Download Boosting ATCA Hardware Platform Management

Boosting ATCA Hardware Platform Management for Power Subsystems
A new set of extensions to the AdvancedTCA specification includes additional means for power
management and supports a new generation of sensors for more comprehensive monitoring and
control of power consumption.
by Mark Overgaard, Pigeon Point Systems
More than ten years after its adoption on the last day of 2002, AdvancedTCA will get a
significant set of extensions later this year in PICMG 3.7, the AdvancedTCA Base Extensions
specification, now nearing completion within PICMG. Among other things, these extensions
enable larger and more powerful ATCA shelves, such as a single shelf with 16 double-wide
boards (each board drawing up to 800 watts!), with 8 of them installed on each side of a dualsided shelf.
A subset of these extensions relate to hardware platform management for the power-related
aspects of these shelves, which potentially consume more than 12 kW each. Many of the
extensions described here could be added via software changes to the hardware platform
management layer of some currently shipping ATCA platforms.
One area of these management enhancements concerns “power geography,” rounding out the
capability for shelves and boards to self-describe their power characteristics to enable more
accurate operational tracking and diagnosis. These power geography enhancements also include
coverage for AC-powered shelves, where the base ATCA specification covers only DC-powered
shelves.
Another extension area is enhancing the sensor facilities of the Intelligent Platform Management
Interface (IPMI) so that sensor values larger than 8 bits can be represented. One key goal is to
enable integration of the wide ranging sensors defined by PMBus, an open protocol based on I2C
for communicating with power conversion and other power-related devices. PMBus-compatible
physical sensors are already being implemented in ATCA power subsystems from the bulk AC
power supplies at the shelf level to the -48V power input modules on individual ATCA boards
and the 12V point of load controllers that provide power for specific board subsystems.
Leveraging the Power Geography
Figure 1 shows the power subsystem of an example PICMG 3.7 shelf that includes four doublewide board slots. In each of those slots, a blue connector—called a Zone 1 connector in ATCA—
on the backplane mates with a corresponding connector on the board to provide management and
power connections for that board. Each board spans two slots, but connects to the backplane in
only one of them. A slot position without a Zone 1 connector, for instance slot ØA1, is called a
void slot. This example is assumed to show just one side (side A) of a dual-sided PICMG 3.7
shelf; the slot numbers include an “A” to indicate that.
There are four -48V feeds in the example shelf, two from each of the Power Entry Modules
(PEMs); each feed supplies one of the two power inputs on each board. The PEMs themselves
are each powered by three AC Power Supply Units (PSUs).
In a shelf built in accordance with the PICMG 3.7 enhancements, all elements shown here are
fully self-describing, including which PSUs are connected to which PEMs, and which feeds from
those PEMs are connected to which board inputs. With current ATCA, in contrast, AC PSUs are
not addressed at all and the power path from a specific PEM output to a specific board input in a
particular slot is not self-describing. The missing information can complicate power management
and problem diagnosis in a shelf.
Figure 2 shows another extension in PICMG 3.7 where new or augmented data structures record
the voltage ranges for which boards and shelves are designed and within which they are expected
to operate in a particular installation. The design voltage ranges are established at the factory and
the expected voltage ranges are configured when a shelf is installed and connected to a power
source. In PICMG 3.7, it is possible to build shelves and boards that support a more restricted
voltage range than ATCA. For example, designing for the enterprise voltage range (-43V to 57V) can allow cost savings compared to supporting the full ATCA legacy voltage range (-39V
to -72V).
What happens if a board designed for a restricted range (such as the enterprise voltage range
supported by the board installed in slot A4 in this example) is installed in a shelf that implements
a wider voltage range? With the information available in PICMG 3.7, a shelf manager could
choose not to enable power to such a board at all, to avoid risking an unacceptable operating
environment for the board.
What if the example shelf shown in the figure is installed with no power source for PEM B? The
expected voltages for the two feeds from that PEM would all be configured as zero. A PICMG
3.7 board can use a new Intelligent Platform Management Interface (IPMI) command to query
the shelf manager for the expected voltages on its feeds. The shelf manager uses power
geography data to identify the correct feeds and retrieves the expected voltages that have been
configured for those feeds. In this case, when a board in the shelf detects zero volts on an input
feed that has an expected voltage of zero, there is no need to raise an alert.
Extended IPMI Sensors in Hardware Platform Management
IPMI sensors that provide numeric sensor values are based on an 8-bit raw data value and a set of
coefficients that can be applied to that value by a client application. This produces a processed
value with appropriate scaling to reflect physical measurement units such as volts or amperes.
For some physical measurements (including the temperatures typically measured in an ATCA
shelf), 8 bits of raw data is sufficient for management purposes.
