Inter-Integrated Circuit: The Useful Communication Bus

Published:2011/5/2 22:29:00 Author:Phyllis From:SeekIC

I2C, known as Inter-Integrated Circuit or TWI (Two Wire Interface), is a multi-master serial single-ended computer bus invented by Philips for attaching low speed peripherals to a motherboard, , cellphone, or embedded system. I2C is pulled up with resistors by using two bidirectional open-drain lines, Serial Data Line and Serial Clock,. The common used voltages are +5V or +3.3V although systems with other voltages are permitted.

The reference design of I2C includes a 7-bit address space with 16 reserved addresses, so a maximum of 112 nodes can communicate on the same bus. The common bus speed of I2C can be mainly divided into two types: the standard mode of 100 kbit/s and the low-speed mode of 10 kbit/s, however, the arbitrarily low clock frequencies are also allowed. Recent revisions of I2C can host more nodes and work at faster speeds. These speeds are more widely used on embedded systems than on PCs. There are also other features like 16-bit addressing. The maximum number of nodes is limited by the address space, and also by the total bus capacitance that restricts practical communication distances to a few meters.

Pure I2C systems are able to support arbitrary message structures except System Management Bus. Under normal circumstances, I2C and System Management Bus can not define message semantics, but there are some exceptions such as messages addressed to the I2C general call address or to the SM Bus Alert Response Address, and messages involved in the SM Bus Address Resolution Protocol for dynamic address allocation and management.

I2C is suitable for peripherals where simplicity and low manufacturing cost overweigh speed. A particular of I2C is a microcontroller that can control a network of device chips by two general input/output pins and software. The peripherals can be added or removed from the I2C bus at any time, which makes it ideal for applications that require components hot swapping.

The package size and pin count of an integrated circuit decided the manufacturing cost with a smaller package usually weighs less and consumes less power, which is especially important for cellphones and portable computing. I2C buses are becoming more and more popular.

One weakness of I2C is the assignment of slave addresses. 7-bit is too few to prevent address collisions between devices and manufacturers rarely provide enough pins for the full slave address. Another shortage of I2C is the speed. Because of those limits, few I2C bus segments have even a dozen devices. It’s common for systems to have several such segments. The related integrated circuit is KTD1414.

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