Digital Electronics
Logic Gates
Logic Chips
Data Sheets
74LS00 Family

Data Sheets

In the last section we used a 7400 chip to construct an S-R flip-flop, even though the job would have been easier with a chip containing two (or more) NOR gates... or, for that matter, a chip which already implements an S-R flip-flop that we can just use as it is. Engineering is like that: sometimes the job is fitting what we want to the components and technology we have at the time. I'm going to introduce a number of other useful members of the 7400 logic family in the next section, which will give you more design options. But there are thousands of logic gate choices available for you to use in a circuit design, and I certainly can't cover all of them! At some point finding the right chip requires you doing some research among the choices available to you. And in order to do that, you need to be able to read a manufacturer's data sheet and understand what it is telling you about a particular part.

While you are working through this section of the course, you should open the data sheet shown to the right. This is an actual data sheet from Texas Instruments, Inc., describing the 7400 I've already introduced, in Adobe Acrobat format. Having this open on your computer, or printed out, will be helpful during this section as I will keep using it as an example.

Circuit description

A data sheet is the document created by the manufacturer of an integrated circuit in order to describe the IC to an engineer. It is intended to let you understand the part well enough to design a circuit using that part. Since the manufacturer wants you to buy their parts (particularly if you are going to manufacture high-volume products that will use lots of their parts), the manufacturer attempts to make the data sheets both thorough and accurate. But the data sheet generally starts out with two things: a fairly simple overview of the circuit's function, and the part's pin-out.

In the sample data sheet, the circuit description includes a two-sentence explanation of the part, near the bottom of the first page, as well as a truth table ("function table") and a logic diagram toward the bottom of the second page. The text description describes the NAND function, first as
    Y = NOT(A AND B)
(the dot represents the AND function, while the overbar above the "A AND B" expression negates it), and then as
    Y = NOT(A) OR NOT(B)
(the plus sign represents the OR function; note that this is not the same as binary addition).


The data sheet for an integrated circuit usually contains the part pin-out fairly early in the document. As I described in the previous section, this is a diagram of the part's pins, their numbers and their names.

The pin-out for the 7400 appears in the upper left corner of the first page. It differs from the one I presented in two ways: it is rotated so that pin 1 is in the upper left corner, and the logic symbols are not drawn inside the box. Neither layout is more "right" than the other; in fact, my pin-out drawing pretty closely matches the one in the Digital Logic Pocket Data Book, also from Texas Instruments! That one was a quick reference to the pins and their function; in a data sheet that goes into more detail, omitting the logic diagrams and orienting the part with pin 1 to the top are common conventions.

But looking at the data sheet, you quickly observe that there are in fact four different pin-outs shown, and they do not agree! The upper left one matches the part we used in the last section, but what tells us that it is the "correct" one, and why are there others?

The answer lies within an important fact: manufacturers often use a single data sheet to describe multiple similar components. This one data sheet actually describes a total of twenty components you could order from Texas Instruments. (The complete list is on the second page.) The four different pinouts describe differences in packages. We are working with the 7400 in a plastic DIP package; the first paragraph of the data sheet lists the package options and identifies our package as type "N." (Note that the "N" designation is specific to Texas Instruments; different vendors use different notations for package types.) Each pin-out diagram has a description above it of which parts and packages match the diagram; the upper left one lists the "N" package type for the "SN7400."

Part families

The upper right corner of the data sheet claims that this data sheet applies to six different parts: SN5400, SN54LS00, SN54S00, SN7400, SN74LS00, and SN74S00. What is all that about?

These different parts are each members of different families. They are all two-input NAND gates with enough similarities to fit in a single data sheet, but actually these six parts are different at the semiconductor level. The part we've been talking about is the SN7400; "SN" is a part number prefix used by Texas Instruments. (Other vendors may make equivalent parts, and their part number prefix may not be the same. For example, ON Semiconductor also uses "SN" as a prefix for the 7400 families, while Fairchild uses "DM" and IDT uses no prefix, as some examples. The 7400 logic families are available from a number of different manufacturers, which is one of the nice things about these parts.)

The SN7400 belongs to the original 7400 family of TTL logic. The 74S00 family and 74LS00 family each offer some improvements over the original 7400 parts, with different electrical characteristics. Texas Instruments also offers these part families in rugged, extended-temperature versions. They gave these variants a "54" family designation instead of "74," and these are again available in all three sets of electrical characteristics.

