I'm sorry to bring this up, because I know it must have been discussed
many times before, but I am confused. The description of the "Enhanced
Serial Download Circuit" implies that the onboard diodes in the PICs
are not Schottkys. But, if this is so, I can not see how the standard
serial input circuit (10k to ground and 22k to input) can possibly work
with, say, a +15/-15 V serial input. At Vdd=5V, Vss=0V, the current
through the 22k resistor would be about .5 mA. With a non-Schottky
diode this would give a drop of about 0.6V. Thus, +5.6/-0.6V after the
diodes. But, the absolute max/min input voltages for the PIC under
these conditions are +5.3/-0.3V. How can this work? Are there really
Schottky diodes onboard? If so, why the external Schottky diode in the
enhanced circuit?
Ed Grens
many times before, but I am confused. The description of the "Enhanced
Serial Download Circuit" implies that the onboard diodes in the PICs
are not Schottkys. But, if this is so, I can not see how the standard
serial input circuit (10k to ground and 22k to input) can possibly work
with, say, a +15/-15 V serial input. At Vdd=5V, Vss=0V, the current
through the 22k resistor would be about .5 mA. With a non-Schottky
diode this would give a drop of about 0.6V. Thus, +5.6/-0.6V after the
diodes. But, the absolute max/min input voltages for the PIC under
these conditions are +5.3/-0.3V. How can this work? Are there really
Schottky diodes onboard? If so, why the external Schottky diode in the
enhanced circuit?
Ed Grens