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The System Management Bus (abbreviated to SMBus or SMB) is a single-ended simple two-wire
bus A bus (contracted from omnibus, with variants multibus, motorbus, autobus, etc.) is a road vehicle that carries significantly more passengers than an average car or van. It is most commonly used in public transport, but is also in use for cha ...
for the purpose of lightweight communication. Most commonly it is found in computer motherboards for communication with the power source for ON/OFF instructions. It is derived from
I²C I2C (Inter-Integrated Circuit, ), alternatively known as I2C or IIC, is a synchronous, multi-controller/multi-target (master/slave), packet switched, single-ended, serial communication bus invented in 1982 by Philips Semiconductors. It is w ...
for communication with low-bandwidth devices on a
motherboard A motherboard (also called mainboard, main circuit board, mb, mboard, backplane board, base board, system board, logic board (only in Apple computers) or mobo) is the main printed circuit board (PCB) in general-purpose computers and other expand ...
, especially power related chips such as a laptop's rechargeable battery subsystem (see Smart Battery System). Other devices might include temperature, fan or voltage sensors, lid switches, clock generator, and RGB lighting. PCI add-in cards may connect to an SMBus segment. A device can provide manufacturer information, indicate its model/part number, save its state for a suspend event, report different types of errors, accept control parameters and return status. The SMBus is generally not user configurable or accessible. Although SMBus devices usually can't identify their functionality, a new PMBus coalition has extended SMBus to include conventions allowing that. The SMBus was defined by
Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the devel ...
and Duracell in 1994. It carries clock, data, and instructions and is based on
Philips Koninklijke Philips N.V. (), commonly shortened to Philips, is a Dutch multinational conglomerate corporation that was founded in Eindhoven in 1891. Since 1997, it has been mostly headquartered in Amsterdam, though the Benelux headquarters is ...
'
I²C I2C (Inter-Integrated Circuit, ), alternatively known as I2C or IIC, is a synchronous, multi-controller/multi-target (master/slave), packet switched, single-ended, serial communication bus invented in 1982 by Philips Semiconductors. It is w ...
serial bus protocol. Its clock frequency range is 10 kHz to 100 kHz. (PMBus extends this to 400 kHz.) Its voltage levels and timings are more strictly defined than those of I²C, but devices belonging to the two systems are often successfully mixed on the same bus. SMBus is used as an interconnect in several platform management standards including: ASF, DASH, IPMI. SMBus is used to access DRAM configuration information as part of serial presence detect. SMBus has grown into a wide variety of system enumeration use cases other than power management.


SMBus/I²C Interoperability

While SMBus is derived from I²C, there are several major differences between the specifications of the two busses in the areas of electricals, timing, protocols and operating modes. smbus.org smbus.org nxp.com 090429 maxim-ic.com


Electrical


Input Voltage (''VIL'' and ''VIH'')

When mixing devices, the I²C specification defines the input levels to be 30% and 70% of the supply voltage ''VDD'', which may be 5 V, 3.3 V, or some other value. Instead of relating the bus input levels to ''VDD'', SMBus defines them to be fixed at 0.8 and 2.1 V. SMBus 2.0 supports ''VDD'' ranging from 3 to 5 V. SMBus 3.0 supports ''VDD'' ranging from 1.8 to 5 V.


Sink Current (''IOL'')

SMBus 2.0 defines a ‘High Power’ class that includes a 4 mA sink current that cannot be driven by I²C chips unless the pull-up resistor is sized to I²C-bus levels. NXP devices have a higher power set of electrical characteristics than SMBus 1.0. The main difference is the current sink capability with ''VOL'' = 0.4 V. *SMBus low power = 350 μA *SMBus high power = 4 mA *I²C-bus = 3 mA SMBus ‘high power’ devices and I²C-bus devices will work together if the pull-up resistor is sized for 3 mA.


Frequency (''FMAX'' and ''FMIN'')

The SMBus clock is defined from 10–100 kHz while I²C can be 0–100 kHz, 0–400 kHz, 0–1 MHz and 0–3.4 MHz, depending on the mode. This means that an I²C bus running at less than 10 kHz will not be SMBus compliant since the SMBus devices may time out. Many SMBus devices will however support lower frequencies. SMBus 3.0 adds 400 kHz and 1 MHz bus speeds.


Timing

*SMBus defines a clock low time-out, TIMEOUT of 35 ms. I²C does not specify any timeout limit. *SMBus specifies TLOW:SEXT as the cumulative clock low extend time for a slave device. I²C does not have a similar specification. *SMBus specifies TLOW:MEXT as the cumulative clock low extend time for a master device. Again I²C does not have a similar specification. *SMBus defines both rise and fall time of bus signals. I²C does not. *The SMBus time-out specifications do not preclude I²C devices co-operating reliably on the SMBus. It is the responsibility of the designer to ensure that I²C devices are not going to violate these bus timing parameters.


