Digital circuit inputs turn on thesholds, turn on voltage?

[danny davis]

When troubleshooting digital electronics gates , etc.

The inputs to turn on the gates are different, To turn the gates on some of them are
1.) +5 volts
2.) +2 volts
3.) +1 volt

CMOS gates turn on voltage is what?

What are the turn on voltages have u seen and how would u know what is the turn on voltage for different turn on theshold voltages for gates?

 

[FvM]

The logic threshold of symmetrical CMOS logic gates (CD and HC series, not HCT series) is typically at mid-supply, but can be between 30% and 70% of supply voltage according to specifications. You'll find detailed information in datasheets.

 

[ebclr]

The best way is check on the datashet the the IC in question, there will be very detailed information,

 

[danny davis]

Without using Datasheets, how would u know?

Some CMOS logic gates are 30% of supply voltage ? Some CMOS logic gates are 70% of supply voltage?

TTL logic gates are 100% of supply voltage only?

Don't you have to look if the logic inputs have pull up resistors or pull down resistors to know if the threshold voltage is going to be sinking or sourcing to turn on the logic gate?

 

[BradtheRad]

Informative charts which show the different ranges of logic levels:

http://www.interfacebus.com/voltage_threshold.html

Don't you have to look if the logic inputs have pull up resistors or pull down resistors to know if the threshold voltage is going to be sinking or sourcing to turn on the logic gate?

Internally the input is at a high impedance. It should be easy to pull high or low by an incoming signal.

A logic input needs to see a definite high or low. If the input sees a high impedance, then you can't be certain what it will do. It may even pick up 60-cycle hum from its surroundings, and you'll see an inexplicable 60 Hz pulse train from the output, for no apparent reason.

Sometimes a device will have inputs which are not very sensitive, and it will need to be fed an incoming signal which pulls it higher or lower than normal.

 

[danny davis]

Thanks For the chart, but i don't understand it well

CMOS Input threshold voltage levels
1.) 0.2 Volt
2.) 1.3 volt
3.) 2.5 volt
4.) 3.7 volt
5. ) 4.7 volt
6.) 5 volt volte

TTL/CMOS Input threshold voltage levels
1.) 0.2 volt
2.) 0.8 volt
3.) 1.5 volt
4.) 2.0 volt
5.) 4.7 volt
6.) 5 volt

TTL Input threshold voltage levels
1.) 0.35 Volt
2.) 0.8 volt
3.) 1.5 volt
4.) 2.0 volt
5.) 3.3 volt
6.) 5 volt

How does a troubleshooting tech, know about all of these when troubleshooting logic gates from different TTL, CMOS chips?

Doesn't having a pull up or down resistor change the threshold voltage from low to high?

Can't a pull up or pull down resistor invert a low to a high or a high to a low?

 

[ebclr]

The table suplyed have all you need.

You must at least read the left colun off the graph

VCC = Power Suply
VOH = Typical tension output for level 1
VIH = Minimum tension to be considerad level 1
VT = Transition on undefenied zone ( avoid stay there )
Vil = Maximum tension on input to be considered 0 level
VOL = Tension on outup for 0 level

AS clear showed on the graph this changes according the family, But again consult the datasheet as the professional do.

 

[danny davis]

So a troubleshooting tech, can't know that the logic chip's inputs are working unless he has the datasheets?

- - - Updated - - -

So a troubleshooting tech, can't know that the logic chip's inputs are working unless he has the datasheets?

 

[BradtheRad]

The big advantage of digital operation is that the signal is noise-free. Or is supposed to be noise-free.

Any level-shifting circuits, or pull-up/pull-down resistors, etc., have to be designed so as not to create ambiguous volt levels.

The IC's are supposed to operate in accord with the standards. If they are hooked up properly, then they tend to work reliably.

It is not common to mix TTL and cmos. Therefore when troubleshooting a circuit, a technician will see whether it is TTL or cmos.

It's okay if he needs to look at the chart, to refresh his memory about the range of volt levels.

 

[Audioguru]

Without using Datasheets, how would u know?

EVERYBODY looks at the range of spec's on datasheets. Why don't you??

Some CMOS logic gates are 30% of supply voltage ? Some CMOS logic gates are 70% of supply voltage?

Yes because each one is different.

TTL logic gates are 100% of supply voltage only?

Absolutely not! Their datasheets show the range of input threshold voltage.

Don't you have to look if the logic inputs have pull up resistors or pull down resistors to know if the threshold voltage is going to be sinking or sourcing to turn on the logic gate?

You need to LEARN about typical logic families.
An old TTL input floats high. The input low current is fairly high as shown on the datasheets.
A Cmos input must never be allowed to float, it must be made high or low. Its input current is almost nothing.

 

[danny davis]

You need to LEARN about typical logic families.

What families should I know about?

A Cmos input must never be allowed to float, it must be made high or low

So TTL chips don't use or need pull up or pull down resistors? only CMOS?

common to mix TTL and cmos. Therefore when troubleshooting a circuit, a technician will see whether it is TTL or cmos.

How would a technician know the IC chips are either TTL or CMOS? what other signs or rule of thumb?

 

[BradtheRad]

CMOS IC's use less current and can run on a large range of supply V. They are convenient for battery power. They were originally given numbers in the 4000's.

TTL is the family of IC's which typically have a number in the 7400's. They use a power supply of 5V, and draw more current.
The original TTL is decades old and not likely to be seen on modern equipment. More efficient versions are available, which have LS or HC, etc., in the middle of the number.

 

[Audioguru]

Many years ago I learned TTL from The TTL Cookbook. Then I learned Cmos from The Cmos Cookbook. I still have and use The Cmos Cookbook but I haven't seen TTL ICs nor the cookbook for decades.

 

[danny davis]

So TTL IC chips don't use pull up or pull down resistors, only CMOS IC chips?

 

[Audioguru]

A Cmos logic IC output goes high or goes low.
Cmos logic ICs are usually driven from the output of another Cmos logic IC so they do not need pullup or pulldown resistors.
If an input of a Cmos logic IC is not used then it floats high or low or in between which might cause the circuit to malfuntion. Then connect the unused input directly to ground or to the positive power supply voltage (determine if you want it to be high or low) then a pullup or pulldown resistor is not needed.

 

[danny davis]

Oh ok
so why would a designer mix TTL and CMOS logic gates In a circuit?
circuit? Why would a designer choose TTL over CMOS gates or vise versa? What does TTL gates do differently than CMOS gates?

 

[Audioguru]

Long ago there were RTL and DTL logic ICs that were fairly slow.
Then original TTL came which used a LOT of power and a regulated 5V supply but was fairly fast.
Then LS TTL came which uses a little less power than original TTL but is much faster.
Then original Cmos came which is slow but uses almost no power and a wide range of supply voltage.
Then very fast High Speed Cmos came that uses very low power and a 2V to 7V supply.

Many teachers talk about only TTL because that is the only logic family they know about.

 

[danny davis]

So it's about how fast and the supply voltage

many circuits mix TTL and CMOS chips, why even use TTL if its slow and needs a lot of power

 

[Audioguru]

So it's about how fast and the supply voltage

many circuits mix TTL and CMOS chips, why even use TTL if its slow and needs a lot of power

Nobody designs with antique TTL anymore. TTL was fast, not slow. Its high amount of current made it charge and discharge stray capacitance fast.
High Speed Cmos is just as fast, uses very low current and works with a supply from 2V to 7V.

 

[danny davis]

TTL works with 5 volts or 12 volts

Is there anything that TTL can do or can't that CMOS can't do or can do?