By doing this the input pins voltage will be pulled up and the voltage will be nearly equivalent to Vcc. To get rid of this above scenario add a resistor to both the input pins and connect them to Vcc. In worst cases the noises and EM waves will produce fluctuating voltage making the entire system unstable. In this instant Electrical noise or EM waves from the surrounding will induce some voltage in these pins and as a result there are high chances that the input voltage falls into that indeterminate region of 0.8 to 2v and thereby pushing our entire system to fail.
When the switch is not connected the pins are said to be in a floating state which means no defined voltage is exhibited in it. Now take a look at the above circuit where a switch is connected to the input pins of OR gate. The last case is too bad for designing a digital circuit, since it may make the entire circuit to fail and your design will do no good. Output could be either 0 or 1 and we can’t predict them. The indeterminate region is the pitfall, this means when input voltage falls between from 0.8 to 2v the Gate will not understand it and it will act in an undesirable manner.For the Gate to read Logic 0 – Input voltage range must be between 0 to 0.8v.For the Gate to read Logic 1 – Input voltage range must be between 2v to 5v.Referring to input voltage levels, you can observe As you can observe in both output and input diagrams, there is a voltage range for each logic states. The above image shows the logic level diagram of TTL logic family of +5Vcc. TTl, CMOS, RTL, DTL are few of the families and out of which TTL and CMOS are quite famous and commonly used.Ĭircuits Library - 220+ practical circuits There are many logic family exists in digital systems. Logic level is nothing but the voltage range which decides how an input or output in a digital circuit is interpreted either as a “1” – high state or “0” – low state. To explain this further, we need to understand about Logic families and how each family differs from each other. In short the purpose of Pull up or Pull down resistor to give keep the input of digital pins at a stable state – 1 in case of Pull up resistor and 0 in case of Pull down resistor.
These resistors are pulling down the voltage of inputs pins close to the level of GND. Now take a look at the above circuit diagram, here the resistors R1 and R2 act as pull down resistors. These resistors are pulling up the voltage of input pins to the level of VCC. Refer the above circuit diagram the resistors R1 and R2 is the Pull up resistors. The purpose of these resistors is to bring up the input pins equivalent to the voltage of Ground or VCC. These are common resistors that connects the digital input pins to VCC or Ground. WHAT IS PULL UP OR PULL DOWN RESISTORS:
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