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Communicating via UART at 3.3V levels with a 5V VCC?

 
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seidleroni
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Communicating via UART at 3.3V levels with a 5V VCC?
PostPosted: Tue Apr 10, 2007 9:27 am     Reply with quote
I'm currently using the PIC18F252, and I plan on communicating with a low voltage system (maxstream 802.15.4 systems) which require a 2.8-3.4V UART voltage. The PIC has no problem interpreting this voltage as a '1', but the maxstream may have problems getting 5V on its receive line from the PIC. Is there any way to reduce the voltage from 5V to the acceptable range for the Maxstream system?

Is there any options available in software to change the UART voltage? I'm trying to not use a voltage divider if necessary.

Has anyone else worked with the Maxstream systems and sent UART commands at 5V and had problems?

Regards,

Seidleroni
frequentguest
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PostPosted: Tue Apr 10, 2007 9:41 am     Reply with quote
This has been discussed on the forum before. Search the forum.
seidleroni
Guest
PostPosted: Tue Apr 10, 2007 2:15 pm     Reply with quote
I have been looking for a while, but I do not see any posts about it. Could you direct me to which one you're thinking of?

Seidleroni
kevcon



Joined: 21 Feb 2007
Posts: 142
Location: Michigan, USA

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PostPosted: Tue Apr 10, 2007 2:25 pm     Reply with quote
http://www.ccsinfo.com/forum/viewtopic.php?t=30433
newguy



Joined: 24 Jun 2004
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PostPosted: Tue Apr 10, 2007 2:25 pm     Reply with quote
http://www.ccsinfo.com/forum/viewtopic.php?t=30433

By the way, this topic was still on the first page of posts when you originally asked this question.
f
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PostPosted: Tue Apr 10, 2007 2:28 pm     Reply with quote
i tried to post that before, but guests can't post links. I (optimistically) hoped seidleroni would be able to find that his/herself.
Guest
PostPosted: Tue Apr 10, 2007 3:21 pm     Reply with quote
I looked at those posts and I see that there is no way to do it in software. I will just use the 5V to 3.3V resistor divider circuit.

Thanks!

jon
Jannie Hamman



Joined: 16 Sep 2003
Posts: 6
Location: South Africa

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Obvious answer?!
PostPosted: Wed Apr 11, 2007 2:13 am     Reply with quote
Run the PIC from 3V3. It works fine....
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Cheers,

Jannie
ckielstra



Joined: 18 Mar 2004
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Re: Obvious answer?!
PostPosted: Wed Apr 11, 2007 3:00 am     Reply with quote
Jannie Hamman wrote:
Run the PIC from 3V3. It works fine....
This might be true for a hobby project but are you willing to risk angry customers because you knowingly made a bad design in a commercial product?

The standard PIC18F252 is only garuanteed to work from 4.2 to 5.5V. The PIC18LF252 is specified for lower voltages down to 2.0V but at a lower operating frequency defined as:
Figure 22-2 of PIC18Fxx2 datasheet wrote:
FMAX = (16.36 MHz/V) (VDDAPPMIN – 2.0V) + 4 MHz
Note: VDDAPPMIN is the minimum voltage of the PICmicro® device in the application.

For 3.3V this means you can run the LF chip at a maximum of 25.3MHz.
jaime



Joined: 25 Nov 2005
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PostPosted: Wed Apr 11, 2007 3:08 am     Reply with quote
http://www.circuitcellar.com/pastissues/articles/Tom99/figure1.htm
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Max



Joined: 15 Dec 2003
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PostPosted: Wed Apr 11, 2007 6:23 am     Reply with quote
I use the circuit.... is good Very Happy

http://www.sisteck.com/download/3vto5v.jpg
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