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28 page 
ADM1075 Data Sheet Rev. A | Page 28 of 52 PMBus INTERFACE The I2C bus is a common, simple serial bus used by many devices to communicate. It defines the electrical specifications, the bus timing, the physical layer, and some basic protocol rules. SMBus is based on I2C and aims to provide a more robust and fault-tolerant bus. Functions such as bus timeout and packet error checking are added to help achieve this robustness, along with more specific definitions of the bus messages used to read and write data to devices on the bus. PMBus is layered on top of SMBus and, in turn, on I2C. Using the SMBus defined bus messages, PMBus defines a set of standard commands that can be used to control a device that is part of a power chain. The ADM1075 command set is based upon the PMBus™ Power System Management Protocol Specification, Part I and Part II, Revision 1.2. This version of the standard is intended to provide a common set of commands for communicating with dc-to-dc type devices. However, many of the standard PMBus commands can be mapped directly to the functions of a hot swap controller. Part I and Part II of the PMBus standard describe the basic commands and how they can be used in a typical PMBus setup. The following sections describe how the PMBus standard and the ADM1075 specific commands are used. DEVICE ADDRESSING ADM1075-2. The PMBus address is seven bits in size. The upper five bits (MSBs) of the address word are fixed and are different for each model, as follows: • ADM1075-1: Base address is 00100xx (0x10) • ADM1075-2: Base address is 00110xx (0x18) is used to select one of four possible addresses for a given model. The ADR pin connection selects the lowest two bits (LSBs) of the 7-bit address word (see Table 6). Table 6. PMBus Addresses and ADR Pin Connection Value of Address LSBs ADR Pin Connection 00 Connect to VEE 01 150 kΩ resistor to VEE 10 No connection (floating) 11 Connect to VCAP SMBus PROTOCOL USAGE SMBus defined bus protocols. The following SMBus protocols are implemented by the ADM1075: • Send byte • Receive byte • Write byte • Read byte • Write word • Read word • Block read PACKET ERROR CHECKING The ADM1075 PMBus interface supports the use of the packet error checking (PEC) byte that is defined in the SMBus standard. The PEC byte is transmitted by the ADM1075 during a read transaction or sent by the bus host to the ADM1075 during a write transaction. The ADM1075 supports the use of PEC with all the SMBus protocols that it implements. The use of the PEC byte is optional. The bus host can decide whether to use the PEC byte with the ADM1075 on a message- by-message basis. There is no need to enable or disable PEC in the ADM1075. The PEC byte is used by the bus host or the ADM1075 to detect errors during a bus transaction, depending on whether the trans- action is a read or a write. If the host determines that the PEC byte read during a read transaction is incorrect, it can decide to repeat the read if necessary. If the ADM1075 determines that the PEC byte sent during a write transaction is incorrect, it ignores the command (does not execute it) and sets a status flag. Within a group command, the host can choose to send or not send a PEC byte as part of the message to the ADM1075. |
