Firmware Emulation of an I2C Slave Device
If your embedded processor has a few spare MIPs and a spare GPIO port, it can communicate on this handy 2-wire bus.
by Steve Kolokowsky, Cypress Semiconductor

Many embedded microprocessors contain a two-wire serial interface, which looks remarkably similar to Philips I²C (I2C) interface. In many designs these two pins are an unused access point into the microprocessor. This article will provide a detailed look at how to implement the slave side of the I2C interface using bit banging. Soon you will be able to use two GPIOs on your own processor for inter-processor communication, which will save a lot of pins over an 8-bit parallel bus.

The I2C standard is covered by patents owned by the Philips Semiconductor spin-off, NXP, created in 2006. In October 2000, Philips was aggressively enforcing these patents, filing a lawsuit against six semiconductor companies. However, according to NXP's website, "I²C licenses under the remaining patents in the program will be free of any royalties, for any use of the patents after October 1, 2006, without any prejudice to any claims for past use whatsoever. Also the fee for obtaining an I²C slave address allocation will not be affected and will continue to apply until further notice."

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