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ADM1027 Scheda tecnica(PDF) 10 Page - Analog Devices |
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ADM1027 Scheda tecnica(HTML) 10 Page - Analog Devices |
10 / 56 page REV. A –10– ADM1027 Notes 1. It is possible to read a data byte from a data register without first writing to the address pointer register if the address pointer register is already at the correct value. However, it is not possible to write data to a register without writing to the address pointer register, because the first data byte of a write is always written to the address pointer register. 2. In Figures 6 to 8, the serial bus address is shown as the default value 01011(A1)(A0), where A1 and A0 are set by the address select mode function previously defined. 3. In addition to supporting the send byte and receive byte protocols, the ADM1027 also supports the read byte protocol (see System Management Bus specifications Rev. 2.0 for more information). 4. If it is required to perform several read or write operations in succession, the master can send a repeat start condition instead of a stop condition to begin a new operation. ADM1027 WRITE OPERATIONS The SMBus specification defines several protocols for different types of read and write operations. The ones used in the ADM1027 are discussed below. The following abbreviations are used in the diagrams: S – START P– STOP R– READ W – WRITE A– ACKNOWLEDGE A – NO ACKNOWLEDGE The ADM1027 uses the following SMBus write protocols: Send Byte In this operation, the master device sends a single command byte to a slave device, as follows: 1. The master device asserts a start condition on SDA. 2. The master sends the 7-bit slave address followed by the write bit (low). 3. The addressed slave device asserts ACK on SDA. 4. The master sends a command code. 5. The slave asserts ACK on SDA. 6. The master asserts a stop condition on SDA and the transaction ends. For the ADM1027, the send byte protocol is used to write a register address to RAM for a subsequent single byte read from the same address. This is illustrated in Figure 9. S SLAVE ADDRESS WA A P 12 3 4 5 6 REGISTER ADDRESS Figure 9. Setting a Register Address for Subsequent Read If it is required to read data from the register immediately after setting up the address, the master can assert a repeat start con- dition immediately after the final ACK and carry out a single byte read without asserting an intermediate stop condition. Write Byte In this operation, the master device sends a command byte and one data byte to the slave device, as follows: 1. The master device asserts a start condition on SDA. 2. The master sends the 7-bit slave address followed by the write bit (low). 3. The addressed slave device asserts ACK on SDA. 4. The master sends a command code. 5. The slave asserts ACK on SDA. 6. The master sends a data byte. 7. The slave asserts ACK on SDA. 8. The master asserts a stop condition on SDA to end the transaction. This is illustrated in Figure 10. S SLAVE ADDRESS WA 12 3 4 5 6 A DATA AP 78 REGISTER ADDRESS Figure 10. Single Byte Write to a Register ADM1027 READ OPERATIONS The ADM1027 uses the following SMBus read protocols: Receive Byte This is useful when repeatedly reading a single register. The register address needs to have been set up previously. In this operation, the master device receives a single byte from a slave device, as follows: 1. The master device asserts a start condition on SDA. 2. The master sends the 7-bit slave address followed by the read bit (high). 3. The addressed slave device asserts ACK on SDA. 4. The master receives a data byte. 5. The master asserts NO ACK on SDA. 6. The master asserts a stop condition on SDA and the trans- action ends. In the ADM1027, the receive byte protocol is used to read a single byte of data from a register whose address has previously been set by a send byte or write byte operation. S SLAVE ADDRESS RA DATA A P 12 3 4 5 6 Figure 11. Single Byte Read from a Register Alert Response Address Alert Response Address (ARA) is a feature of SMBus devices, which allows an interrupting device to identify itself to the host when multiple devices exist on the same bus. The SMBALERT output can be used as an interrupt output or can be used as an SMBALERT. One or more outputs can be connected to a common SMBALERT line connected to the master. If a device’s SMBALERT line goes low, the following procedure occurs: 1. SMBALERT is pulled low. 2. Master initiates a read operation and sends the alert response address (ARA = 0001 100). This is a general call address that must not be used as a specific device address. 3. The device whose SMBALERT output is low responds to the alert response address, and the master reads its device address. The address of the device is now known and it can be interrogated in the usual way. 4. If more than one device’s SMBALERT output is low, the one with the lowest device address will have priority, in accordance with normal SMBus arbitration. |
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