At 07:32 AM 7/18/2013, Kerry Wentworth wrote:
>The switch points for ones and zeros for a (non Schmidt trigger) input is
><=3D 1/3 VCC =3D 0
>  >=3D 2/3 VCC =3D 1
>  > 1/3 VCC and < 2/3 VCC =3D unknown

That is NOT correct for the PIC 12-bit and 14-bit micros that I am=20
using.  The threshold of
PIC input pin that is NOT Schmitt Trigger is about 1.3Vdc when the=20
PIC is running from a 5Vdc supply.

That may be different on modern PICs but the data sheet will tell all=20
(if you spend a month finding the right spot in the data sheet).

This low input-threshold voltage is completely TTL-compatible and is=20
also the reason why RC-Time measurements are usually started with=20
Vcap at Vdd and discharging towards Ground rather than the other way around=
..

Note that this input threshold has a fairly steep knee (lots of gain=20
in the input stage), which makes PIC digital-input pins so good at=20
doing RC Time measurements.

dwayne

--=20
Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
www.trinity-electronics.com
Custom Electronics Design and Manufacturing

--=20
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