At 06:42 AM 20/12/2011, you wrote:

>Howdy,
>
>I can't find this info in the datasheet. I measured (on a 3.3Vdd)=20
>1.30V and 1.64V,
>but I'd like to know if there are any official infos about these values.

The data sheet gives no minimum or maximum (except the maximum implied
by the difference between the max and min thresholds). Even the thresholds
are "characterized but not tested". No guarantees. If you need guarantees,
you can use something external.

Much earlier chips gave a typical value of 0.15*Vdd, which is about 50%
more than you measured. Depending on your analog skills it could be a bit
tricky to make this measurement, and errors will tend to cause it to
read a lower value for the hysteresis (iow, it will trigger on noise).

I would power the chip on a proper PCB with ground plane, using an internal
clock and with good bypassing on the power. Put maybe 100 ohms in series wi=
th
the input being tested, then 1uF film to the common ground point, and
100K to the voltage source (measure across the capacitor). Presumably
you're driving an output with a program that reads the pin- use another
port and don't draw significant current (eg. drive the gate of a 2N7000
driving a separately (maybe battery) powered LED).

>Also, by making the (digital) input deliberately "float", I measured=20
>no anomaly in
>the current drawn by the MPU. Perhaps the schmitt trigger prevents=20
>this to happen?

The current will vary with the input state, but it should not get
very high. Have a look at the NXP datasheet for the 74HC14 to get an idea
of how it will likely behave wrt Vdd and input voltage (the numbers may not=
 be
the same, of course).

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the rewar=
d"
speff@interlog.com             Info for manufacturers: http://www.trexon.co=
m
Embedded software/hardware/analog  Info for designers:  http://www.speff.co=
m



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