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Sun May 6 08:57:18 HKT 2012

d TX pins of the SIM908 and PIC18F2685 can be directly connected to each ot=
What do you think? Perhaps it is time to find my old breadboard and try it.=
I think it should work perfectly fine from SIM908 to PIC with just a trace.=
However, the SIM908 probably has an absolute maximum voltage specification =
that would not allow anything above 3.3 V on any pin. Thus, the other way, =
from PIC to SIM908, would not work with just a simple trace. Look around yo=
ur data sheet for that limit. The existing diode (not shown in your schemat=
ic, but mentioned in email), is surely still necessary to protect the SIM90=
8 from too much voltage. If there is a resistor in there as well as the dio=
de, then that&#39;s probably necessary as well, but I can&#39;t remember wh=
at&#39;s typical. If you have the schematic for the other board, drop me a =
link and I&#39;ll take a look.<br>

In general, a device cannot work when one of its pins has a higher voltage =
than its own power input pin. If you connect a higher voltage, sometimes cu=
rrent will flow backwards through the chip, out the power pin, and raise yo=
ur 3.3 V supply to 4 V or more. This will cause your regulator to heat up a=
nd possibly create other symptoms. It can be especially bad if other chips =
are not rated for 4 V power.<br>

I had a board where an unused chip was exposed to improper voltages, and it=
 caused problems for quite a while. I never thought to check that chip. It =
was only when I removed the chip and noticed that all the random problems w=
ent away that I realized one of its pins was seeing a voltage higher than t=
he supply. Had I checked the 3.3 V supply rail before, I would have seen th=
at it sometimes went above 4 V, and that would have been a clue, but I didn=
&#39;t think of that until I had practically solved the problem.<div class=
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