How to Read a Datasheet


Datasheets are instruction manuals for electronic components. They (hopefully) explain exactly what a component does and how to use it. Unfortunately these documents are usually written by engineers for other engineers, and as such they can often be difficult to read, especially for newcomers. Nevertheless, datasheets are still the best place to find the details you need to design a circuit or get one working.

A datasheet’s contents will vary widely depending on the type of part, but they will usually have most of the following sections:

The first page is usually a summary of the part’s function and features. This is where you can quickly find a description of the part's functionality, the basic specifications (numbers that describe what a part needs and can do), and sometimes a functional block diagram that shows the internal functions of the part. This page will often give you a good first impression as to whether potential part will work for your project or not:

A pinout lists the part’s pins, their functions, and where they’re physically located on the part for various packages the part might be available in. Note the special marks on the part for determining where pin 1 is (this is important when you plug the part into your circuit!), and how the pins are numbered (the below parts are numbered counterclockwise). You'll find some acronyms here: VCC is the supply voltage (commonly 5V or 3.3V), CLK is clock, CLR is clear, OE is output enable, etc. These acronyms should be spelled out later in the datasheet, but if not, try Google or Wikipedia. If a pin has a star next to it or a line over the name, that's an indication that the pin is active low which means that you'll pull the pin low (0V) to activate it, rather than H (VCC):

Detailed tables of electrical specifications follow. These will often list the absolute maximum ratings a part can withstand before being damaged. Never exceed these or you'll be replacing a possibly expensive part!

You'll also see the more normal recommended operating conditions. These may include voltage and current ranges for various functions, timing information, temperature ranges, bus addresses, and other useful performance information. The below excerpt contains a good example where the fine print can help you out: "Note 3" in this set of specifications states that "All unused inputs of the device must be held at VCC or GND to ensure proper device operation." This is a reminder to tie all unused inputs H or L to prevent them from "floating" between H and L which can make your circuit malfunction and be difficult to debug:

Some parts will have one or more graphs showing the part’s performance vs. various criteria (supply voltage, temperature, etc.) Keep an eye out for "safe zones" where reliable operation is guaranteed:

Truth tables show how changing the inputs to a part will affect its output. Each line has all the part's inputs set to specific states, and the resulting output of the part. "H" means that input is a logical high (usually VCC), "L" means a logical low (usually GND), "X" means the chip doesn't care what the input is (could be H or L), and an arrow means that that you should change the state of that pin from L to H or H to L depending on the arrow direction. This is called "clocking" an input, and many chips rely on this for proper operation:

Timing diagrams show how data should be sent to and received from the part, and what speed it should be sent / received. These are typically laid out with various inputs and outputs as horizontal lines, showing the logic transitions that happen to those lines over time. If the trace dips down, that's a L input or output. If the line rises higher, that's a H input our output. Timing specifications are laid out as arrows between transitions (names are referenced back to timing numbers in the electrical specs), and vertical bars or arrows will link related transitions:

Complex parts will have extensive application information. This varies depending on the part, but may include detailed descriptions of pin functions, how to communicate with the part, lists of commands, memory tables, etc. This is often very useful information, so read through it carefully:

Some datasheets will include example schematics for various circuits that can be built around the part. These are often very useful building blocks for interesting projects, so be sure to look through them:

Some parts are sensitive to the way they’re built into a circuit, and the datasheet will provide layout considerations. These can range from noise-reduction techniques, to dealing with thermal issues, to mechanical mounting considerations as with the accelerometer below. This all tends to be very good advice, that if followed from the start will lead to the most trouble-free circuits. Likewise, if you don't follow this advice, your circuit may have problems later on that can be hard to diagnose, and harder to fix:

At the end of many datasheets is packaging information, which provides accurate dimensions of the packages a part is available in. This is very useful for PCB layout, see our EAGLE tutorials for information on creating a new part footprint:

Finally, a few of our customers have correctly pointed out that datasheets are subject to having errors just like anything else, and running into one of these errors can be frustrating to say the least. To reduce this possibility, be sure you have the latest version of a datasheet before doing any serious work. These are available at the manufacturer's website; we at SparkFun do our best to keep our datasheet links up to date, but things can and do slip through the cracks (please let us know if they do!) Also check for errata documents, which are updates and corrections to a part's specifications often found after the part went to production. And if nothing else helps, many manufacturers have applications engineers you can contact to get help on hard-to-solve problems.

When working with a new part for the first time, or when deciding which part to use for your project, it’s a very good idea to read that part’s datasheet from beginning to end, paying close attention to the fine print. You’ll often come up with a bit of knowledge or a shortcut that will save you hours of grief later on.

Example Question

A datasheet is the best place to find:

A.  What voltage a part needs to run

B.  How fast a part will run

C.  How to communicate with a part

D.  All of the above

The correct answer is D: all of the above. Pretty much any technical information you need to know about a part should be somewhere in the datasheet. The trick is finding it in the fine print.

Comments 3 comments

  • It may be worth showing an example of a pcb footprint diagram, which is not always provided but is different from (and more useful than) the mechanical package diagram. Also, there are many “standard” packages, and often there will a separate datasheet for the package itself somewhere on the manufacturer’s site. It’s worth a look.<br />
    <br />
    On the subject of packages: beware the TO92! Often what is shown is the pinout “looking up” from below the chip, which is the exact opposite of what you probably expect.<br />
    <br />
    Also: beware of undocumented features. If the graphs section doesn’t cover your proposed parameters, or there’s a suspicious gap explaining exactly how some feature might work–chances are the manufacturer hasn’t tested this, might not know the answer, and even if you figured out some behavior for one particular chip, it won’t be guaranteed to be the same in all manufactured chips. Specs not in the datasheet may change at any time. The datasheet is usually good about describing what the expected variation in a parameter is, but I find that a common “rookie mistake” is to measure the behavior of one particular unit, and assume the results hold for all such chips manufactured by the company. There are device-to-device variations, lot-to-lot, month-to-month — and often even revision to revision: the manufacturer might redesign your product from the ground up (perhaps they’ve got a new process running in the factory) and as long as the new devices matches the specs guaranteed in the datasheet they won’t bother to tell you. It’s your risk if you dependent on anything not written down.

  • This is great for us noobs. Thanks!<br />
    <br />
    The picture in the timing diagrams section seems to be missing at the moment.

  • Reading datasheets is not as easy as it looks. A lot of times the information you want is buried in the document.
    Before you even choose a part for a project, read the datasheets provided about the IC/device you want to use. Understand the datasheet and know where keep points are. I recommend highlighting keep information, or marking pages for quick reference.
    One thing I see a lot of new hobbyists do is choose a part and then after running into issues and talking with others realize they chose the wrong part. Please, spend a few hours doing proper research before throwing money away. I will usually spend about 1-2 hours PER PART of research before compiling a BOM.