# Pull-up Resistance / Pull-Up Resistors

Logistics doors are used extensively in digital electronics today. Care should be taken to ensure that the Input and output (OUTPUT) of these logic doors provide the correct signal status and the expected switching condition. the digital circuit must be set correctly to high or low for it to function properly.

Logic gates, we know that any digital logic circuit is the most basic building block. Using combinations of and gate, OR door and NOTE door, which are three basic doors, we can create quite complex combination circuits. However, these digital circuits can have only one of two logial situations called the logial "0" or the logial "1" state.

These logic states are represented by two different voltage levels, in which any voltage below a certain level is considered the logic of "0" and any voltage above another level is considered the logic of "1". Sometimes entries in a digital logic gate or circuit may not be within the range at which logic can be perceived as input "0" or logic "1". In this case, the digital circuit may be triggered incorrectly because the door or circuit does not recognize the correct input value. If the output should be "1", it may not be "0" if it should be "1" or "0".

For example, consider the digital circuit above. Two keys," a "and" b", represent inputs to a general logic gate. With the "A" switch turned off, the entry "A" is connected to the soil, (0v) or the logic level "0" (low). Likewise, when the "b" switch is turned off, the input "B" is connected to the soil, the logic level "0" is (low) and this is the right situation that we need.

However, when the "A" switch is turned on, what will be the value of the voltage applied to the input, "A"? We assume that the "A" switch is open circuit and therefore the input "A" will be +5V (high) as it does not short-circuit the soil, but this may not be the case. Since the input is no longer effectively connected from a defined high or low condition, it may not be possible for the input to remain at any voltage level. In this case, the circuit may cause problems while working. At this point, PULL-UP and PULL-DOWN resistance saves our lives.After connecting these resistors to the circuit, it becomes possible to obtain a precise output signal value.

## Pull-up Resistance

The most common method of making sure that the inputs of digital logic doors and circuits cannot self-displace and swim is to connect unused pins directly to the soil (0V) for a fixed low "0" input.or nor doors) or directly to the Vcc (+5V) for a fixed high "1" input (and NAND doors).

Using these two pull-up resistors, the input depends on the +5 V feed through an effective pull-up resistance. Inputs are also connected to 5V through resistance. As soon as the key is closed, the entrance will be connected to the soil. On the other hand, this situation will be prevented in the best way because the current passage will be reduced to almost never before (as determined by the Ohm Act). When the "A" or "B" switches are turned off, the (open) input shorts out the soil (low) by creating a logic "0" condition, as before in the input.
Although the connection between VCC and input (or output) is the preferred method for using a pull-up resistance, another question that comes to mind is what is the value of the necessary resistance.

### Calculation of Pull-up Resistance Value

Digital logic circuits work using two binary states, normally represented by two different voltages: high voltage for logic "1" and Low Voltage for logic "0". However, these two voltage values vary according to the circuit used. Even this situation alone can be a problem in itself. For example, for the TTL 74LSxxx series digital logic gates, the voltage ranges representing the logic level "1" and the logic level "0" are as follows:

The minimum voltage required to make the input value "1" is 2V. The maximum voltage required to make the input value "0" precisely is 0.8V.

In other words, TTL 74LSxxx input signals between 0V and 0.8V are considered "low". Input signals between 2.0V and 5.0V are considered "high". Any voltage between 0.8V and 2.0V is not recognized as a log "1" or a log "0".

### Single Door Pull-up Resistance Value

Then, using the Ohm law, the maximum tensile resistance required to reduce 3 volts for a single TTL 74LS series logic gate will be 150kΩ. Although this calculated value will work, it leaves no room for error as voltage drop is at maximum and input current is minimal throughout the resistance. Ideally, we want a logic "1" to be as close as possible to the Vcc and guarantee 100% that the door is a high (logic-1) input over pull-up resistance.

## Pull-down Resistance

A Pull-down resistance works in the same way as the previous pull-up resistance, but this time the entrance to the logic gates is connected to the ground. With the operation of a mechanical switch, the signal can be drawn to the "HIGH" state. This pull-down resistance configuration is especially useful for meters and digital circuits such as flip-flops, which often require a positive single shot trigger to cause a situation change.

### Single Door Pull-down Resistance Value

The maximum pull-down resistance value is then calculated as 2kΩ. Again, as with pull-up resistance calculations, this 2kω resistance value leaves no room for error as voltage drop is at maximum. Therefore, if the resistance is too large, it can cause Voltage Drop along pull-down resistance.