it determines whether the request to the slave is to read or write data during the next cycles. Devices are addressed using a 7-bit address (coordinated by Philips) transfered as the first byte after the so-called start condition. The bus is multi-master capable, and a particular device implementation can act as either master or slave at different times. e., they initiate a transfer), or as a slave (they only act when being called by a master). There are formulas in the datasheet that help selecting the pullups.ĭevices can either act as a master to the bus (i. The pullups must be small enough to recharge the line capacity in short enough time compared to the desired maximal clock frequency, yet large enough so all drivers will not be overloaded. All devices participating in the bus are connected together, using open-drain driver circuitry, so the wires must be terminated using appropriate pullup resistors. The two-wire interface consists of two signal lines named SDA (serial data) and SCL (serial clock) (plus a ground line, of course).
This is essentially the same called I2C by Philips, but that term is avoided in Atmel's documentation due to patenting issues. Some newer devices of the ATmega series contain builtin support for interfacing the microcontroller to a two-wire bus, called TWI.
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