Just like the different package options required different pin-out drawings, there are separate specs in the data sheets for the six different family versions. The specs for the 7400 (and 5400) start on page four, the specs for the 74LS00 (and 54LS00) are on page five, and the specs for the 74S00 (and 54S00) are on page six. For now just note that for simplicity you generally want to build circuits using all parts from the same family (7400, 74LS00, and so on) but with careful attention to the data sheets you can mix families.

Absolute maximum ratings

A data sheet usually has a section called "absolute maximum ratings," which basically describes limits beyond which you cause damage to the part. Note that these are not operating conditions. The distinction is that when you violate operating conditions, the part may not work the way it is supposed to, but when you violate absolute maximum ratings, you have potentially damaged or destroyed the part, and it may not work the way it is supposed to ever again.

In this data sheet we see that supplying the chip with more than seven volts is deadly, as is connecting greater than 5.5V to any input. We also see that there are temperatures below and above which the part may be damaged. These would seem to be no big deal (do you expect to work in a room that violates these conditions?) except that a soldering iron could heat the chip hotter than that. The acceptable thermal characteristics for soldering a component are themselves a complex topic, and manufacturers often offer separate documents describing the sorts of thermal stresses the part can accept in soldering. I only mention this to point out that taking a great deal of time soldering directly to a component, should you try this, is another way to ruin a part.

Recommended operating conditions

The recommended operating conditions describe the environment for which the part was designed to work: that is, the ground rules for using the part and expecting the right results. These tables use a number of conventions for naming their specifications, and many data sheets expect that you understand what these conventions mean.

This data sheet lists the operating specification for VCC. There are three columns for the specs, marked "min" (minimum), "nom" (nominal or ideal), and "max" (maximum). Not every item uses all three columns, but the VCC spec does: it shows us that the minimum voltage is 4.75V, the nominal voltage is 5V and the maximum voltage is 5.25V.

Next we see two specifications for what, as an input, constitutes valid logic levels. The VIH spec tells us that a signal must be at least 2V for us to recognize a logic "1," while the VIL spec tells us that a signal must be no higher than 0.8V for us to recognize a logic "0." These tell us that the 7400 accepts a fairly wide range of logic "1" input voltages, but also tells us that between 0.8V and 2V there is a "gray area" where we don't know which logic level will be seen. (Ordinarily this only applies to the time it takes an input to change voltage levels.)

The next pair of specifications describe the ability of the output of the 7400 to pass electrical current through the output transistors. IOH tells us that a logic "1" output will allow up to 0.4mA of current to flow to the voltage supply, and IOL tells us that a logic "0" output will allow up to 16mA of current to flow to ground. We also say that the output "sources" 0.4mA and "sinks" 16mA of current. An application that tried to pass larger amounts of current could cause the chip to not function properly; a 7400 cannot drive an electric motor, for example, which might draw hundreds of milliamps.

The last spec in this section, TA, tells us that the part is only expected to operate between zero and seventy degrees Celcius. This means that this part is no good for an outdoor application; you would need an extended-temperature part (like the 5400; look at the temperature specs for that component).

Electrical characteristics

The recommended operating conditions are what conditions youare supposed to provide to the chip. The electrical characteristics are the other side of the contract: if the part is being treated properly, this is how it will behave.

This table includes specs for VOH and VOL, which are the voltage levels for logic "1" and logic "0," respectively. The middle column for these specs is marked "typ" (typical); we expect a logic "1" to be around 3.4V (no lower than 2.4V) and we expect a logic "0" to be around 0.2V (no higher than 0.4V).

The table also includes specs for IIH and IIL, which are the amount of current drawn in the detection of logic level "1" and logic level "0," respectively. We expect the 7400 to consume a maximum of 0.04mA of current when presented with a logic "1" input, or consume a maximum of 1.6mA of current when presented with a logic "0." (The IIH spec listed µA as the units; one µA is 1,000 mA.)

Data sheets typically offer an ICC spec, which is the amount of power the part consumes from the power supply. The 7400's power consumption is quite dependent upon its inputs, so the data sheet breaks out an ICCH spec for when the inputs are all "0" and an ICCL spec for when the inputs are all "1." The maximum current consumed by the 7400 is the greater of the two specs, or 22mA.

I skipped two of the specs on the 7400 data sheet, and different data sheets can include plenty of other specs beyond what we discussed. However, for the purposes of our discussion the ones I've covered are the essentials.

