Rainbow Electronics MAX34440 - Manual (Page 19)

PMBus 6-Channel Power-Supply Manager
MAX34440
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
(t
TO ), 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.