Is my understanding Digital devices all screwy, i'm talking about their datasheets
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Posted by Mr. J D on August 27, 2006, 12:38 am
Please log in for more thread options How do you figure out what a digital gate senses as HIGH and LOW? I thought it was labeld VIH, VIL, VOH, and VOL. But ive been told that by setting VIH you can set what VOH is. Totally confused by this. I thought there was a specified range, and that was what you used. According to other people you could, "set", and have a VIH at 1 Volt. How do these setting really work? | |||||||||||||
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Posted by Robert Baer on August 27, 2006, 1:26 am
Please log in for more thread options Mr. J D wrote: Circlotrons until he gets dizzy; that will send both of you to Alffa Centurie. | |||||||||||||
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Posted by Roger Hamlett on August 27, 2006, 10:42 am
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> How do you figure out what a digital gate senses as HIGH and LOW? I
They don't.
> thought it was labeld VIH, VIL, VOH, and VOL. But ive been told that by > setting VIH you can set what VOH is. Totally confused by this. I > thought there was a specified range, and that was what you used. > According to other people you could, "set", and have a VIH at 1 Volt. > How do these setting really work? The only thing with a 'settable' ViH, such as you describe, would be a comparator, and some level translators. However on most chips, ViH, may well be dependant on other parameters. For instance, this is a spec from one chip: Vddmin = 3.3v For Vdd < 4.5v ViH = 0.25*Vdd+0.85v Vdd = 4.5 to 5.5v ViH = 2v VddMax = 5.5v So for this chip, Vdd, can vary from 3.3v to 5.5v. At 4v, ViH, will be 1.85v. The minimum ViH, for this chip, would be 1.675v, and the maximum 2v. There is a slight 'hiatus' in the definition at 4.5v, with the defined ViH level, jumping by 75mV, from 4.499v to 4.5v. There are some 'multi family' level translator chips, where the ViH, may depend not only on the supply voltage, but on external components (resistors/jumpers), and it may be one of these that is being referred to. These effectively have comparators with adjustable thresholds. Best Wishes | |||||||||||||
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Posted by John Larkin on August 27, 2006, 1:19 pm
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>How do you figure out what a digital gate senses as HIGH and LOW? I
>thought it was labeld VIH, VIL, VOH, and VOL. But ive been told that by >setting VIH you can set what VOH is. Totally confused by this. I >thought there was a specified range, and that was what you used. >According to other people you could, "set", and have a VIH at 1 Volt. >How do these setting really work? You can't "set" the input threshold, except by varying the supply voltage Vcc. Most CMOS devices typically have an actual, very sharp high/low decision level a bit below 0.5 of Vcc. But the datasheet gives levels VIH and VIL that are *guaranteed* to be solid high and low. HCT type devices have a lower decision level, typically about 1.4 volts at Vcc = 5. Schmitt triggers have hysteresis and behave a bit differently from regular CMOS inputs. Get a few CMOS gates and experiment! John | |||||||||||||
> thought it was labeld VIH, VIL, VOH, and VOL. But ive been told that by
> setting VIH you can set what VOH is. Totally confused by this. I
> thought there was a specified range, and that was what you used.
> According to other people you could, "set", and have a VIH at 1 Volt.
> How do these setting really work?
>