8-bit AVR Microcontrollers

Combining Several TWI Modes

In some cases, several TWI modes must be combined in order to complete the desired action. Consider for example reading data from a serial EEPROM. Typically, such a transfer involves the following steps:

  1. 1.The transfer must be initiated.
  2. 2.The EEPROM must be instructed what location should be read.
  3. 3.The reading must be performed.
  4. 4.The transfer must be finished.

Note that data is transmitted both from master to slave and vice versa. The master must instruct the slave what location it wants to read, requiring the use of the MT mode. Subsequently, data must be read from the slave, implying the use of the MR mode. Thus, the transfer direction must be changed. The master must keep control of the bus during all these steps, and the steps should be carried out as an atomical operation. If this principle is violated in a multi-master system, another master can alter the data pointer in the EEPROM between steps 2 and 3, and the master will read the wrong data location. Such a change in transfer direction is accomplished by transmitting a REPEATED START between the transmission of the address byte and reception of the data. After a REPEATED START, the Master keeps ownership of the bus. The flow in this transfer is depicted in the following figure:

Figure 1. Combining Several TWI Modes to Access a Serial EEPROM