In the power area, however, there is often a need to size data values for worst-case conditions
while maintaining accuracy for finer granularity assessments. The physical devices used to
monitor power-related properties in some currently shipping ATCA shelves capture greater than
8-bit data for voltage, current and power measurements. However, that data has to be reduced to
8-bit data for reporting via IPMI. For example, the 16-bit hardware sensor in some currently
shipping PEMs can measure power in 0.2W increments, but when reduced to 8-bit data the IPMI
sensor can only report power in 51W increments, thus sacrificing potentially important
accuracy.
The PMBus architecture, which was designed for and by power monitoring specialists, defines
two fundamental raw value formats. The PMBus direct format uses 16-bit signed raw values
while the linear format uses an 11-bit mantissa and a 5-bit binary scale factor— both signed
integers. Like IPMI, PMBus defines a set of coefficients and a conversion formula based on
those coefficients to calculate processed values of sensor measurements. Furthermore, the
PMBus coefficients and conversion formula, while similar to IPMI’s, are sufficiently different
that PMBus-based sensors cannot be accurately represented with the existing IPMI coefficients
and conversion formula.
Figure 3 shows the main levels of ATCA power subsystems, from the onboard power domains
(such as one supporting a particular CPU socket), through overall board-level power, to the
PEMs and PSUs that supply feeds in the overall shelf. As noted in the figure, there are existing
power monitoring devices and products at all these levels that either: 1) support greater than 8-bit
data without using PMBus or 2) implement PMBus and therefore trigger a need to accurately
represent the measurements collected by PMBus-based sensors.
Extended IPMI Sensors in PICMG 3.7
Based on the needs described above, PICMG 3.7 defines facilities for extended sensors that
support raw data values that are two to eight bytes in size, versus the one-byte limit for raw data
values in normal IPMI sensors. For any given sensor, the size of raw values is fixed within that
range, as appropriate for the measurements that particular sensor needs to represent. For instance,
an extended IPMI sensor representing a physical PMBus current sensor that returns raw data in
the 16-bit direct format could be configured for two-byte raw data. The coefficients for an
extended IPMI sensor are represented in the same fixed size as the raw data.
In addition, an extended IPMI sensor can be configured to use either the normal IPMI conversion
formula (with extended size coefficients) or the PMBus conversion formula. The combination of
larger raw data values and the option to use the PMBus conversion formula allows PMBus
sensor values to be fully and accurately represented by PICMG 3.7’s extended IPMI sensors.
With the PMBus linear format, specific raw values in a stream of sensor measurements could
potentially have any values for the 11-bit mantissa and 5-bit scale factor fields. When such raw
values need to be converted as extended IPMI raw values, the scaling factor needs to be applied
at that time. Given a binary scaling factor ranging from 16 bits of negative scaling to 15 bits of
positive scaling, a scaled 11-bit mantissa can require up to 42 bits. The 11-bit mantissa could be
shifted left by 15 bits (with a positive scaling factor of 01111b) or right by 16 bits (with a
negative scaling factor of 10000b). Therefore the raw value storage must be at least 42 bits in
size. As a result, an extended IPMI sensor based on a PMBus linear format data needs to be
configured for at least six-byte raw data, rounding up from 42 bits to the next 8-bit boundary of
48 bits.
The measurement of energy consumption is one area where the extended IPMI sensor facility is
especially important. An energy consumption measurement can be considered the sum of a series
of instantaneous power measurements over a known period of time. The longer the measurement
time, the larger this sum can be, with a corresponding increase in the number of bytes needed to
represent it. With the 64-bit raw sensor values supported in extended IPMI sensors, it should be
feasible to implement realistic energy consumption measurements with reasonable assumptions
on how frequently a client application needs to retrieve an accumulated energy consumption total
to ensure that a rollover of the 64-bit integer sum is not missed, thus avoiding an incorrect energy
reading. Client applications can use floating point computation to maintain arbitrarily large sums
with acceptable accuracy. Most microcontrollers used as local IPMI management controllers do
not have floating point support in hardware and IPMI assumes that only integer arithmetic is
supported.
The Service Availability Forum’s Hardware Platform Interface (HPI) is widely, but not
universally, used as the foundation of system management applications above an ATCA shelf
manager. At the HPI level, sensor values are represented natively as either already processed 64bit integers or floating point numbers. Any processing implied by IPMI or PMBus conversion
formulas and coefficients is applied in the HPI implementation layer below the application code.
When that implementation layer is augmented to support the extended IPMI sensors here,
existing HPI applications can use the resulting sensor values without needing any changes.
Pigeon Point Systems, Oceanside, CA. (760) 757-2304. [www.pigeonpoint.com].