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AN213 suggestion

 
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dlh



Joined: 15 Dec 2006
Posts: 265
Location: ~Cincinnati
Posted: 29 August 2008, 12:21 PM    Post subject: AN213 suggestion Reply with quote
About 5 months back, Tibbo discontinued their EM202 NetModule. They promised a drop-in replacement by July 1 but it has yet to materialize so I am redesigning to use the Lantronix XPort.

All of the XPort pins are +5V tolerant (-03 model only) but the RS232 output is only 2.4V for logic high so I need to shift the level. I am modifying an existing design and have minimal space available to fit both a 3.3V regulator, caps and a level shifter. Since I am interfacing with the ZX-40a UART, I need a non-inverting level shifter. I will likely use a single OpAmp but it might be helpful if AN213 included a non-inverting transistor switch circuit.
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dkinzer
Site Admin


Joined: 03 Sep 2005
Posts: 2493
Location: Portland, OR
Posted: 29 August 2008, 14:57 PM    Post subject: Re: AN213 suggestion Reply with quote
dlh wrote:
[I]t might be helpful if AN213 included a non-inverting transistor switch circuit.
It does. See Figure 15 on page 11. The circuit that converts low voltage output to 5-volt output is reproduced below.

non-inverting level converter.jpg
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Non-inverting level converter.
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non-inverting level converter.jpg
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dlh



Joined: 15 Dec 2006
Posts: 265
Location: ~Cincinnati
Posted: 29 August 2008, 16:01 PM    Post subject: Reply with quote
Ahhh! I just didn't read far enough - quitting after seeing the inverting switch on p6 but no non-inverting version of it. Thanks.
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spamiam



Joined: 13 Nov 2005
Posts: 661
Posted: 30 August 2008, 13:46 PM    Post subject: Reply with quote
Wow, this just shows how little my instincts are worth.

I would have supposed that when the 3.3v device set its pin to "high" or tristate, it would see something close to the 5 V due to the 5v pull up on the opther side. Granted, the transistor is switched off because there will be no base current, but I would have thought that the floating voltage at the pin on the 3.3v device would approach 5v.

Just goes to show that sometimes you need to try things out before assuming that good (or bad) things will happen!

-Tony
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dkinzer
Site Admin


Joined: 03 Sep 2005
Posts: 2493
Location: Portland, OR
Posted: 30 August 2008, 15:15 PM    Post subject: Reply with quote
spamiam wrote:
I would have supposed that when the 3.3v device set its pin to "high" or tristate, it would see something close to the 5 V due to the 5v pull up on the opther side.
With no base current, the collector-emitter path is a very high resistance. If the low-voltage output is, in fact, in a tri-state condition, you're right that it will tend toward 5 volts. On the other hand, if it is being driven high (i.e. to 3.3V) then it will absorb the small amount of current flowing through the transistor without changing the output level.
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