At 10:56 AM 2/16/01 +1300, you wrote:
>>         Aren't the ST threshholds proportional to Vcc? I suspect
>> they are 0.8*Vcc and 0.2*Vcc or 4.0V and 1.0V. This seems
>> relatively close to what you measured.
>
>My mistake also for saying ST limits are 0.2V and 0.8V, of
>course it should be 0.2Vcc and 0.8Vcc, but I don't want that
>to detract from what is puzzling me
>
>The figures I measured are nowhere near 4V and 1V. With a
>negative-going voltage the top figure is ~1.60V and the lower
>is ~1.50V = only ~0.1V of hysteresis (plus there's the ability to
>sneak past the 1.60V IRQ point without the PIC noticing). With
>a positive-going voltage they are 3.11V and 1.48V respectively
>= 1.63V of hysteresis, which is more reasonable although 50%
>away from the datasheet max/min. Why the big difference between
>-ve and +ve going voltages with INTEDG set appropriately ?

The minimum and maximum figures on the data sheet are the
values that Microchip guarantees will be recongnized as a 1 or 0.
Your measurements are within the range, so it is doing what it
is supposed to be doing.

The amount of hysteresis Vhys is listed as 0.1V typical on the
current data sheet (no limits given).

Note also from the schematic (Page 15) that the RB0 input buffer will
generally have *different* thresholds from the ST interrupt
input, they are really two different circuits connected to the
same input pin. There is *no* guarantees on how they will overlap
in their thresholds.

Best regards,


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