3.3V UAR'S VS 5V

[GNU]

Hello,
I want to comminicate two UARTs : 3.3V (UART1 Vinmax = 3.7V) with a TTL
one (UART2 5V) from 9600Baud to 250Kb/s like this :

UART1 (TTL 5V) UART2 (3.3V)
TX1 ------------> RX2
RX1 <----------- TX2

From TX2->RX1, i think there's no problčme. My question is how to
interface Tx1&RX2, The first sends 5v to Rx2 wich accepts only 3.3
signals. Need your help please.
Thanks.
Tarek

 

[Roger Hamlett]

"GNU" <Reply_In_NG_Please> wrote in message newspr82ge7ya0l9kb4@alata...

Hello,
I want to comminicate two UARTs : 3.3V (UART1 Vinmax = 3.7V) with a TTL
one (UART2 5V) from 9600Baud to 250Kb/s like this :

UART1 (TTL 5V) UART2 (3.3V)
TX1 ------------> RX2
RX1 <----------- TX2

From TX2->RX1, i think there's no problčme. My question is how to
interface Tx1&RX2, The first sends 5v to Rx2 wich accepts only 3.3
signals. Need your help please.
Thanks.
Tarek
You may have problems in both directions. 5v to 3.3v, can be (crudely)

fixed, by adding a series resistor, and a clamp diode to prevent the
incoming signal going significantly above the 3.3v supply rail. In the other
direction, it will depend on the input 'high level' voltage of the 5v part,
and how high the 3.3v part will pull the signal. A lot of the time it will
work, _but_ is usually close to the limits for the parts. So (for instance),
the 5v part may well have an 'input high' voltage of 2.4v, while the 3.3v
part, typically gives perhaps 2.5v out. For safety though, you should really
take the 'worst case' figures in both cases, and this will often show that
there is a potential for problems.
If this is for a one off system, where poor reliability can be accepted, use
the resistor/clamp, and direct connection the other way, and simply try it.
If this is for a commercial product which is meant to be reliable, instead
look at a level translator, like the Maxim 3372/3 products.
A single package here, provides the required shift in both directions.

Best Wishes

 

[Wouter van Ooijen (www.vo]

UART1 (TTL 5V) UART2 (3.3V)
TX1 ------------> RX2
RX1 <----------- TX2

I used HCT buffers, running from the receiving voltage.

Also check the Philips I2C documents for a trick using FETs to
interface between parts of a circuit running different voltages. Their
trick is bi-direction, which you don't need, but it won't hurt either.


Wouter van Ooijen

-- ------------------------------------
http://www.voti.nl
PICmicro chips, programmers, consulting

 

[GNU]

On Sat, 05 Jun 2004 02:14:39 GMT, Wouter van Ooijen (www.voti.nl)
<wouter@voti.nl> wrote:

UART1 (TTL 5V) UART2 (3.3V)
TX1 ------------> RX2
RX1 <----------- TX2

I used HCT buffers, running from the receiving voltage.

Also check the Philips I2C documents for a trick using FETs to
interface between parts of a circuit running different voltages. Their
trick is bi-direction, which you don't need, but it won't hurt either.


Wouter van Ooijen

-- ------------------------------------
http://www.voti.nl
PICmicro chips, programmers, consulting
-----

Thanks your answers.
What i need to know, is about using a zener diode Z3V3 ? The beaudrate is
between 9600Baud and 250Kb/s.
Thanks.