Is my understanding Digital devices all screwy, i'm talking about their datasheets

[Mr. J D]

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?

 

[Homer J Simpson]

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?

Aaahh, grasshopper, very interesting questions.

 

[Graham W]

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?

VOH is an output specification for the device. VOHmin is the guaranteed
lowest value of the high output signal under specified conditions.

VOL, similarly, is the low value output signal.

 

[Ancient_Hacker]

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?

I don't know of any mainstream logic family where you can "set" the
high or low thresholds. Perhaps you should ask whoever told you this
for more details.

The exception is the old CMOS family, which could run from 3 to 18
volts, and the threshold was always really close to 1/2 Vcc.

Anyway, one hardly ever has to worry about the exact thresholds,
ubnless yo're trying to do some analog trickery using digital parts.
In 97% of digital design, you just try to run short wires and aim to
keep the outputs swinging cleanly, -- that's enouugh to more than
satisfy the inputs.

 

[mc]

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?

I have no idea what they're talking about. Your original understanding is
correct.

If there are chips where you can "set" the levels, they are not the normal
logic families.

 

[Rich Webb]

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.

Kind of. Some logic families, e.g. the 4000-series, can run from a
relatively supply range. The input and output thresholds change
propotionally with the change in supply voltage.

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?

There is no general answer; it's family-specific. Some only work for a
relatively narrow supply range, others accept a much wider range.

 

[Lostgallifreyan]

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?

It looks to me as if you've got a confusion between analog and digital
logic.

Analog logic usually works between values of 0 and 1, linear and
continuous. The actual voltage may vary, if it's supplied with 18 volts
(quite likely, as that allows fine detail without too much noise), the 18 V
is still a 1 logically.

Likewise in digital logic, whatever thwe supply voltage is, the high level
(which is very close to the supply voltage) is a 1. The difference is that
to eliminate noise, the digital logic switches between that 1 and a 0 very
close above 0 volts.

Inputs can be set high or low (or anywhere between 0 and 1 on analog
logic). How they affect outputs depends entirely on the number of inputs,
and the types of logic each one controls.

Digital logic can be reduced to structures built from AND, NOT and OR
gates.

Analog logic is based on op-amps, analog multipliers, etc.

 

[Marc Guardiani]

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?

The logic family used by DDR and DDR II memories effectively allows the
user to "set" (actually reference) the switch point of the logic.
However, in practice, this is always set to half the supply. You can go
to www.jedec.org to download the DDR spec for free.

You can also go to any large FPGA manufacturer (e.g. www.altera.com or
www.xilinx.com) and read about the various I/O specs they support that
also allow the logic switching threshold to be "set". However, for the
various standards, there are specific voltages that have to be used for
the reference.

Marc