26.3.2 General TWI Bus Concepts

The TWI provides a simple, bidirectional, two-wire communication bus consisting of a serial clock line (SCL) and a serial data line (SDA). The two lines are open-collector lines (wired-AND), and pull-up resistors (Rp) are the only external components needed to drive the bus. The pull-up resistors provide a high level on the lines when none of the connected devices are driving the bus.

The TWI bus is a simple and efficient method of interconnecting multiple devices on a serial bus. A device connected to the bus can be a master or slave, where the master controls the bus and all communication.

Figure 26-2 illustrates the TWI bus topology.

An unique address is assigned to all slave devices connected to the bus, and the master will use this to address a slave and initiate a data transaction.

Several masters can be connected to the same bus, called a multi-master environment. An arbitration mechanism is provided for resolving bus ownership among masters, since only one master device may own the bus at any given time.

A device can contain both master and slave logic, and can emulate multiple slave devices by responding to more than one address.

A master indicates the start of a transaction by issuing a START condition (S) on the bus. An address packet with a slave address (ADDRESS) and an indication whether the master wishes to read or write data (R/W) are then sent. After all data packets (DATA) are transferred, the master issues a STOP condition (P) on the bus to end the transaction. The receiver must acknowledge (A) or not-acknowledge (A) each byte received.

Figure 26-3 shows a TWI transaction.

The master provides the clock signal for the transaction, but a device connected to the bus is allowed to stretch the low-level period of the clock to decrease the clock speed.