Two wire serial interfaces are included in following AVR microcontroller families: ATmega8x, ATmega16x, ATmega163x, ATmega32x, ATmega323x, ATmega64x, and ATmega128x. TWI interface is a “Philips†standard I2C. Using TWI interface you can connect up to 128 devices using only two wires: clock (SCL) and data (SDA). Only two pull-up resistors on each line are needed this interface to work properly.
I2C interface circuit is open collector. This means if one of all devices has low level signal on a line, then it is “0â€, and if all devices have high impedance state, then signal is considered to be high “1â€. More details about TWI interface you can find on any ATmega datasheet.
One of my examples Interfacing AD7416 digital temperature sensor you can find here: Analog Devices Digital temperature sensor AD7416
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