science

Power Management Bus

Power Management Bus is a 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 Power Management Bus rather than just read about it. In short: The Power Management Bus (PMBus) is a variant of the System Management Bus (SMBus) which is targeted at digital management of power supplies. Like SMBus, it is a relatively slow speed two wire communications protocol based on I²C.

Key takeaways

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

Reference excerpt

The Power Management Bus (PMBus) is a variant of the System Management Bus (SMBus) which is targeted at digital management of power supplies. Like SMBus, it is a relatively slow speed two wire communications protocol based on I²C. Unlike either of those standards, it defines a substantial number of domain-specific commands rather than just saying how to communicate using commands defined by the user.

Overview The first part gives an overview with particular reference to SMBus, while the second part goes into detail about all the commands defined for PMBus devices. There are both standardized commands and manufacturer specific commands. Conformance requirements for PMBus are minimal, and are described in Part I of the specification. See the PMBus 1.1 specification for full details.

Comparison to SMBus At the lowest level, PMBus follows SMBus 1.1 with a few differences. This information is presented in more detail in Part I of the PMBus specification:

400 kHz bus speeds are allowed (vs. the 100 kHz limit of SMBus) In PMBus, blocks may include up to 255 bytes (vs. the 32-byte limit of SMbus). As in SMBus 2.0, only seven-bit addressing is used. Some commands use the SMBus 2.0 block process calls. Either the SMBALERT# mechanism or the SMBus 2.0 host notify protocol may be used to notify the host about faults. PMBus devices are required to support a Group Protocol, where devices defer acting on commands until they receive a terminating STOP. Since commands can be issued to many different devices before that STOP, this lets the PMBus master synchronize their actions. An "extended command" protocol is defined, using a second command byte to add 256 more codes each for both standard and manufacturer-specific commands.

PMBus commands The PMBus command space can be seen as exposing a variety of readable, and often writable, device attributes such as measured voltage and current levels, temperatures, fan speeds, and more. Different devices will expose different attributes. Some devices may expose such attributes in multiple "pages", as for example one page managing each power supply rail (maybe 3.3V, 5V, 12V, −12V, and a programmable supply supporting 1.0–1.8V). The device may set warning and fault limits, where crossing a limit will alert the host and possibly trigger fault recovery. Different devices will offer different capabilities. The ability to query a PMBus 1.1 device about its capabilities may be particularly useful when building tools, especially in conjunction with the ability to store user data in the devices (e.g. in EEPROM). Without such a query capability, only error-prone external configuration data is available. Part II of the PMBus specification covers every standard PMBus command. It also describes the models for managing output power and current, managing faults, converting values to and from the formats understood by a given device, and accessing manufacturer-provided information such as inventory data (model and serial number, etc.) and device ratings.

Linear11 Floating-Point Format PMBus defines its own 16-bit floating-point format, termed "Linear11".

N = Signed Exponent Y = Signed Mantissa Value Represented = Y × 2N Unlike the half-precision floating-point format (which also uses 16 bits in total) and other typical float formats, a signed 11-bit mantissa is used rather than an unsigned fraction with a separate sign bit. Similarly, the exponent is stored as a signed 5-bit number rather than a more typical biased unsigned number. This has the following implications:

The sign of the resulting number uniquely depends on bit 2 of the high byte, rather than the most significant bit of the high byte. Because both values are stored as signed numbers, it is necessary to explicitly sign-extend both values when decoding the number. However, this makes the encoding process simpler. There is no representation for negative zero. Inverting the sign of the resulting number must take into account some special edge cases: The sign of the result can be inverted with an 11-bit two's complement operation, if and only if Y ≠ -1024. When Y = -1024, the sign inversion process must produce Y = 512, N = N + 1, if and only if N remains less than 32. The most negative number is represented with Y = -1024 and N = 31. There is not a positive representation for this number.

Patenting issues In January 2008, Power-One was awarded a win in a patent infringement suit between them and Artesyn Technologies for the latter's PMBus enabled converters. Power-One claims that PMBus applications need a license from them. Potential PMBus users should investigate the issue for themselves. See external links.

See also I²C (I2C) System Management Bus (SMBus) Advanced Configuration and Power Interface (ACPI) List of network buses

External links

Official Official website Official PMBus specifications (free)

Other PMBus–panacea or hype? is an article that's informative about the context of PMBus, written by the specifications' editor. Article from Electronic Design describes the lawsuit between Power-One and Artesyn Technologies, accessed 24 July 2013.

Worked examples

Example 1 — a first encounter with Power Management Bus

Start with the simplest possible case. Write down what Power Management Bus claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In 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 Power 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 Power 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 Power Management Bus

In research: Power Management Bus appears in 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 Power 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: Power Management Bus is common in secondary-school and first-year university syllabi. It links to neighbouring topics Battery charging, Serial buses, so understanding it makes those chapters shorter.
In everyday life: Look for Power 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.

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How to study Power Management Bus in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Power 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 Power Management Bus out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Power Management Bus in simple terms?

Why does Power Management Bus matter?

Because it connects several 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 Power 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 Power Management Bus.