Hmmm... was not aware that the USART TX was open-drain, but this can=20
make things much simpler.  I'll have to investigate that.

FWIW, each line is single direction always -- they are not bidirectional=20
lines.  The line that goes 3.3V-->5V is what I was worried about, as=20
when the 3.3V TX output goes high, it would only go up to 3.3V, whereas=20
the 5V RX input wants to see 4.0V. But this is moot (and solved) if it's=20
open drain.

I do remember that the 3.3V PIC's RX and TX pins are both 5V tolerant,=20
but now sure if the 3.3V side will clamp it.  I'll have to test this=20
with my existing prototype setup I think.

Cheers,
-Neil.


On 8/7/2012 3:03 PM, Joe Wronski wrote:
> On 8/7/2012 2:47 AM, PICdude wrote:
>> In the 3.3V-->5V direction, it seems the 5V RX1 input (18F24K22) is a
>> Schmitt trigger, and the electrical specs for that in the datasheet
>> are minimum input-High voltage of 0.8Vdd, so 4.0V, and thus the 3.3V
>> signal won't work.
>>
> Eh?    How is an open drain output from the 3.3V side *not* going to
> meet the 4V input spec on the 5V side, if the line is pulled up to 5V?
> Unless I'm missing something, the 5V  tolerant feature of the 3.3V part
> means you can just pull both lines up to 5V.
> But if the 3.3V input side clamps the 5V output, the 5V side would get
> confused when it's master?
>
> Joe W

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