Preliminary C8051F120/1/2/3 C8051F124/5/6/7 - KEMT FEI TUKE

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C8051F120/1/2/3 C8051F124/5/6/7 Preliminary drain, the bus will be pulled LOW. The master attempting the HIGH will detect a LOW SDA and give up the bus. The winning master continues its transmission without interruption; the losing master becomes a slave and receives the rest of the transfer. This arbitration scheme is non-destructive: one device always wins, and no data is lost. 19.2.2. Clock Low Extension SMBus provides a clock synchronization mechanism, similar to I 2 C, which allows devices with different speed capabilities to coexist on the bus. A clock-low extension is used during a transfer in order to allow slower slave devices to communicate with faster masters. The slave may temporarily hold the SCL line LOW to extend the clock low period, effectively decreasing the serial clock frequency. 19.2.3. SCL Low Timeout If the SCL line is held low by a slave device on the bus, no further communication is possible. Furthermore, the master cannot force the SCL line high to correct the error condition. To solve this problem, the SMBus protocol specifies that devices participating in a transfer must detect any clock cycle held low longer than 25 ms as a “timeout” condition. Devices that have detected the timeout condition must reset the communication no later than 10 ms after detecting the timeout condition. 19.2.4. SCL High (SMBus Free) Timeout The SMBus specification stipulates that if the SCL and SDA lines remain high for more that 50 µs, the bus is designated as free. If an SMBus device is waiting to generate a Master START, the START will be generated following the bus free timeout. Page 228 DS008-0.8-AUG02 © 2002 Cygnal Integrated Products, Inc.
Preliminary C8051F120/1/2/3 C8051F124/5/6/7 19.3. SMBus Transfer Modes The SMBus0 interface may be configured to operate as a master and/or a slave. At any particular time, the interface will be operating in one of the following modes: Master Transmitter, Master Receiver, Slave Transmitter, or Slave Receiver. See Table 19.1 for transfer mode status decoding using the SMB0STA status register. The following mode descriptions illustrate an interrupt-driven SMBus0 application; SMBus0 may alternatively be operated in polled mode. 19.3.1. Master Transmitter Mode Serial data is transmitted on SDA while the serial clock is output on SCL. SMBus0 generates a START condition and then transmits the first byte containing the address of the target slave device and the data direction bit. In this case the data direction bit (R/W) will be logic 0 to indicate a "WRITE" operation. The SMBus0 interface transmits one or more bytes of serial data, waiting for an acknowledge (ACK) from the slave after each byte. To indicate the end of the serial transfer, SMBus0 generates a STOP condition. Figure 19.4. Typical Master Transmitter Sequence S SLA W A Data Byte A Data Byte A P Interrupt Interrupt Interrupt Interrupt Received by SMBus Interface Transmitted by SMBus Interface S=START P=STOP A=ACK W=WRITE SLA = Slave Address 19.3.2. Master Receiver Mode Serial data is received on SDA while the serial clock is output on SCL. The SMBus0 interface generates a START followed by the first data byte containing the address of the target slave and the data direction bit. In this case the data direction bit (R/W) will be logic 1 to indicate a "READ" operation. The SMBus0 interface receives serial data from the slave and generates the clock on SCL. After each byte is received, SMBus0 generates an ACK or NACK depending on the state of the AA bit in register SMB0CN. SMBus0 generates a STOP condition to indicate the end of the serial transfer. Figure 19.5. Typical Master Receiver Sequence S SLA R A Data Byte A Data Byte N P Interrupt Interrupt Interrupt Interrupt Received by SMBus Interface Transmitted by SMBus Interface S = START P=STOP A=ACK N = NACK R=READ SLA = Slave Address © 2002 Cygnal Integrated Products, Inc. DS008-0.8-AUG02 Page 229
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Preliminary C8051F120/1/2/3 C8051F124/5/6/7 - KEMT FEI TUKE

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