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'[PIC] pulling it high'
2009\06\29@013212 by solarwind

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If I connect my 3.3 V microcontroller to my 5 V LCD module, and the
uC's output is low (TRIS = 0, PORT = 0), and the LCD pulls the line
high to 5 V, what will happen? Will the 5 V sink kill the uC?

-- [ solarwind ] -- http://solar-blogg.blogspot.com/

2009\06\29@021910 by solarwind

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On Mon, Jun 29, 2009 at 6:31 AM, solarwind<spam_OUTx.solarwind.xTakeThisOuTspamgmail.com> wrote:
> If I connect my 3.3 V microcontroller to my 5 V LCD module, and the
> uC's output is low (TRIS = 0, PORT = 0), and the LCD pulls the line
> high to 5 V, what will happen? Will the 5 V sink kill the uC?

What if I were to attach a 10K pullup to 5 V on the uC's pin? Would
that do damage? I'm still trying to get the fundamentals down.

2009\06\29@040554 by Chris McSweeny

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The uC will pull the line down to VDD +0.3V. Provided that doesn't
result in more current than the uC can sink (check the datasheet -
IIRC 20mA is the normal limit) you'll be fine. Certainly no problem
with a 10k pullup.

Chris

2009\06\29@113415 by Harold Hallikainen

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> If I connect my 3.3 V microcontroller to my 5 V LCD module, and the
> uC's output is low (TRIS = 0, PORT = 0), and the LCD pulls the line
> high to 5 V, what will happen? Will the 5 V sink kill the uC?
>

There are a couple issues here. First off, you're pulling an output that
is is programmed low high. This is not good for the output, but it would
probably survive due to current limiting in the chip.

The second output is pulling the pin to 5V when the chip is running on
3.3V. Most PICs have clamp diodes from the I/O pins to Vcc, so you'll
overcurrent these clamp diodes and cause problems (possibly a failed
chip). However, some PICs have 5V tolerant inputs. If that's the case, the
clamp diodes are either not there or drive a zener at a higher voltage
(not sure which). Though these pins are designed to accept 5V inputs, you
can also use them as open drain outputs to drive 5V logic (with pull-ups
to 5V) by setting TRIS to 1 instead of LAT to 1. The LAT bits always stay
at 0. When you want to output a 1, you write a 1 to TRIS, allowing the
pull-up to pull the line up. If you want a 0, you write a 0 to TRIS,
turning on the pull-down transistor in the PIC and pulling the line down.

The 24H series, as I recall, has an "open drain" bit on port configuration
that disables the pull-up transistor making this sort of operation a bit
more automatic.

Harold



--
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2009\06\29@140252 by Jan-Erik Soderholm

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>> If I connect my 3.3 V microcontroller to my 5 V LCD module, and the
>> uC's output is low (TRIS = 0, PORT = 0), and the LCD pulls the line
>> high to 5 V, what will happen? Will the 5 V sink kill the uC?
>>

How could that happen ?
If the LCD drives the lines, it must have the R/W pin high.
And in that case the PIC shouldn't drive the pins at the
same time (either low or high), of course.


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