More 74HC gates

The 74HC10, 11, 20, 21, 27, 30, 7266 and 132: three-, four- and eight-input gates, an XNOR and a Schmitt-trigger NAND.

They sit beside the quad gates.

They work like the rest of the family: real VCC and GND pins that have to be wired, nothing at all until they are, inputs that switch at half the supply, and an inertial propagation delay, so a pulse shorter than the delay never reaches the output. An unknown or floating input only makes an output unknown when it could change the answer: a 0 on any input of a NAND or AND settles it, and a 1 on any input of a NOR does. Aliases: 7410, 7411, 7420, 7421, 7427, 7430, 74132, SN74HC10, SN74HC20, CD74HC30, CD74HC7266, 74LS10, 74LS20, 74LS30, 74HCT10, triple 3-input NAND, dual 4-input NAND, 8-input NAND, XNOR, Schmitt NAND.

74HC10: triple 3-input NAND gate

Three NAND gates of three inputs each. Y is low only when all three inputs are high. Propagation delay: 10 ns, the SN74HC10 typical at 4.5 V and 50 pF.

Pin	Name	What it does
1, 2, 13	1A, 1B, 1C	Gate 1 inputs.
12	1Y	Gate 1 output.
3, 4, 5	2A, 2B, 2C	Gate 2 inputs.
6	2Y	Gate 2 output.
9, 10, 11	3A, 3B, 3C	Gate 3 inputs.
8	3Y	Gate 3 output.
7	GND	Ground.
14	VCC	Supply.

Gate 1 is split around the package: its third input 1C is pin 13 and its output 1Y is pin 12.

74HC11: triple 3-input AND gate

The 7411: three AND gates of three inputs each, on exactly the 74HC10's pinout (the table above). Y is high only when all three inputs are high. Propagation delay: 10 ns, the SN74HC11 typical at 4.5 V and 50 pF.

74HC27: triple 3-input NOR gate

The 7427: three NOR gates of three inputs each, again on the 74HC10's pinout. Y is high only when all three inputs are low, and a 1 on any input settles it low. Propagation delay: 10 ns, the SN74HC27 typical at 4.5 V and 50 pF.

74HC20: dual 4-input NAND gate

Two NAND gates of four inputs each. Y is low only when all four inputs are high. Propagation delay: 14 ns, the SN74HC20 typical at 4.5 V and 50 pF.

Pin	Name	What it does
1, 2, 4, 5	1A, 1B, 1C, 1D	Gate 1 inputs.
6	1Y	Gate 1 output.
9, 10, 12, 13	2A, 2B, 2C, 2D	Gate 2 inputs.
8	2Y	Gate 2 output.
3, 11	NC3, NC11	Not connected inside the package.
7	GND	Ground.
14	VCC	Supply.

74HC21: dual 4-input AND gate

The 7421: two AND gates of four inputs each, on the 74HC20's pinout (pins 3 and 11 not connected). Propagation delay: 14 ns, the SN74HC21 typical at 4.5 V and 50 pF.

74HC30: 8-input NAND gate

The 7430: one NAND gate of eight inputs. Y is low only when A to H are all high, which makes it the usual way to detect one particular 8-bit value (invert the bits that should be 0 first). Propagation delay: 10 ns, the CD74HC30 typical at 5 V and 15 pF.

Pin	Name	What it does
1 to 6	A to F	Inputs.
11, 12	G, H	Inputs.
8	Y	Output.
9, 10, 13	NC9, NC10, NC13	Not connected inside the package.
7	GND	Ground.
14	VCC	Supply.

74HC7266: quad 2-input XNOR gate

Four exclusive-NOR gates with push-pull outputs. Y is high when A and B agree, which makes it a one-bit equality test. (The 74HC266 is the open-drain version; this is not it.) It is not the 74HC86 pinout: the outputs are pins 3, 4, 10 and 11. Propagation delay: 9 ns, the CD74HC7266 typical at 5 V and 15 pF. An XNOR has no controlling value, so any unknown input gives an unknown output.

