Pull up resistor Let’s look at how pull-up resistors operate and how they might be used.
Consider building a digital circuit in which a push button is required to turn on an LED. You appropriately wire the circuit, attaching one end of the push button to a digital input and the other to ground. When you eventually turn on the power, you notice that the Light blinks on and off without you having to press the button.
If you’ve ever seen something like this, it’s because you failed to include a pull up resistor in your digital circuit. So, what is a pull up resistor? What is it and how do you utilise it?
Pull Up Resistor and How Do You Use One?
What Is a Pull up Resistor?
A pull up resistor is a resistor that is added to a digital circuit to prevent undesired signals from interfering with the logic or programming of the circuit. It is a method of biassing or pulling an input line to positive or VCC when there is no other active device driving the line. By connecting the line to VCC, you effectively set the line’s default state to 1 or true.
Establishing a default state for all input pins is critical to avoid random signals generated when the pin is floating. When an input pin is separated from an active source, such as ground or VCC, it enters a floating state.
Pull up resistors are commonly utilised in microcontroller and single-board computer digital circuits.
How a Pull Up Resistor Works in a Circuit
Pushing a momentary switch on a digital circuit causes the circuit to shut and communicate true or high to the microcontroller. Nevertheless, disabling the switch does not prevent the input pin from transmitting such signals.
This is because disconnecting the connection through a switch means it is no longer linked to anything other than air. This places the line in a floating state, where signals from the environment can force the pin to elevate high at any time.
To prevent these stray signals from registering in your circuit, you must inject enough voltage into the input line to keep it recording high when ground is no longer detected. You cannot, however, directly connect VCC to the input line since the circuit will short as soon as the switch/sensor links the line to ground.
You’ll need to use a resistor to avoid shorting out the pull-up voltage. The correct resistor value ensures that the floating line has enough voltage to increase high while remaining low enough not to short the circuit prematurely. The quantity of resistance depends on the logic type used in your circuit.
Explaining Logic Families
To accurately calculate the resistance value of your pull-up resistor, you must first understand what logic type your circuit employs. The resistance value of your pull-up resistor will be determined by the logic family used in your circuit.
There are several sorts of logic. Here are a few examples:
If you’re not sure which logic family your circuit employs, it’s quite probable that it employs CMOS or TTL logic families, as ECL and DTL have long been obsolete. Chips with the prefixes “74” or “54” are normally TLL chips, whereas processors with the prefixes “CD” or “MC” are CMOS chips. If you’re still unclear, you can quickly determine which logic family your controller employs by conducting a simple web search for its data sheet.
How to Calculate the Pull Up Resistor Value
- We can now proceed to determine values for our pull-up resistor now that you understand the various sorts of logic families and their minimum on and maximum off voltages.
- Three values are required to compute the right resistor value. The minimum on voltage of the logic family your circuit uses, the circuit’s supply voltage, and the input leakage current, which you may check on the data sheet or by using a multimeter.
- Once you’ve gathered all of the variables, just enter them into the following formula:
- (supply voltage – logic high voltage) / input leakage current = resistance value
If your circuit employs TTL and the input line draws 100uA at 5V. TTL requires a minimum of 2V to rise high and a maximum of 0.8V to raise low. This means that the correct value exiting our pull-up resistor should be between 3V and 4V because the voltage must be greater than 2V but less than our supply voltage of 5V.
- Our provided values are:
- 5V is the supply voltage.
- 4V logic high voltage
- Leakage current at the input = 100A or 0.0001A
- Let’s plug the variables into the formula now that we have them:
- (5 volts – 4 volts) / 100 amps = 10,000 ohms
- Our pull-up resistor must have a resistance of 10,000 ohms (10 kilohms or 10k).
How to Use a Pull-Up Resistor in a Circuit
Pull-up resistors are commonly employed in digital circuits to prevent undesirable interference with the digital programming of the circuit. If the digital circuit includes switches and sensors as input devices, pull-up resistors can be used. Moreover, pull-up resistors are only useful if the input pins are linked to ground. If the input pins are linked to VCC, pull-down resistors may be used instead.
Locate the input line that connects to an input device before using a pull-up resistor. Once identified, use the previously described procedure to compute the value of your resistor. If your circuit does not require high accuracy, resistor values ranging from 1k to 10k can suffice.
Now that you have the correct resistor value, connect one end of the pull-up resistor to VCC and the other to the input device and the MCU. Congratulations! You should now understand what a pull-up resistor is and how to utilise one.
Solidify Your Knowledge Through Experience
Locate the input line that connects to an input device before using a pull-up resistor. Once identified, use the previously described procedure to compute the value of your resistor. If your circuit does not require high accuracy, resistor values ranging from 1k to 10k can suffice.
Now that you have the correct resistor value, connect one end of the pull-up resistor to VCC and the other to the input device and the MCU. Congratulations! You should now understand what a pull-up resistor is and how to utilise one.
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