Frequently Asked Questions

PMBus is a non-proprietary standard for communication with power converters of all types. The PMBus specification is in three parts.

• Part I describes how data is moved from one device to another. It is based on the SMBus specification with a few additional requirements specific to the PMBus.
• Part II of the PMBus specification describes the command language, data formats including numerical formats, how communication faults are handled, and other requirements.
• Part III of the PMBus specification was introduced with Revision 1.3. Part III describes the AVSBus which provides a fast and simple interface for logic devices (such as microprocessors, ASICs, and FPGAs and other programmable logic devices) to quickly control their supply voltage. AVSBus also provides for some monitoring of the power converter attached to the devices. The physical layer of the AVSBus is very much like the common SPI.

AVSBus (Adaptive Voltage Scaling Bus) is a part of the PMBus specifications. The AVSBus specification describes a power management bus design to allowed complex logic devices, like processors and large FPGAs, to control their supply voltages as fast as possible.

When a logic device wants to enter an energy saving mode, like a hibernation or sleep state, it generally lowers its supply voltage to a minimum value. The faster the voltage can be changed from the normal or high performance operating value to the energy saving value, the more energy can be saved. An important part of how long it takes to reduce the voltage is the communication delay from the logic device to the power converter. AVSBus, which can operate at speeds up to 50 MHz, is designed to minimize the communication delay and maximize energy savings.

In the other direction, when a logic device wants to return to normal or high performance operation, it is important that the voltage be increased to the new operating voltage as quickly as possible in order to maximize performance. Again, AVSBus is designed to minimize the communication delay so that the power converter can respond to the voltage change request in the least time possible.

Companies that are current PMBus Adopters are eligible, but not required, to participate in the PMBus Standards Working Group. Companies interested in participating in the PMBus Standards Working Group should send an email message toadmin@smiforum.org with the name and contact of the person that they would like to represent them in the Working Group.

In general, each PMBus Adopter may have one representative on the group although this is at the discretion of the Chair of the Working Group. When voting on any matter, each PMBus Adopter company gets one vote.

The PMBus web site, http://pmbus.org/, provides supporting materials. On the home page, in addition to the specification, Application Profiles & Notes are available under the Specifications tab and several presentations on PMBus are available under the Resources tab.

The current PMBus specifications are available at no cost, although registration with your name, email, and company is required, from the PMBus web site, http://pmbus.org/.

The I²C specification only describes the physical layer, timing, and flow control of a two-wire bus. The I²C specification does not describe the format of messages (like the SMBus protocols) and does not describe the content of the messages.

The PMBus specification is a complete power management protocol. It includes how to get the bit and bytes from one device to another (transport) as well as describing a command language that gives meaning to those bits and bytes.

In principle, yes, although there is no specification to do so. The original PMBus specification was in two parts, transport and command language, to allow for the possibility of using another bus, such as RS-485, to send and receive PMBus commands.

At this time, device manufacturers must self-certify that their devices are compliant to the PMBus specification.

There is no central authority that controls the addresses of PMBus devices. Manufacturers are free to use any of the non-reserved addresses for their devices. This places the responsibility for assuring that there are no address conflicts on the system engineer designing with PMBus devices.

On the PMBus web site, http://pmbus.org/, the Products tab leads to a list of PMBus adopter companies. Click on Product By Category to see a list of particular products, or conversely, click on Products By Company to see a list of PMBus products from a specific company.

There is a growing number of technical notes, design tools, adapters and demonstration kits available to support PMBus system development. Many of our adopter companies provide tools that are specifically tailored to support their PMBus-compliant product offerings, which can be found under the Products tab of the PMBus web site. Other universal development tools are also available from third-party suppliers.

Disclaimer: This partial listing of suppliers and products is provided as a convenience and is not an endorsement of any of these suppliers or products by the System Management Interface Forum. Please contact your supplier directly for a complete review of PMBus support information.

The minimum required signals are the clock, SMBCLK, and data, SMBDAT.

Optional PMBus signals include the CONTROL signal, which acts as a hardwired on/off signal, and SMBALERT#, which is a signal for a PMBus device to notify the controller or system host that it has information or needs attention, such as in the cause of a fault condition. Another optional signal is WP (write protect), an input to a PMBus device that can control write access to a PMBus device.

The CONTROL signal is a hardwired on/off signal. The signal name “control” was chosen because many power supply manufacturers were already using signal names like ENABLE or REMOTE ON/OFF.

The CONTROL signal can be configured to be active high (CONTROL pulled high to start the converter) or active low (CONTROL pulled low to start the converter) by using the ON_OFF_CONFIG command.

One use of the CONTROL signal is as an emergency off signal. Under normal conditions, the converter would be turned on and off over the bus using the OPERATION command. In the case of a system fault requiring the system to be powered down immediately, the CONTROL signal could be set to the “Not Run” or “Off” value to cause all converters to turn off or start their power down sequence immediately.