Pin	Name	What it does
1, 2	1A, 1B	Gate 1 inputs.
3	1Y	Gate 1 output.
4	2Y	Gate 2 output.
5, 6	2A, 2B	Gate 2 inputs.
8, 9	3A, 3B	Gate 3 inputs.
10	3Y	Gate 3 output.
11	4Y	Gate 4 output.
12, 13	4A, 4B	Gate 4 inputs.
7	GND	Ground.
14	VCC	Supply.

74HC132: quad 2-input NAND gate with Schmitt-trigger inputs

The 74HC00's pinout and logic with hysteresis on every input, the same model as the 74HC14. Each input reads high once it has climbed past VT+ and low once it has dropped below VT-, and holds between the two, so a slow or noisy edge gives one clean output edge. The thresholds are the SN74HC132 typicals at 4.5 V (VT+ 2.5 V, VT- 1.6 V), scaled with the supply: 2.78 V and 1.78 V at 5 V. An input that has not left the gap between them since power-up reads unknown. Propagation delay: 18 ns, the SN74HC132 typical at 4.5 V and 50 pF.

With one input tied high a gate is a Schmitt inverter. A resistor from its output back to that input and a capacitor from the input to ground make an oscillator, as on the 74HC14, and the other input then gates it: low stops it with the output high, high lets it run.

Pin	Name	What it does
1, 2	1A, 1B	Gate 1 inputs.
3	1Y	Gate 1 output.
4, 5	2A, 2B	Gate 2 inputs.
6	2Y	Gate 2 output.
9, 10	3A, 3B	Gate 3 inputs.
8	3Y	Gate 3 output.
12, 13	4A, 4B	Gate 4 inputs.
11	4Y	Gate 4 output.
7	GND	Ground.
14	VCC	Supply.

Properties

model, which part number the chip is:

• 74HC10, 74HC11, 74HC20, 74HC21, 74HC27 and 74HC30 offer the 74HC, 74HCT and 74LS versions (for example 74HC10, 74HCT10, 74LS10).
• 74HC7266 offers 74HC7266 and 74HCT7266.
• 74HC132 offers only 74HC132. The 74HCT132 and 74LS132 switch at TTL thresholds that do not scale with the supply, which this model does not have.

The 74HC version runs from 2 V to 6 V and its inputs switch at half the supply. The 74HCT version needs 4.5 V to 5.5 V and reads TTL input levels, switching at 1.4 V, so it takes a 3.3 V signal as a clean high. The 74LS version needs 4.75 V to 5.25 V, switches at 1.4 V too, drives its output high only to about 3.4 V (1.6 V under the supply), and reads a floating input as high, as an open TTL input really does. The pins, the logic and the delay are the same in every version: the delay is the 74HC figure throughout.

What the model gets right

Controlling values: a NAND with one input low is high however many of its other inputs are floating. The delays are each chip's own data sheet typical, and they are inertial. The 74HC132 reads its inputs as voltages, so a capacitor charging slowly crosses each threshold exactly once.

What it does not model

One typical delay for every gate and both directions, with no dependence on supply, load or temperature, and no 74LS or 74HCT delays of their own. No supply current and no output current limit. The 74HC132 sits on the typical thresholds, where a real chip can be anywhere in a much wider range, so a real RC oscillator built on one can run well away from the simulated frequency.

Common mistakes

Forgetting VCC and GND. Leaving unused inputs of a 74HC gate floating: tie them to VCC on a NAND or AND (or to another input of the same gate) and to GND on a NOR. Counting on a floating input reading high: only the 74LS version does that. Wiring a 74HC7266 like a 74HC86, and connecting the NC pins of a 74HC20 or 74HC30 as if they were inputs.

See it in action

Browse the example circuits at /templates.