Switching characteristics

The last set of specs for the 7400 (top of page five) is the switching characteristics. This quantifies the response time of the part. The 7400 is a simple part, so the only timing specifications are for propogation delays: when an input changes, how long does the output take to also change?

The tPLH spec says that when the input conditions change such that the output must switch from "0" to "1," there is a delay of around 11ns (22ns maximum) for the change to occur from the time when the inputs stabilized. (1ns is one billionth of a second.) The tPHL spec says that when the input conditions change such that the output must switch from "1" to "0," the delay is around 7ns (15ns maximum).

More complex parts can have more complex switching characteristics, but we will wait until the section on "timing diagrams" before getting into that topic any further.

Matching up the specs

Probably by now your eyes are glazing at all this generally unexciting information. My point here is not to equip you for a semiconductor trivia contest, but rather to enable you to match up data sheets and see if parts can work together.

If you connect the output of a 7400 to the input of another 7400 (or the same one, for that matter), you are comparing the requirements and behaviors of the two ends of that wire. If the output is a logic "1," the 7400 is providing a voltage no less than 2.4V and somewhere less than 5V (we don't know the maximum, except that it can't possibly exceed the power supply voltage because where would the voltage come from?). The connected input needs to see a voltage above 2V and which must not exceed 5.5V... so this is a good fit. That is, the allowed range is greater than the possible range. If the output is a logic "0," the 7400 is providing a voltage no greater than 0.4V. The connected input needs to see a voltage below 0.8V, so again the fit is good.

When the output is "1," the output will source as much as 0.4mA, while the connected input only wants a maximum of 0.04mA. When the output is "0," the output will sink as much as 16mA, while the connected input only wants a maximum of 1.6mA. These comparisons are the basis for the claim that a TTL output can feed ten "TTL loads;" there is enough current at both states to meet the requirements of ten inputs from the same family.

Now, mixing TTL parts together, you would expect that the inputs and outputs were designed to be compatible, and the ten TTL load rule gives you an easy rule of thumb, so you don't need to match up the specs. However, what if you need to mix parts from different families? Suppose, for example, that you needed to connect the output from a 74LS00 to the input of a 7400. Is that going to work? Now you need to match up VOH and VOL on the 74LS00 with VIH and VIL on the 7400. (Doing this you find that the specs are a little different, but still within range.) And you need to match up IOH and IOL on the 74LS00 with IIH and IIL on the 7400. (You find that it will work... but the ratio of IOH to IIL is only 5:1, meaning that you can only drive a maximum of five 7400 inputs, not ten.

This sort of matchup is what I meant earlier when I said that you can mix part families if you pay attention to the data sheets to see what will (and won't) work.

Other information

A data sheet can have other sorts of information as well. Before leaving this section I'll touch on a few other things you might encounter.

Ordering information tells you how to find the correct part number for the component you want. In the 7400 data sheet, we know that we want the 7400, and we want the DIP package. The ordering information table on page two tells us the correct Texas Instruments part number is SN7400N, and that the parts are shipped in quantity in a plastic tube. (In case you were wondering, only large volume purchases would go straight to Texas Instruments. If you want to buy a single SN7400N, go to a parts distributor like Digi-Key and look up that specific number. Or, if you wanted just a 7400 in DIP form and don't care what manufacturer made it, go to a electronics hobbyist supplier like Jameco and just order a 7400.)

A schematic sometimes appears in a data sheet, to describe the workings of the part to an electrical engineer. The 7400 data sheet shows the schematic for the TTL, LS and S versions on page three. For our purposes this is not important, but if you know enough electronics to make sense of them you may find them interesting.

Test conditions are often detailed, to qualify the methodology used in testing the specs in the data sheet. The 7400 data sheet devotes page seven to describing how the tests are to be performed.

Package outlines may appear in a data sheet, to show the physical dimensions of a component. The 7400 comes in a variety of packages, so pages eight through fifteen give the package drawings. (Many manufacturers do not include this in their data sheets, instead referring you to separate documents that give these drawings, since a package like a 14-pin DIP is used on a lot of different parts.)

Programming information appears on data sheets for more complex parts used in computer systems, where you need to not only know the electrical interface to the component but also the things the part can be made to do through the interface.

And lastly, data sheets frequently include timing diagrams, which are the topic of a later section of this course.

Next: 74LS00 Family

Previous: Logic Chips

Copyright ©2003-2006, Mark Bereit. All rights reserved.