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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!

  • in reply to shabaz

    It might be worth considering different connectors for each voltage.

    I wonder if there are any RS232 devices (+-12V) still out there to worry about?

    I have built a couple of different USB-to-serial PCBs and each time I do it I figure I'll build in some extra features, but when a new application comes along there is always something non-ideal about my previous designs - usually size or connect compatibility.

    All signing, all dancing is a nice idea, as long as it meets he original requirements well.

  • in reply to shabaz

    Lets see - if going through hole is like going from touch screen back to pushbutton then wire-wrap must be like rotary dial. (point-to-point wiring would be like switchboard phoning?)

  • in reply to dougw

    Lol. Speaking of rotary dial I still have some phones like that : ) They are too beautiful outside and inside to throw out. Just a shame they are so impractical to use now, dialing so many digits..

    We saw so much interesting vintage technology at Bletchley and the Computer Museum when was around. It was a lot of fun. If you ever visit the UK it would be great to arrange a trip, especially if Mark could re-visit too! I'm fairly sure we could spend weeks there and not get bored..

  • in reply to shabaz

    I'm fairly certain we could find lots to look at.

     

    I once saw this use for a rotary phone.

    https://www.sparkfun.com/tutorials/51

     

    Brilliant idea.

    I'm tempted to make one into a cordless phone around home.

     

     

    Mark

  • in reply to shabaz

    In the town where I grew up our phone number was exactly 3 digits. You could easily remember all the numbers you needed to call - or you could dial a random number and the person answering would know the number you were trying to reach. Phonebook? We are talking pamphlet.

    The sparkfun rotary cell phone concept mentioned by Mark could be smart enough to use the rotary sequence as a 2 or 3 digit speed dial....

    The UK is definitely high on my list of places to visit - more points of interest and higher density of interesting places than just about anywhere on the planet - and I can almost speak the language.

  • CP2103 fits except for the encapsulated...

    Powered from USB, and taking VCC from the board (arduino, RPi, BBB...) to connect it to the VCCIO of the CP2103

  • in reply to gihu

    Hi gihu!

     

    Very nice part. I should add it to the list here, it looks closely related to the CP2102 (I think the main difference is the EPROM on the 2103 vs the PROM on the 2102, but I need to read the datasheet more thoroughly).

    Unfortunately as you say, the package is the thing that doesn't fit. The through-hole requirement is quite limiting. I also want to avoid having the extra wire if possible, not too sure yet until I try it out.

    With a 3.3V logic out it is ok for 3.3V and 5V UARTS (perhaps not some older SBCs if the 5V levels are not compatible : ( but hoping to just heavily restrict current to try to make it safe for 1.8V UARTs too (no idea yet if this is practical, need to do some sums), also making it safe for both boards if RX/TX are accidentally swapped. The receive side just needs to be tolerant, which is easier to accommodate.

    It is only for development purposes, not production, so I don't overly mind if it doesn't work at high bitrates or with all historical boards. This way I can stick to through-hole parts entirely I think, and have something that doesn't need thinking before connecting. And maybe a proto area, so people can customize or add their own functionality (since there is I2C and ADC capability on the MCP2221), or the RS232 capability that Doug suggests, since that would be handy too. Example documented adaptations for RS232, MIDI etc would be cool.

  • in reply to shabaz

    Maybe it would be posible emulate the IC that says with two rail-to-rail opamp like MCP6041 comparator, and voltage dividers with resistors.

    I am searching, but the only compators working from 1V8 that I have found,are SMD image, too slow, or both.

  • in reply to gihu

    Can I ask why you would want to emulate the IC rather than just use the one that's available? Is this to try and get down to proper 1V8 operation? The other advantage of the one I linked to is it's opto-isolated and so provides for potentially quite a robust interface for something that will likely be plugged into many different boards.

  • in reply to rachaelp

    Agree, the opto-isolation is cool. Just as a side note, not related to what I'll implement, but just wanted to share what I'd found kind-of related to gihu's suggestion, I was browsing op-amp circuits and found this really unusual one for bidirectional capability, based on positive feedback (PDF doc) http://www.ti.com/lit/an/slaa148/slaa148.pdf

    page  14.

     

    Cool thing is in an SMD circuit where just a single line needs it, you could do it with a SOT23-5 or whatever.