Appendices
B
Interpreting Datasheets
All commercial integrated circuits (and many research integrated circuits)
have documents describing their functionality, data and electrical interface
specifications, and so on. These documents or datasheets are essential to se-
lecting components to design an embedded system, are essential to imple-
menting the hardware and firmware for an embedded system, and are critical
to debugging and evaluating the functionality and electrical behavior of sys-
tems you design. Because they are so essential to embedded system design,
implementation, and evaluation, you will invariably need to read through
parts of tens of different datasheets, on hundreds of occasions, in completing
a single system design. Knowing how to most efficiently read a datasheet is
therefore an invaluable skill, and will save you time and headache. Knowing
what to exp ect from a good datasheet will also make it possible for you to
create good datasheets when you design your own component, whether that
component is a module that integrates multiple of-the-shelf ICs on a circuit
board for use as a component in other designs, or a research integrated circuit
that you plan to provide to other research groups.
B.1 Datasheets, User Guides/Manuals, Errata and their Quality
There are three main kinds of documents that you will need to reference for a
typical integrated circuit such as a microcontroller, sensor, display, or FPGA,
and the datasheet is just one of them.
The first document you invariably need to turn to in considering a new
component in a design, or in understanding the parts of a design imple-
mented by someone else after reading its schematic, is the device datasheet.
Typically, the datasheet provides a summary of the functionality of a device
(e.g., a Digital Output MEMS Accelerometer in the case of the Analog devices
ADXL362
1
accelerometer), its capabilities, (e.g., High resolution: 1 mg/LSB in
1
Analog Devices
2014.
the case of the ADXL362), its supply voltage operating range (e.g., 1.6 V to
3.5 V supply range for the ADXL362), its device package (3 mm × 3.25 mm ×
4 phillip stanley-marbell
1.06 mm package for the ADXL362), and much more. Typically, all of this in-
formation is contained on the first page„ followed by a block diagram, and
further details as detailed in Section
B.2 below.
A reference manual, user guide, or user manual on the other hand typically
contains a detailed description of the (digital) behavior of an integrated cir-
cuit and is most commonly a separate document in the case of microcon-
trollers and microprocessors. For example, for the NXP/Freescale KL03
microcontroller family, there are separate datasheets (e.g., Kinetis KL03 32
KB Flash 48 MHz Cortex-M0+ Based Microcontroller
2
) and a separate reference
2
Freescale Semiconductor
2014a.
manual (KL03 Sub-Family Reference Manual
3
).
3
Freescale Semiconductor
2014b.
Both datasheets and reference manuals will typically contain a revision
number or date, and it essential to have copies of the newest version (which
might include fixes of errors) as well as copies of older versions (to under-
stand why you may have made a given design decision based on erroneous
information in a datasheet). Both will also typically contain a section (typ-
ically at the end of the document) with a history of changes made to the
current and previous versions of the document. Typically (but not always),
the different versions of a manual are given different filenames by the man-
ufacturer, so it is easy to keep the different versions without overwriting old
versions upon downloading a new one. For this reason, it is advisable to
only download datasheets directly from the manufacturer. There are vari-
ous web pages online that bill themselves as one-stop shops or repositories
for datasheets. These sites have no incentive to abide by these guidelines,
and are usually setup as a way to attract advertising. Given the significant
amounts of time that you could waste due to incorrect documentation, and
given the possibility of physical hard that can result from embedded systems,
it is wise to get datasheets directly from a device’s manufacturer.
In addition to errors in documentation, there might be design errors or
implementation errors in actual integrated circuits themselves. Most manu-
facturers issue a separate document, the errata, to detail these problems and
to give work-arounds if there are any.
The quality of datasheets varies widely across manufacturers. For the
last decade or so, datasheets for displays have by and large been the most
difficult to decipher (to put it politely). Examples of companies with high
quality datasheets include Texas Instruments (TI) and Analog Devices (ADI).
B.2 The Anatomy of a Typical Datasheet
A typical datasheet consists of eight parts:
➊ The document revision number. This is not really a section on its own but
is important enough to call explicit attention to it here.
foundations of embedded systems 5
➋ The summaries and block diagram, typically on the first page.
➌ The absolute maximum ratings.
➍ The electrical specifications (typical power dissipation, currents, etc.)
➎ The pinout / pin names and descriptions.
➏ The core functionality: Memory map for microcontrollers, registers and
I2C addresses for sensors, etc.
➐ The mechanical specifications and package variants, recommended land
patterns, etc.
➑ The document change history.
Knowing that these parts exist for most datasheets and the approximate order
in which they occur, will help you quickly get to the information you need,
saving you time and invariably giving you a better understanding of the
integrated circuit in question.
B.2.1 The summary and block diagram
The summary section of a datasheet is typically the first place you will turn
to when making a choice about whether or not to use a given integrated cir-
cuit. The remainder of a typical datasheet will provide further details of the
electrical properties described in the summary section, while the reference
manual will typically provide the details on the non-electrical properties of a
component such as details of its behavior. These non-electrical properties are
typically summarized in a visual block diagram.
B.2.2 The absolute maximum ratings and other electrical specifications
The next section after the opening summary is typically a section covering the
extremes of the conditions under which the device will operate, such as the
maximum supply voltage (this is typically always specified) or the maximum
accelerations that a device is rated to withstand.
The section on absolute maximum ratings is typically separate from a sec-
tion that details the typical electrical operating conditions and properties,
such as power dissipation of an integrated circuit but some datasheets may
lump this information with the information on worst-case recommended op-
erating conditions.
6 phillip stanley-marbell
B.2.3 The pinout / pin names and descriptions
Following the worst-case and typical electrical operating conditions and prop-
erties typically comes a description of the pinout of a device. For devices
with a small number of pins, the pinout description, which is typically given
as a table, can also be a handy summary of the device’s functionality. The
description of a device’s pins may sometimes include what looks like a dia-
gram of the chip: This is not the diagram you are interested in when creating
a printed circuit board layout; the mechanical properties and dimensions of
the device package (typically in multiple variants) is usually described in pre-
cise detail at the end of the datasheet in a section typically under the heading
of mechanical specifications.
B.2.4 The core functionality
The main section of a typical datasheet describes the devices functionality. In
contrast to the kind of information found in a reference manual, the infor-
mation in a datasheet is typically usually brief. For digital or mixed-signal
components, the datasheet is typically focused more on the analog and elec-
trical properties than on the digital behavior.
B.2.5 The mechanical specifications
The penultimate section in most datasheets is the section describing the
mechanical properties of the device’s package: package dimensions, pin/-
pad/ball dimensions and locations. These are typically provided as a combi-
nation of engineering drawings and tables, specified to sub-millimeter preci-
sion.
When creating a printed circuit board, you might be tempted to create a
device footprint that exactly matches these dimensions. Be careful: there is
typically a separate mechanical drawing that described the pads that should
be created on a printed circuit board in order to enable reliable manufacturing
and reliable operation. These descriptions are typically referred to as the
recommended land patterns, and may be specified in the datasheet or might be
available in a separate document from the device’s manufacturer.
B.2.6 The document change history
Almost all data sheets go through one or more revisions after their initial
release by the manufacturer, and the final section of a typical datasheet will
list the revisions the document has gone through and the changes made in
each revision.
