computer science

System Management Bus

System Management Bus is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand System Management Bus rather than just read about it. In short: The System Management Bus (SMBus or SMB) is a single-ended simple two-wire bus for the purpose of lightweight communication. Most commonly it is found in chipsets of computer motherboards for communication with the power source for ON/OFF instructions.

Key takeaways

System Management Bus belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect System Management Bus to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of System Management Bus from memory before moving on to harder problems.

Reference excerpt

The System Management Bus (SMBus or SMB) is a single-ended simple two-wire bus for the purpose of lightweight communication. Most commonly it is found in chipsets of computer motherboards for communication with the power source for ON/OFF instructions. The exact functionality and hardware interfaces vary with vendors. It is derived from I²C for communication with low-bandwidth devices on a motherboard, especially power related chips such as a laptop's rechargeable battery subsystem (see Smart Battery System and ACPI). Other devices might include external master hosts, temperature sensor, fan or voltage sensors, lid switches, clock generator, and RGB lighting. Peripheral Component Interconnect (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, return status over SMBus, and poll chipset registers. 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 and Duracell in 1994. It carries clock, data, and instructions and is based on Philips' I²C 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: Alert Standard Format (ASF), Desktop and mobile Architecture for System Hardware (DASH), Intelligent Platform Management Interface (IPMI). SMBus is used to access DRAM configuration information as part of serial presence detect (SPD). 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.

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 any other value. Instead of relating the bus input levels to VDD, SMBus defines them to be fixed. SMBus 2.0 defines VIL,max at 0.8 V and VIH,min at 2.1 V, and supports a VDD ranging from 3 to 5 V, while in SMBus 3.0, the levels are defined at 0.8 and 1.35 V, with a 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 to 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 (ACPI) specifications.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with System Management Bus

Start with the simplest possible case. Write down what System Management Bus claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to System Management Bus before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about System Management Bus ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of System Management Bus

In research: System Management Bus appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses System Management Bus in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: System Management Bus is common in secondary-school and first-year university syllabi. It links to neighbouring topics Battery charging, Computer-related introductions in 1995, Computer buses, so understanding it makes those chapters shorter.
In everyday life: Look for System Management Bus outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study System Management Bus in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what System Management Bus means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain System Management Bus out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is System Management Bus in simple terms?

Why does System Management Bus matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study System Management Bus?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on System Management Bus.