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SC16IS740 Datasheet(PDF) 39 Page - NXP Semiconductors |
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SC16IS740 Datasheet(HTML) 39 Page - NXP Semiconductors |
39 / 63 page SC16IS740_750_760 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2011. All rights reserved. Product data sheet Rev. 7 — 9 June 2011 39 of 63 NXP Semiconductors SC16IS740/750/760 Single UART with I2C-bus/SPI interface, 64-byte FIFOs, IrDA SIR An address on the network is seven bits long, appearing as the most significant bits of the address byte. The last bit is a direction (R/W) bit. A ‘0’ indicates that the master is transmitting (write) and a ‘1’ indicates that the master requests data (read). A complete data transfer, comprised of an address byte indicating a ‘write’ and two data bytes is shown in Figure 21. When an address is sent, each device in the system compares the first seven bits after the START with its own address. If there is a match, the device will consider itself addressed by the master, and will send an acknowledge. The device could also determine if in this transaction it is assigned the role of a slave receiver or slave transmitter, depending on the R/W bit. Each node of the I2C-bus network has a unique seven-bit address. The address of a microcontroller is of course fully programmable, while peripheral devices usually have fixed and programmable address portions. When the master is communicating with one device only, data transfers follow the format of Figure 21, where the R/W bit could indicate either direction. After completing the transfer and issuing a STOP condition, if a master would like to address some other device on the network, it could start another transaction by issuing a new START. Another way for a master to communicate with several different devices would be by using a ‘repeated START’. After the last byte of the transaction was transferred, including its acknowledge (or negative acknowledge), the master issues another START, followed by address byte and data — without effecting a STOP. The master may communicate with a number of different devices, combining ‘reads’ and ‘writes’. After the last transfer takes place, the master issues a STOP and releases the bus. Possible data formats are demonstrated in Figure 22. Note that the repeated START allows for both change of a slave and a change of direction, without releasing the bus. We shall see later on that the change of direction feature can come in handy even when dealing with a single device. In a single master system, the repeated START mechanism may be more efficient than terminating each transfer with a STOP and starting again. In a multimaster environment, the determination of which format is more efficient could be more complicated, as when a master is using repeated STARTs it occupies the bus for a long time and thus preventing other devices from initiating transfers. Fig 21. A complete data transfer S P SDA SCL 0 to 6 78 ACK 002aab046 START condition STOP condition address R/W 0 to 6 78 data ACK 0 to 6 78 data ACK |
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