Transmit - UART from Scratch - Part 1

This is the first part in a new side series to implement a TTL UART from scratch. I've changed my mind several times on adding a serial port to my homebrew cpu but I'd already given it quite a lot of thought. I figure it would be useful to build one even if I don't use it for the final build.

In this video I build a simple transmit circuit, it constantly sends a single character (Or the output of a counter), not hugely useful in it's own right but a good foundation to build on.

For test and demonstration I use an FTDI usb to UART cable, you could just as easily connect it to a Raspberry PI, an Arduino or a regular serial port (with a level shifter).

[cbmeeks]

I think this is one of your best videos. You essentially created a crude UART with only four chips and a canned oscillator.

[brandonmack]

I look forward to the receive and interface videos.

And yes, definitely add this to your CPU! That'll be very cool.

[olavl]

Lovely bit of tinkering. And yes to all the suggestions ;-)

[AfterHoursEngineering]

That is soooo cool! I would have never have thought of doing that. :-)

[jerril]

Thanks James. This is a perfect companion to the CPU. UART is one of those terms I heard way back, I know their function, but never really considered it from this perspective.Take Care.

[louisferreira]

These videos are really good and informative. What would really help those who are still learning, is if you added a Shematic at the end of the video so it makes it easier for us to understand whats going on.

[kyleeames]

I watched this thinking it was part of the planned enhanced IO for the CPU build. That would be so cool. In any case, asynchronous serial data transmission is quite an impressive feat. I look forward to seeing what you do next :D

[twobob]

Kind of thing you could cobble together with bits (no pun intended) after the world ends to interface with old gear. Very nice explanation

[poojasharma-rwfm]

Thank you sir for your CPU series you showed us practically how CPU works....we appreciate your effort....god bless you...

[uptech]

You’re awesome! And I love it by the minute even more of your work!

[FrankGevaerts]

Lucky you! I had to change some software as recently as two years ago to support 7 bit serial :)
Nice work!

[andrewwatts]

Lovely experiment, and yes to more tests!

[cedAuz]

Great as always James. To answer your question, I'd like to see you build a full fledged bi-directional serial port connected to your cpu (using interupts for instance) so that you can have some sort of serial user interface to it. It's ok for me if you hardwire the speed such as 115kb/s 8N1 because there is no great value in handling all the details of variable baud rate or parity or other. But then you would need some sort of CPU supervisor to handle the interface.

[Architector]

Awesome! Now i'm waiting for the receiver part!

[Homebrew_CPU]

Very nice! Makes me feel just a little guilty for using off-the-shelf UARTs for my Magic-1 HomebrewCPU.

[lawrencemanning]

Awesome as always. Receive is going to be a bit more tricky I think, since you have the synchronisation to deal with, ie the loading has to start on an external pulse. I'd like to see all of the build in at least this level of detail. :) Makes me want to write a simple UART in VHDL, something I've not yet attempted.

[alistairpotts]

Lovely video, as usual, thank you! I'd like to see this getting hooked up to your CPU, mainly because I want to see how you handle buffering, interrupts and crossing clock domains.

I see a couple of other people have commented on the Elegoo breadboards, and I'd second their concerns. I've used 15 of them so far in my CPU prototyping, and about half have been fine, the other half terrible (very poor contacts that make or break contact as you add components at the other end of the board, for example). The BB830 ones I switched too recently were a revelation. Fingers crossed yours don't give you trouble.

[michaelheuss]

I’d love to see a UART circuit hooked up to your CPU with code to echo the input out to your little LCD display. It would be neat and probably very useful for future coding projects on it.

[PaulasTechStuff]

FAB! would love to see your CPU sending & receiving data from serial.

[PeetHobby]

Cool, maybe I will use this principal for my transistor-based computer.
7:29 Indeed, any quad flipflip out the 7400 serie will do the trick, no special need for a 74**193, I guess that a 74**193 costs more because the extra circuitry inside.