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5V UARTs on SBCs?

shabaz

Hi!

I'm thinking of turning this project into a PCB:

Building a USB UART Serial Adapter

 

As part of that, I thought a few bits of functionality could be added at low cost:

(a) an MSP430 programmer

(b) perhaps a voltage monitoring capability since there is an ADC on the MCP2221

 

However, I wanted it to be a super-simple project. The current project uses jumpers to select 5V or 3.3V logic level UARTs. One solution could be

a switch, but it would be nice to not have a switch either.

To eliminate that, I'm thinking of making the Rx pin 5V tolerant, but leave the Tx pin at 3.3V logic levels. This should therefore be compatible with any boards

that use 3.3V levels, but also the 5V Arduino Uno (it should be fine with 3.3V logic level input according to the ATmega datasheet).

(I'll also make it somewhat tolerant to accidentally swapping round of Rx and Tx).

 

My question is, does anyone know, are there any popular 5V boards that would not be OK with 3.3V logic level?

 

(I know now there are 1.8V boards too, like the Odroids, but that is a complication too far, so this board will only handle 3.3V and have the tolerance for 5V).

 

Thanks!

  • Add Dragon 410C as 1.8 volt too for GPIO.

    Clem

  • in reply to clem57

    Hi Clem,

    Thanks!

     

    Looks like more and more boards may offer 1.8V than I thought. (Just remembered Edison would be the same).

  • in reply to clem57

    One thing that could be feasible (to keep it simple to use with no switches) is to make it unlikely to cause harm if accidentally connected to a 1.8V UART.

    This project is only intended up to 115200 baud, so the current can be limited a fair bit without causing risk of data error. If I reduce it even further but have a

    bypass for high frequencies then it might even work with 1.8V for extended periods.

    I'll have to think this through..

  • The main purpose of the design is great - and useful.

     

    For additional options, I'd say: Don't add anything that requires an additional component or code.

    It's good to provide hack points in the design where the design can be extended or hacked - where it makes sense and if it doesn't impact the initial purpose.

     

    This opinion can be ignored image

  • in reply to Jan Cumps

    Hi Jan,

     

    Wise words from you as always. It makes sense. Keep it simple, do one thing well rather than many things poorly, and provide some hack points (maybe some small perf-board array of holes, to allow interfacing or extending). All great ideas.

  • You can get bi-directional level translator IC's that will run either side from differing voltages. You could use one of these with one side powered from your circuit and the other side either powered by or have it's voltage slaved to the target boards power. This will give a wide voltage range of operation without having to manually switch anything and it's just one simple IC so very simple to implement.

     

    Best Regards,


    Rachael

  • in reply to rachaelp

    Hi Rachael!

     

    I should have mentioned but I forgot, the plan is an entirely through-hole project, and most of the level translators are SMD except for the large LVC244/245. I'm still open to doing it that way (I have an FTDI cable which too requires connecting to the supply so that it can figure out the logic levels) but I'm wondering if it can be avoided nowadays since modern microcontrollers/SBCs seem to have 3.3V logic tolerant inputs even if they are running at 5V. I still need to figure out some trick for the 1.8V SBCs though : ( Possibly relying on the fact that the baud rate won't be higher than 115200 (that is the limit of the MCP2221) and limiting current a lot at the expense of waveform shape). I don't know if there would be long-term damage, I need to go some calculations.

    It would also save the headache of double-checking it is connected to the right level, e.g. the Odroid U3 is 1.8V serial, whereas the Odroid C1 is 3.3V, etc.

    Maybe it's not possible though..

     

    As for the input side to the project, that will be easier, a couple of BJTs to provide the high voltage tolerance into the MCP2221.

  • in reply to shabaz

    Ah, through hole! Yes I haven't done anything that was entirely through hole for so long I forget that lots of things I take for granted in SMT aren't available! I've just done some searching and limiting my results to through hole and it really does make for quite a challenge. I think it probably makes you have to be more inventive to find ways around the limitations of what is available.

     

    So with that in mind, I have spent a few mins searching and haven't done any real in depth analysis of whether this would work but.... ignoring it says I2C in the datasheet title, if you read further I think it can be used for generic logic signals (I have never used this part, I just found it on Digikey) how about this as a possible level translation device?

     

    http://www.ixysic.com/home/pdfs.nsf/www/CPC5902.pdf/$file/CPC5902.pdf

     

    It's through hole and two channels so I think should work for your UART so long as you are only using TX/RX although you could use additional devices for other UART signals and your speed is within the I2C limits of 400KHz so that shouldn't be an issue. It only goes down to 2.7V but it's likely easier to make a 2.7V output work with 1.8V logic.

     

    Best Regards,


    Rachael

  • in reply to rachaelp

    Going through-hole these days feels a lot like going to a burger place and seeking vegetarian dishes : )

    Interesting find, I'd not seen this part before.. I'll have to read up and experiment a bit, hopefully this weekend.

  • in reply to shabaz

    going to a burger place and seeking vegetarian dishes : )

    And nothing that a good steak wouldn't fix (as I keep telling my son)

     

    A quick search came up with this

    Arduino Playground - I2CBi-directionalLevelShifter

    https://www.intersil.com/content/dam/Intersil/documents/an16/an1637.pdf

     

    I know the fet version should be fast enough.

     

    Mark