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ADM1022ARQ Datenblatt(PDF) 8 Page - Analog Devices |
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ADM1022ARQ Datenblatt(HTML) 8 Page - Analog Devices |
8 / 19 page REV. 0 ADM1022 –8– If ADD is left open-circuit the default address will be 0101100. The facility to make hardwired changes to A1 and A0 allows the user to avoid conflicts with other devices sharing the same serial bus; for example, if more than one ADM1022 is used in a system. The serial bus protocol operates as follows: 1. The master initiates data transfer by establishing a START condition, defined as a high-to-low transition on the serial data line SDA while the serial clock line SCL remains high. This indicates that an address/data stream will follow. All slave peripherals connected to the serial bus respond to the START condition, and shift in the next eight bits, consisting of a 7-bit address (MSB first) plus an R/ W bit, which deter- mines the direction of the data transfer, i.e., whether data will be written to or read from the slave device. The peripheral whose address corresponds to the transmitted address responds by pulling the data line low during the low period before the ninth clock pulse, known as the Acknowl- edge Bit. All other devices on the bus now remain idle while the selected device waits for data to be read from or written to it. If the R/ W bit is a 0, the master will write to the slave device. If the R/ W bit is a one, the master will read from the slave device. 2. Data is sent over the serial bus in sequences of nine clock pulses, eight bits of data followed by an Acknowledge Bit from the slave device. Transitions on the data line must occur during the low period of the clock signal and remain stable during the high period, as a low-to-high transition when the clock is high may be interpreted as a STOP signal. The number of data bytes that can be transmitted over the serial bus in a single READ or WRITE operation is limited only by what the master and slave devices can handle. 3. When all data bytes have been read or written, stop conditions are established. In WRITE mode, the master will pull the data line high during the 10th clock pulse to assert a STOP condition. In READ mode, the master device will override the acknowledge bit by pulling the data line high during the low period before the 9th clock pulse. This is known as No Acknowledge. The master will then take the data line low during the low period before the 10th clock pulse, then high during the 10th clock pulse to assert a STOP condition. Any number of bytes of data may be transferred over the serial bus in one operation, but it is not possible to mix read and write in one operation, because the type of operation is determined at the beginning and cannot subsequently be changed without starting a new operation. In the case of the ADM1022, write operations contain either one or two bytes, and read operations contain one byte, and perform the following functions: To write data to one of the device data registers or read data from it, the Address Pointer Register must be set so that the correct data register is addressed, then data can be written into that register or read from it. The first byte of a write operation always contains an address that is stored in the Address Pointer Register. If data is to be written to the device, then the write operation contains a second data byte that is written to the reg- ister selected by the address pointer register. This is illustrated in Figure 11a. The device address is sent over the bus followed by R/ W set to 0. This is followed by two data bytes. The first data byte is the address of the internal data register to be written to, which is stored in the Address Pointer Register. The second data byte is the data to be written to the internal data register. When reading data from a register there are two possibilities: 1. If the ADM1022’s Address Pointer Register value is unknown or not the desired value, it is first necessary to set it to the cor- rect value before data can be read from the desired data register. This is done by performing a write to the ADM1022 as before, but only the data byte containing the register address is sent, as data is not to be written to the register. This is shown in Figure 11b. A read operation is then performed consisting of the serial bus address, R/ W bit set to 1, followed by the data byte read from the data register. This is shown in Figure 11c. 2. If the Address Pointer Register is known to be already at the desired address, data can be read from the corresponding data register without first writing to the Address Pointer Reg- ister, so Figure 11b can be omitted. NOTES 1. Although 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, 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 11a to 11c, the serial bus address is shown as the default value 01011(A1)(A0), where A1 and A0 are set by the three-state ADD pin. 3. The ADM1022 also supports the Read Byte protocol, as described in the System Management Bus specification. |
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