Pull up and Pull down resistors - eclubiitk/EClub-Handbook GitHub Wiki
When working with digital circuits, one needs to make sure that every input is clearly defined in the form of a particular value: either HIGH or LOW.If the inputs of the digital circuit are not with in the range of values by which they can be clearly separated as HIGH or LOW triggers,then it may lead to the sensing of a false trigger or self bias that will lead to the circuit not working properly.
For example, consider the digital circuit above(Fig.1). The two switches, “a” and “b”, represent the inputs to a generic logic gate. When switch “a” is closed (ON), input “A” is connected to ground, (0v) or logic level “0” (LOW) and likewise, when switch “b” is closed (ON), input “B” is also connected to ground, logic level “0” (LOW) and this is the correct condition we require.However, when switch “a” is opened (OFF), what will be the value of the voltage applied to input “A”, HIGH or LOW?
We assume it will be +5V (HIGH) as switch “a” is open-circuited and therefore input “A” is not shorted to ground, but this may not be the case. As the input is now effectively unconnected from either a defined HIGH or LOW condition, it has the potential to “float” about between 0V and +5V (Vcc) allowing the input to self–bias at any voltage level whether that represents a HIGH or a LOW condition. This may lead to an open(HIGH) switch to be assessed as a closed(LOW) switch that may lead to a change in the output that is obtained at the logic output(Q).
Then to prevent accidental switching of digital circuits, any unconnected inputs called floating inputs should be tied to a logic 1 or logic 0 as appropriate for the circuit. We can easily do this by using what are commonly called Pull-up Resistors and Pull-down Resistors to give the input pin a defined default state, even if the switch is open, closed or there is nothing is connected to it.
By using two pull-up resistors, one for each input, when switch A or B is open (OFF), the input is effectively connected to the +5V supply rail via the pull-up resistor. The result is that as there is very little input current into the input of the logic gate, very little voltage is dropped across the pull-up resistor so nearly all the +5V supply voltage is applied to the input pin creating a HIGH, logic 1 condition.
When switches A, or B are closed, (OFF) the input is shorted to ground (LOW) creating a logic 0 condition as before at the input.We are not shorting out the supply rail as the pull-up resistor only passes a small current (as determined by Ohms law) through the closed switch to ground. If we do not use a resistor then a high value of current will pass through the circuit and that could damage the electronic devices involved in the circuit.
By using a pull-up resistor in this way, the input always has a default logic state, either 1 or 0, high or low, depending on the position of the switch, thus achieving the proper output function of the gate at Q and therefore preventing the input from floating about or self-biasing giving us exactly the switching condition we require.
A Pull-down resistor works in the same way as the previous pull-up resistor, except this time the logic gates input is tied to ground, logic level 0(LOW) or it may go HIGH by the operation of a mechanical switch. This pull-down resistor configuration is particularly useful for digital circuits like latches, counters and flip-flops (TODO: ADD Link) that require a positive one shot trigger when a switch is momentarily closed to cause a state change.
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