NAND gates have two inputs. To better understand what they do, first look at the underlying AND gate: when both inputs are high, then the result is high, else low. Here is the truth table for an AND gate:
| A | B | A AND B |
|---|---|---|
| 0 | 0 | 0 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 1 |
A NAND gate is an AND gate with an added NOT, inverting the result. So whenever an AND gate would be true, the NAND gate is false - and vice versa.
Here is the truth table for a NAND gate:
| A | B | A NAND B |
|---|---|---|
| 0 | 0 | 1 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 0 |
When both inputs are high, then the result is low, else high - the exact opposite of an AND gate.
Use Cases
A NAND gate is flexible and can be used for many different operations, depending on how you wire it.
For example, a NAND gate can invert a signal (flipping the state of a GPIO around) simply by connecting the signal to both inputs of the gate.
Key Aspects When Selecting NAND Gates
There are many different NAND chips available that differ in just a few aspects:
| Property | Note |
|---|---|
| Voltage | make sure the component supports the voltage you need. Some components require a minimum voltage of 5V and cannot be used with 3.3V circuitry |
| Current | If you want to directly drive electronic components off the component, i.e. a LED, make sure it can sink and/or source the required current. Typically, components can sink and source small currents of up to 10mA. Anything beyond requires additional transistors |
| Number of Gates | Most chips come with a number of NAND gates. The popular CD4011 for example comes with four NAND gates |
| Trigger | Simple NAND gates require clean logic levels to work properly which is not an issue in digital circuits. When input signals are not clean, may contain noise or may not always have clear and distinct high and low voltages, then built-in Schmitt Triggers (like in the CD4093) help: they use hysteresis and make sure than indistinct intermediate voltages may cause oscillations |
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(content created Aug 28, 2024)