be used for data transfers inside a circuit board. But as the IoT market expands, it will serve a growing need to transfer data collected from a wide range of mechanical, environmental, biometric, and health sensors.

The use of I3C could expand to a new range of sensors in PCs, drones, and virtual reality headsets. It could be used for 3D cameras on drones capturing high-resolution images or for speeding up communications inside autonomous cars.

The new communications bus could also shrink the size of wearables, smart devices, and boards like Raspberry Pi. A lot of space is taken up by I2C, UART, and SPI buses on Raspberry Pi, and the I3C will merge all of them into one, which should reduce the pin count. Beyond saving space, it'll also reduce the cost of making devices.

The specification supports numerous sensor classifications and functions, including accelerometers, touch screens, time-of-flight cameras, ultrasonic sensors, transducers, and actuators, MIPI Alliance said in an email.

I3C can also be used to interface sensors used for near-field communications, haptics feedback, and infrared or ultraviolet sensing.

MIPI Alliance has a track record of providing proven standards and boasts members including Google, Intel, AMD, Qualcomm, and Sony. Virtually every chipmaker uses I2C in some form and will benefit from I3C. Google's now defunct Project Ara relied on an MIPI Alliance standard called UniPro for snap-on components to communicate.

Compared to a typical I2C implementation, the new I3C protocol provides more bandwidth while consuming up to 10 times less power. The two-wire digital interface supports data transfers from 10Mbps to 39.5Mbps depending on mode, clock rate, and power consumption levels. I2C speeds maxed out at 3Mbps, and while the SPI bus can be faster, it demands more clock speed. I3C is also backward compatible with I2C.

The I3C bus could replace I2C on a wide range of devices and maker boards like Raspberry Pi. But it would involve redesigning the circuitry on the board, which could take time.

Implementers are finding MIPI I3C not significantly different in size from SPI, and not much different from typical I2C implementations. That's because I3C standardizes bus management and control messages, something that formerly had to be handled elsewhere in the design, MIPI said. Rather than dedicating two wires to every device, the whole bus, with multiple devices, runs on two wires.

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Agam Shah

IDG News Service
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