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ADM1021AARQZ-R Datasheet(PDF) 12 Page - ON Semiconductor |
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ADM1021AARQZ-R Datasheet(HTML) 12 Page - ON Semiconductor |
12 / 15 page ![]() ADM1021A http://onsemi.com 12 Figure 15. Writing to the Address Pointer Register Only R/W A6 11 9 SCLK SDATA A5 A4 A3 A2 A1 A0 D7 D6 D5 D4 D3 D2 D1 D0 ACK. BY ADM1021A STOP BY MASTER START BY MASTER ACK. BY ADM1021A 9 FRAME 1 SERIAL BUS ADDRESS BYTE FRAME 2 ADDRESS POINTER REGISTER BYTE Figure 16. Reading Data from a Previously Selected Register A6 11 9 SCLK SDATA A5 A4 A3 A2 A1 A0 D7 D6 D5 D4 D3 D2 D1 D0 NO ACK. BY MASTER STOP BY MASTER START BY MASTER ACK. BY ADM1021A 9 FRAME 1 SERIAL BUS ADDRESS BYTE FRAME 2 DATA BYTE FROM ADM1021A R/W When reading data from a register there are two possibilities: 1. If the ADM1021A’s address pointer register value is unknown or not the desired value, it is first necessary to set it to the correct value before data can be read from the desired data register. This is done by performing a write to the ADM1021A as before, but only the data byte containing the register read address is sent, because data is not to be written to the register. This is shown in Figure 15. 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 16. 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 register, so Figure 15 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; this is because the first data byte of a write is always written to the address pointer register. 2. Remember that the ADM1021A registers have different addresses for read and write operations. The write address of a register must be written to the address pointer if data is to be written to that register, but it is not possible to read data from that address. The read address of a register must be written to the address pointer before data can be read from that register. ALERT Output The ALERT output goes low whenever an out−of−limit measurement is detected, or if the remote temperature sensor is open−circuit. It is an open drain and requires a 10 k W pullup to VDD. Several ALERT outputs can be wire−ANDed together so the common line goes low if one or more of the ALERT outputs goes low. The ALERT output can be used as an interrupt signal to a processor, or it can be used as an SMBALERT. Slave devices on the SMBus cannot normally signal to the master that they want to talk, but the SMBALERT function allows them to do so. One or more ALERT outputs are connected to a common SMBALERT line connected to the master. When the SMBALERT line is pulled low by one of the devices, the following procedure occurs, as shown in Figure 17. Figure 17. Use of SMBALERT MASTER RECEIVES SMBALERT MASTER SENDS ARA AND READ COMMAND NO ACK START ALERT RESPONSE ADDRESS RD ACK DEVICE ADDRESS STOP DEVICE SENDS ITS ADDRESS 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 ALERT 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 ALERT output is low, the one with the lowest device address has priority, in accordance with normal SMBus arbitration. |
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