Another use of the CONTROL signal is to synchronize power up (or power down) sequencing. First, startup delay time and output voltage rise time of all the converters in the system are configured using the TON_DELAY and TON_RISE commands. Then the converters are configured to start when the CONTROL signal is asserted (active high or low as configured with the ON_OFF_CONFIG command). Asserting the CONTROL signal provides a simultaneous “Start” signal to the converters.

When developing the PMBus specification the PMBus Specification Working Group recognized the need for device manufacturers to add their own unique value, albeit at the cost of interoperability with devices from other manufacturers. To accomplish this goal, the PMBus specification designates several command codes as manufacturer-specific. Possible uses of the manufacturer-specific commands are:

• A command to download firmware updates,
• A command that reads back of all of the device status in one command,
• Commands for calibrating additional parameters, or
• A command to download coefficients for a digital compensator.

If a manufacturer does use one or more manufacturer-specific commands, they must provide a full description of the command, including transaction type and data formats, in their product literature.

Two commands are provided that allow a controller or system host to determine the capabilities of PMBus devices on the bus: CAPABILITY and QUERY.

The CAPABILITY command provides basic information about the PMBus. The CAPABILITY command returns information such as:

• Does the device support Packet Error Checking (PEC)?
• Maximum bus speed supported by the device?
• Does the device have an SMBALERT# signal output?
• Which numeric format is used for general purpose data?
• Does the device support the AVSBus?

The QUERY command allows a controller or system host to ask a PMBus device about support for a specific command.

The QUERY command returns information such as:

• Is the command supported or not?
• If the command is supported, does the device allow writes to this command?
• Does the device allow reads of the data associated with the command?
• What is the data format the device uses with this command?

The combination of these two commands, CAPABILTY and QUERY, allows a controller or system host to determine how to configure itself to work with a PMBus device on the bus.

The standard way is for a controller to poll each address in the system and terminate the transaction with a STOP condition after the ACK/NACK response. If an ACK follows the R/W# bit then the controller knows that there is a device at that address. If there is no acknowledge (a NACK) after the R/W# bit then the master knows there is no device at the address.

A faster way is for the system to use ZONE capable devices. The controller can then use a ZONE READ operation to get a response from all the attached devices. This results in a transaction that is only as long as the number of devices on the bus rather than the 127 transactions needed to poll every address. The procedure for discovering all of the devices in a system using the ZONE READ operation is described in detail in PMBus Power System Management Protocol Application Note AN001: Using The ZONE_READ and ZONE_WRITE Protocols, which can be found in the Application Profiles & Notes section under the Specifications tab on the PMBus web site http://pmbus.org/.

The new Zone Protocol is comprised of three features which enable faster bus transactions:

• Zone Configuration
• Zone Write
• Zone Read

A zone is a set of target devices on a shared PMBus that can react to Zone Write and Zone Read operations. Any target device can be assigned to a zone with a configuration command, and a controller can set the Active Write Zone and Active Read Zone to communicate with a specific zone, in much the same way a controller can set the PAGE of a target device to communicate with a subset of its functionality. The Active Write Zone and Active Read Zone do not have to be the same value. The Active Write Zone is often not the same as the Active Read Zone because it is common to “control” one subset of target devices and “monitor” telemetry from a different subset.

“Appendix C. SMBus Device Address Assignments” of the SMBus 3.0 specification gives a detailed list of the available and reserved addresses for both PMBus and SMBus devices.

The SMBus specifications are available from the System Management Bus (SMBus) web site, http://smbus.org/.

The SMBus specification does not give specific criteria for compliance. However, to be compliant a device must meet the requirements given in the specification, including but not limited to:

• Electrical interface,
• Timing, and
• Proper operation with all supported SMBus protocols.

At this time, device manufacturers must self-certify that their devices are compliant to the SMBus specification.

Commercial users of the PMBus trademarks must be PMBus adopters; that is, they must be members of the System Management Interface Forum (SMIF) in good standing and must have signed the PMBus adopters agreement. Commercial use includes, but is not limited to, using the term PMBus, AVSBus, or any PMBus or AVSBus logo on a company’s web site, or using the term PMBus, AVSBus, or any PMBus or AVSBus logo on commercial literature such as product briefs, data sheets, application notes, or sales quotations.

The fee for membership in the SMIF is 3900 US Dollars. (Membership fees are subject to change.)

There are no other fees for using PMBus or PMBus trademarks in a commercial application.

PMBus trademarks may be freely used in non-commercial applications.

The SMIF organization takes no position on the existence or applicability of any patent or its licenses. Patents may exist and users should do due diligence. Multiple PMBus adopters provide solutions. Please refer to your vendor of choice for their statement regarding these matters.

Per the PMBus adopters agreement, “necessary claims” (claims essential to implement the specification) in patents owned by PMBus adopters are automatically licensed to all PMBus adopters royalty-free. There is no list of such patents or claims, nor have we determined what claims of what patents are necessary.