3.3V serial communications with external equipment

[JamesCm]

Hello everyone,

I'm looking for an IC to allow an MCU to safely communicate with external equipment via serial, but instead of RS232 voltage levels it's simply 3.3V (0V / 3.3V logic levels). Does anyone know of any? All I can find are regular buffer/drivers, and the closest I found are CAN transceivers.

Thanks in advance!
James

 

[KlausST]

Hi,

* In a noiseless environment
* with a good data cable
* with suitabke termination
* with limited data rate
* and a suitable receiver circuit
..you may safely communicate.

There are reasons for RS232, RS485, CAN bus....and not to use single ended 3V signals.
Each of the above interfaces has it's benefits and drawbacks..making them suitable for the one application and not suitable for another application.

You need to understand that there are more than voltage levels, like:
* rise / fall rate
* ESD protection
* ground bounce
* driving current
* input filter
* common mode suppression
* cable length
* cable type
* characteristic impedance
* shielding
* device count
* protocol
* parity and redundancy checks
and so on..

So a 3V communication may work ... or not. This is not in our hands. It depends on your situation.

Klaus

 

[danadakk]

I2C, SPI, One Wire .....you have to consider buss loading and as Klauss
mentions cable length and all the other considerations he mentions.

You can always bit bang your own protocol on a pin.


Regards, Dana.

 

[JamesCm]

I understand. But unfortunately due to backwards compatibility it has to be serial communications but without RS232 voltage levels, and 3.3V levels instead. But I'm worried about putting the MCU IO pins on a connector to the outside world, even with ESD protection diodes. So I was looking for pretty much a MAX3232 IC but without RS232 voltage levels, and 3.3V levels instead.

The cable is very short and it's an "at home" environment.

So, I guess the questions are:

1) Is there an IC to do that?
2) Would it actually be ok to use the MCU I/O pins directly on that connector, and there's really no need for an IC for that?

Thanks,
James

 

[KlausST]

Hi,

sadly your new post gives zero new information.
You don´t do yourself a favour by hiding such basic informations like cable length, baud rate, cable type, receiver input stage informations...

What do you expect? Let´s imagine there is a guy who tells you: "Yes, it´s fine to use 3.3V signals". Then he/she gives an answer without sufficient information. You have to try and maybe fail.
You may thorw dices, instead.

So all I can say: maybe.

1) To do what? Slew rate limiting, ESD protection, high current drive, dedicated characteristic impedance drive, and so on?

2) Yes, as long as all is according MCU I/O pin specifications.

Klaus

 

[FvM]

Consider a series resistor for current limiting and surge protection. If the current strength of processor GPIO is insufficient, use buffer ICs.

 

[BradtheRad]

A zener diode limits voltage at an IC pin. If you don't have a 3 or 3.3v, then string together led's or plain diodes. A safety resistor may be needed.

 

[danadakk]

Yes you can use a UART on GPIO pins, thats normally how a processor with internal UART
is used, and then its I/O is passed to a level translator to get the RS232 levels.

If you have concern about ESD environment, and shorts, you can always do this on the
pin externally -

R value to limit pin current when esd tries to take pin outside its supply rails.
1K ohms, depends on com speed. Just do a 5 x RC with C the pin C, and 5 x RC
<< bit time. Use small signal diodes to get fast turn on time. Additional margin
can be had using Schottky diodes.

Note some folks rely on internal parasitic diodes in CMOS and just use the R
on the pin.


Regards, Dana.

 

[JamesCm]

Yes you can use a UART on GPIO pins, thats normally how a processor with internal UART
is used, and then its I/O is passed to a level translator to get the RS232 levels.

If you have concern about ESD environment, and shorts, you can always do this on the
pin externally -

View attachment 171089

R value to limit pin current when esd tries to take pin outside its supply rails.
1K ohms, depends on com speed. Just do a 5 x RC with C the pin C, and 5 x RC
<< bit time. Use small signal diodes to get fast turn on time. Additional margin
can be had using Schottky diodes.

Note some folks rely on internal parasitic diodes in CMOS and just use the R
on the pin.


Regards, Dana.

Well, I looked all over and I guess ESD + resistor is indeed the best option. I'll use an ESD protection IC, very small and very chip. Can't go wrong with it. The zener diode to limit voltage at the IC pin is also interesting, but a short on that output would burn both the diode and the pin. But yes, with a resistor after it, indeed it's a good idea.

Thanks everyone for your posts and your help!
James