Serial bus enhances venerable I2C bus speed
By Margery Conner, Technical Editor -- EDN, May 9, 2006
Since Philips in the early 1980s developed the I2C as a two-wire, low- to medium-speed communication bus, it has become the de facto standard for onboard communication and, when you bolster it with drivers, for board-to-board communication. However, as communication speeds have increased and interboard and even intercabinet communication have become commonplace, the I2C bus has been in need of a speed and drive-capacity overhaul. Hoping to bring users up to date and attract a new user base, Philips has now introduced a family of devices based on the Fm+ (fast-mode-plus) specification. Fm+ tightens the bus tolerances and timing requirements and allows a top speed of 1 MHz versus 400 kHz for the old version and a 10-times-greater-than-normal bus-drive capability of as much as 4000 pF.
“Most of our customers needed to make trade-offs between the bus length, the speed, and the number of devices on the bus,” says Dhwani Vyas, general manager of the interface-product line at Philips Semiconductors. “Fm+ gives a larger playing field: With fewer devices, you can go longer distances.” The new specification also allows for device ID, which allows you to query each device about its manufacturing and revision-level information. In addition, Fm+ allows for three address pins on each device with four states, allowing for as many as 64 device addresses.
“Fm+ is an extension of the I2C spec,” says Steve Blozis, marketing manager for I2C-logic devices at Philips. “The devices will be backward-compatible with I2C buses.” According to Blozis, some backers of other buses similar to I2C, such as the SMBus, have been concerned because Philips owned a patent on the bus, but the basic I2C patent expired in August 2004.
The initial three devices in the new family are the PCA9633 4-bit I2C LED controllers for RGBA (red-green-blue-alpha) color mixing, the PCA9698 advanced 40-bit I2C I/O expander, and the PCA9665 fast-mode plus parallel-bus-t- I2C-bus controllers. Prices range from 55 cents to $2.05.
