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MAX34440 Datasheet(PDF) 19 Page - Maxim Integrated Products |
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MAX34440 Datasheet(HTML) 19 Page - Maxim Integrated Products |
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19 / 43 page  PMBus 6-Channel Power-Supply Manager 19 Host Reads from a Write-Only Command When a read request is issued to a write-only command (CLEAR_FAULTS, STORE_DEFAULT_ALL, RESTORE_DEFAULT_ALL), the device does the following: 1) ACKs the address byte. 2) Ignores the command. 3) Sends all ones (FFh) as long as the host keeps acknowledging. 4) Sets the CML bit in STATUS_BYTE. 5) Sets the CML bit in STATUS_WORD. 6) Sets the DATA_FAULT bit in STATUS_CML. 7) Notifies the host through ALERT assertion (if enabled). Host Writes to a Read-Only Command When a write request is issued to a read-only command, the device does the following: 1) Ignores the command. 2) Sets the CML bit in STATUS_BYTE. 3) Sets the CML bit in STATUS_WORD. 4) Sets the COMM_FAULT bit in STATUS_CML. 5) Notifies the host through ALERT assertion (if enabled). SMBus Timeout If during an active SMBus communication sequence the SCL signal is held low for greater than the timeout duration (tTO), the device terminates the sequence and resets the serial bus. It takes no other action. No status bits are set. PMBus Operation From a software perspective, the device appears as a PMBus device capable of executing a subset of PMBus commands. A PMBus 1.1-compliant device uses the SMBus version 1.1 for transport protocol and responds to the SMBus slave address. In this data sheet, the term SMBus is used to refer to the electrical characteristics of the PMBus communication using the SMBus physi- cal layer. The term PMBus is used to refer to the PMBus command protocol. The device employs a number of standard SMBus protocols to program output voltage and warning/faults thresholds, read monitored data, and provide access to all manufacturer-specific commands. The device supports the group command. The group command is used to send commands to more than one PMBus device. It is not required that all the devices receive the same command. However, no more than one command can be sent to any one device in one group command packet. The group command must not be used with commands that require receiving devices to respond with data, such as the STATUS_BYTE com- mand. When the device receives a command through this protocol, it immediately begins execution of the received command after detecting the STOP condition. The device supports the PAGE command and uses it to select which individual channel to access. When a data word is transmitted, the lower order byte is sent first and the higher order byte is sent last. Within any byte, the most significant bit (MSB) is sent first and the least sig- nificant bit (LSB) is sent last. PMBus Protocol Support The device supports a subset of the commands defined in the PMBus™ Power System Management Protocol Specification Part II - Command Language, Revision 1.1. For detailed specifications and the complete list of PMBus commands, refer to Part II of the PMBus spec- ification available at www.PMBus.org. The support- ed PMBus commands and the corresponding device behavior are described in this document. All data values are represented in DIRECT format, unless otherwise stated. Whenever the PMBus specification refers to the PMBus device, it is referring to the MAX34440 operating in conjunction with a power supply. While the command can call for turning on or turning off the PMBus device, the MAX34440 always remains on to continue com- municating with the PMBus master, and the MAX34440 transfers the command to the power supply accordingly. Data Format Voltage data for commanding or reading the output voltage or related parameters (such as the overvolt- age threshold) is presented in DIRECT format. DIRECT format data is a 2-byte, two’s complement binary value. DIRECT format data can be used with any command that sends or reads a parametric value. The DIRECT format uses an equation and defined coefficients to calculate the desired values. Table 3 shows the coefficients used by the device. Interpreting Received DIRECT Format Values The host system uses the following equation to convert the value received from the PMBus device—in this case, the MAX34440—into a reading of volts, degrees Celsius, or other units as appropriate: X = (1/m) x (Y x 10-R - b) where X is the calculated, real world value in the appro- priate units (V, NC, etc.); m is the slope coefficient; Y is the 2-byte, two’s complement integer received from the PMBus device; b is the offset; and R is the exponent. |
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