Logic gate operation

[embeddedlover]

I had one basic question, which has been bothering me for sometime.

I have a logic gate, in which recommended VCC is min. 4V and max. 5V. I want to keep aside the input logic levels for sometime and wanted to know if i can operate the same logic gate at 3V and get my desired results. Let us assume my logic level on input does not go beyond VCC. Logic gate i want to use are NOR, NAND, INVERTER.

 

[KlausST]

Hi,

there are a lot of logic gates with a specified VCC of 3V.
Just us an approriate one. Everything else is a waste of time.

If you use devices out of specified range:
* some may work some may not
* some may working now but not in a year
* some may work with temperature now, but not at other temperatures.


Klaus

 

[embeddedlover]

If you use devices out of specified range:
* some may work some may not
* some may working now but not in a year
* some may work with temperature now, but not at other temperatures

Completely agree with your views. My doubt has been regarding the device operating below the rated voltage. and that NAND gate internally are FETs. So, operating below specified voltage shouldn't be a problem. I am looking for a clear reason for not able to do it.

I am looking at only possibility that the switching may not occur if operated at lower voltages. As, internally, in a NAND gate, fets are to be switched ON/OFF.

 

[KlausST]

Hi,

if you want to protect against malefunction with too low supply voltage then use a voltage monitor.

Klaus

 

[Audioguru]

Old 74xx TTL logic works with a supply from 4.5V to 5.5V.
CD4xxx ordinary Cmos logic works with a supply from 3.0V to 18.0V.
74HCxxxx high speed Cmos logic works with a supply from 2.0V to 6.0V.

 

[Gorgon]

It also depends on the switching speed you want to use. The lower the Vdd the lower the max speed available. For normal 'slow' speed logic, all families that work on lower voltages, will be good.

One thing you should be aware of is if you mix signals from logic modules with different voltage supplies, then you may need some special level conversion.
For example; if one module working on 3V supply, interact with logic supplied with 5V.

 

[KlausST]

Hi,

The lower the Vdd the lower the max speed available.

I was confused about this, so i looked into fairchild's logic selection guide.

I found that the families for lower voltage (VCX, ULP) have the higher speed.
But within one family it is true: the lower the voltage, the lower the speed.

Klaus

 

[Gorgon]

The goal was to have logic that covered a range of voltages, and as far as I know there are lower speed on lower voltages for these families. This goes for both the 74HCxx and the CD4xxx series.

 

[Audioguru]

CD4xxx and 74HCxxx are Cmos. The higher is the supply voltage then the higher is the current that can charge and discharge internal and stray capacitance faster.

 

[D.A.(Tony)Stewart]

I have a logic gate, in which recommended VCC is min. 4V and max. 5V.
can (I) operate the same logic gate at 3V and get my desired results?

Let me suggest some rules for logic with unstated expectations
1. Never assume if one gate works another will too. Always conform to specs otherwise, the unexpected may happen ( Murphy's Law) There is some variation within a process and also depending on channel size and factory wafer doping process, some speed variations, so it you do not follow "Worst Case" Rules for timing, your design may become the "worst case".

( In the old days ( 70's) if a chip wasn't fast enough ( {"for our desired results"}, we would check if it was Motorola and disqualify them from that part list and only use Fairchild. If it still wasn't fast enough, we'd raise the voltage and still not fast enough, we'd put cold spray on it. ( but that is just for experience, not final design)

Operating below the rated voltage limits means none of the specs are guaranteed, so if your logic dictates, does it toggle, maybe it will, but will it have it be slower, worse unequal slew rates or have much less noise immunity....? or all of the above? Probably.

There are at least 13 different active logic families each with different voltage ranges. None of them have 4 and 5V for min/max.

The lowest driver impedance and fast I have found are on the ARM chip with 25 Ohms ESR on the 3.3V drivers. Old HC4xxx CMOS is usually around 200~300 OHms. You may not care but load & stray capacitance x driver ESR, both affect rise time. If you are doing a lot of synchronous clocking, you have to watch for race conditions with worst case timing. NOrmally a timing analysis was done manually on all production designs. Now they have simulators with Monte Carlo analysis on power consumption and timing etc.

 

[embeddedlover]

Operating below the rated voltage limits means none of the specs are guaranteed, so if your logic dictates, does it toggle, maybe it will, but will it have it be slower, worse unequal slew rates or have much less noise immunity....? or all of the above? Probably.

on the point... this was what i was looking for on the high level.. My guess too is that it may work but not as per the desiredspecifications.

Actually, speed is not a concern for me. I am basically toglling GPIO and that too at a very low rate. this GPIO is gated. So, a point came up whether it works as per the desired functionality if i operate it below the voltage range.

@audio guru

Old 74xx TTL logic works with a supply from 4.5V to 5.5V.
CD4xxx ordinary Cmos logic works with a supply from 3.0V to 18.0V.
74HCxxxx high speed Cmos logic works with a supply from 2.0V to 6.0V.

Thank you for the info

 

[D.A.(Tony)Stewart]

Will it work with crosstalk and stray noise , supply noise is the unknown, load factors and will be vendor and chip design dependant ?

The best choice is choose a logic family that meets your requirements. Like I said there are at least 13 different logic voltage range families, which you can easily find with distributor search drop down lists.