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What is the basic difference between sending a LOW signal vs GND to a circuit? What If I ground instead of sending LOW? Or similarly what is the difference between Vcc and HIGH?

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  • \$\begingroup\$ +0.1V with respect to GND is low for most digital families, although that's not a ground level strictly speaking. Every family of digital ICs has thresholds for high and low. The thresholds are in the datasheets. \$\endgroup\$ – Nick Alexeev Jan 6 '16 at 7:39
  • \$\begingroup\$ Logic inputs are often tied directly to Vcc or Ground when a constant High or Low is required. \$\endgroup\$ – DoxyLover Jan 6 '16 at 7:46
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Take a look at this: -

enter image description here

It's for a 5V logic system and it tells you what the voltage ranges that are acceptable for standard CMOS inputs.

So an input signal that is below 0.8 volts will be regarded as a low and that is a definite. However, if the signal received is (say) 1V it might still be regarded as a low but there is no guarantee that it won't be regarded as a high.

Then you could take this further and look at what the specifications of a driver and receiver are: -

enter image description here

You should be able to see that a logic driver output has a tighter spec than a logic input and this should make sense.

What If I ground instead of sending LOW?

You should be able to see that ground (aka 0V) is in the range of an acceptable "low".

Or similarly what is the difference between Vcc and HIGH?

You should be able to see that Vcc is in the range of an acceptable "high".

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Logic Levels

A digital output will have a datasheet specification for its VOHmin and VOLmax. When driving a logic high output, the voltage could be as low as VOHmin. When driving a logic low output, the voltage could be as high as VOLmax.

A digital input will have a datasheet specification for its VIHmin and VILmax.

  • Input voltage between VCC and VIHmin is received as logic high.
  • Input voltage between VILmax and GND is received as logic low.
  • Input voltage between VIHmin and VILmax is not a valid signal, should never happen except during transitions.
  • Input voltage greater than VCC or less than GND is not valid, and can cause incorrect operation or even permanent damage.
  • Input voltage of GND is interpreted as logic low.
  • Input voltage of VCC is interpreted as logic high.

Ground is the reference point against which voltages are measured: by definition, whatever voltage is on ground is defined as zero volts. Analog circuits care about the signal level no matter how close it may be to ground. Digital circuits only distinguish between "logic high" and "logic low", it doesn't matter what the exact voltage is as long as it is within one of these valid signal ranges.

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low is less than something (check the documentation for what is considered as to be low)

ground is an absolute voltage, ground is almost always low,

VCC is the positive supply voltage, VCC is always considered high, high is above some other limit.

in digital circuits

ground is the lowest voltage, anything close enough to ground is considered low vcc is the highest voltage anything close enough to VCC is considered high

high and low do not meet. there is a range between them where it is not defined if the signal is high or low.

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