Protocols


ACK and NACK usage

There are the following differences in the use of the NACK bus signaling: In I²C, a slave receiver is allowed to not acknowledge the slave address, if for example it's unable to receive because it's performing some real time task. SMBus requires devices to acknowledge their own address always, as a mechanism to detect a removable device's presence on the bus (battery, docking station, etc.) I²C specifies that a slave device, although it may acknowledge its own address, may decide, some time later in the transfer, that it cannot receive any more data bytes. I²C specifies that the device may indicate this by generating the not acknowledge on the first byte to follow. Other than to indicate a slave's device-busy condition, SMBus also uses the NACK mechanism to indicate the reception of an invalid command or datum. Since such a condition may occur on the last byte of the transfer, it is required that SMBus devices have the ability to generate the not acknowledge after the transfer of each byte and before the completion of the transaction. This is important because SMBus does not provide any other resend signaling. This difference in the use of the NACK signaling has implications on the specific implementation of the SMBus port, especially in devices that handle critical system data such as the SMBus host and the SBS components.


SMBus protocols

Each message transaction on SMBus follows the format of one of the defined SMBus protocols. The SMBus protocols are a subset of the data transfer formats defined in the I²C specifications. I²C devices that can be accessed through one of the SMBus protocols are compatible with the SMBus specifications. I²C devices that do not adhere to these protocols cannot be accessed by standard methods as defined in the SMBus and
Advanced Configuration and Power Interface Advanced Configuration and Power Interface (ACPI) is an open standard that operating systems can use to discover and configure computer hardware components, to perform power management (e.g. putting unused hardware components to sleep), auto c ...
(ACPI) specifications.


Address Resolution Protocol

The SMBus uses I²C hardware and I²C hardware addressing, but adds second-level software for building special systems. In particular its specifications include an Address Resolution Protocol that can make dynamic address allocations. Dynamic reconfiguration of the hardware and software allow bus devices to be ‘hot-plugged’ and used immediately, without restarting the system. The devices are recognized automatically and assigned unique addresses. This advantage results in a plug-and-play user interface. In both those protocols there is a very useful distinction made between a System Host and all the other devices in the system that can have the names and functions of masters or slaves.


Time-out feature

SMBus has a time-out feature which resets devices if a communication takes too long. This explains the minimum clock frequency of 10 kHz to prevent locking up the bus. I²C can be a ‘DC’ bus, meaning that a slave device stretches the master clock when performing some routine while the master is accessing it. This will notify to the master that the slave is busy but does not want to lose the communication. The slave device will allow continuation after its task is complete. There is no limit in the I²C-bus protocol as to how long this delay can be, whereas for an SMBus system, it would be limited to 35 ms. The SMBus protocol just assumes that if something takes too long, then it means that there is a problem on the bus and that all devices must reset in order to clear this mode. Slave devices are not then allowed to hold the clock LOW too long.


Packet Error Checking

SMBus 1.1 and later define optional Packet Error Checking (PEC). In that mode, a PEC (packet error code) byte is appended at the end of each transaction. The byte is calculated as CRC-8
checksum A checksum is a small-sized block of data derived from another block of digital data for the purpose of detecting errors that may have been introduced during its transmission or storage. By themselves, checksums are often used to verify dat ...
, calculated over the entire message including the address and read/write bit. The polynomial used is x8+x2+x+1 (the CRC-8- ATM
HEC HEC or hec may refer to: Math and science * Habitable Exoplanets Catalog * HEC syndrome, a medical condition characterized by hydrocephalus, endocardial fibroelastosis and cataracts * Highly emetogenic chemotherapy, a term for chemotherapy drug ...
algorithm, initialized to zero).


SMBALERT#

The SMBus has an extra optional shared
interrupt In digital computers, an interrupt (sometimes referred to as a trap) is a request for the processor to ''interrupt'' currently executing code (when permitted), so that the event can be processed in a timely manner. If the request is accepted ...
signal called SMBALERT#, which can be used by slaves to tell the host to ask its slaves about events of interest. SMBus also defines a less common "Host Notify Protocol", providing similar notifications but passing more data and building on the I²C multi-master mode.


Support

SMBus devices are supported by
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,
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,
NetBSD NetBSD is a free and open-source Unix operating system based on the Berkeley Software Distribution (BSD). It was the first open-source BSD descendant officially released after 386BSD was forked. It continues to be actively developed and is a ...
,
DragonFly BSD DragonFly BSD is a free and open-source Unix-like operating system forked from FreeBSD 4.8. Matthew Dillon, an Amiga developer in the late 1980s and early 1990s and FreeBSD developer between 1994 and 2003, began working on DragonFly BSD in ...
,
Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which i ...
,
Windows 98 Windows 98 is a consumer-oriented operating system developed by Microsoft as part of its Windows 9x family of Microsoft Windows operating systems. The second operating system in the 9x line, it is the successor to Windows 95, and was released ...
and newer and
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.


See also

*
List of network buses List of electrical characteristics of single collision domain segment "slow speed" network buses: {, class="wikitable sortable" style="width:100%;" ! Name !! Multidrop !! Max nodes !! Electrical type !! Cable type !! data-sort-type="number", M ...
* Embedded controller (EC) * Super I/O * Low Pin Count (LPC) *
Serial Peripheral Interface The Serial Peripheral Interface (SPI) is a synchronous serial communication interface specification used for short-distance communication, primarily in embedded systems. The interface was developed by Motorola in the mid-1980s and has become ...
(SPI) * Platform Environment Control Interface (PECI) * Host Embedded Controller Interface (HECI) * Power Management Bus (PMBus) * System Management Controller (SMC)


References


External links

*
Official SMBus specifications (free)

SBS forum

SMBus at tech-faq.com
{{Intel technology Computer-related introductions in 1995 Serial buses Out-of-band management Intel products Battery charging Computer hardware standards Computer buses