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ATmega644PA Datasheet(PDF) 234 Page - ATMEL Corporation |
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ATmega644PA Datasheet(HTML) 234 Page - ATMEL Corporation |
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234 / 468 page  234 8152G–AVR–11/09 ATmega164PA/324PA/644PA/1284P • Two or more masters are accessing different slaves. In this case, arbitration will occur in the SLA bits. Masters trying to output a one on SDA while another Master outputs a zero will lose the arbitration. Masters losing arbitration in SLA will switch to Slave mode to check if they are being addressed by the winning Master. If addressed, they will switch to SR or ST mode, depending on the value of the READ/WRITE bit. If they are not being addressed, they will switch to not addressed Slave mode or wait until the bus is free and transmit a new START condition, depending on application software action. This is summarized in Figure 20-21. Possible status values are given in circles. Figure 20-21. Possible Status Codes Caused by Arbitration 20.9 Register Description 20.9.1 TWBR – TWI Bit Rate Register • Bits 7:0 – TWI Bit Rate Register TWBR selects the division factor for the bit rate generator. The bit rate generator is a frequency divider which generates the SCL clock frequency in the Master modes. See ”Bit Rate Generator Unit” on page 215 for calculating bit rates. 20.9.2 TWCR – TWI Control Register The TWCR is used to control the operation of the TWI. It is used to enable the TWI, to initiate a Master access by applying a START condition to the bus, to generate a Receiver acknowledge, to generate a stop condition, and to control halting of the bus while the data to be written to the Own Address / General Call received Arbitration lost in SLA TWI bus will be released and not addressed slave mode will be entered A START condition will be transmitted when the bus becomes free No Arbitration lost in Data Direction Yes Write Data byte will be received and NOT ACK will be returned Data byte will be received and ACK will be returned Last data byte will be transmitted and NOT ACK should be received Data byte will be transmitted and ACK should be received Read B0 68/78 38 SLA START Data STOP Bit 76543210 (0xB8) TWBR7 TWBR6 TWBR5 TWBR4 TWBR3 TWBR2 TWBR1 TWBR0 TWBR Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 00000000 Bit 76543210 (0xBC) TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE TWCR Read/Write R/W R/W R/W R/W R R/W R R/W Initial Value 00000000 |
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