1
\$\begingroup\$

I have UART devices where I can chose various non-standard speeds, but none of them are matching. There are however some speeds, that are a multiple of the other device's speeds. For instance, I could have the rates 150'000 on the receiving end and and 3'000'000 on the sending device. Communication is only in one direction.

Do you think it is possible to hack something together to have the sending device send every bit multiple times and therefore emulate a slower baud rate? I think I would have to disable all options like start- and stop-bits and implement them manually in my fake protocol, so I can stretch these out as well.

Does anyone have experience with this? Could it possibly work? Or am I missing something?

EDIT: To clarify, I am not hiding anything, the numbers are correct: I actually want to talk to ESCs on a drone that expect either 150'000, 300'000 or 600'000 bits/s (DSHOT protocol). The board/micro controller I want to use to control the ESCs does not support these rates, but it would offer a custom baud rate of 3'000'000 bits/s (= 5 x 600'000). It offers others as well, but none match any DSHOT baudrate nor are they a nice multiple. So simply put, the idea is to send every bit 5 times and thus have the faster rate appear 5x slower. Does that make sense? Hence I would have to implement features like start- and stop-bit myself and have them sent 5x instead of once for example.

\$\endgroup\$
  • \$\begingroup\$ How much they differ? Two UART devices may be able communicate with each other if their rate is within about 3%. Otherwise you are out of luck with the UART device, but you may implement it in software by bit-banging. But nowadays it shouldn't be necessary (UARTs are usually flexible enough in their capability to set up arbitrary data rates). Could you tell what are the devices in question? (Someone might have some knowledge of them to help) \$\endgroup\$ – Jubatian Nov 21 '17 at 8:13
  • 1
    \$\begingroup\$ You might be missing something. This is a pretty odd situation, and I think you will get far better solutions if you don't hide what you're trying to configure, and give us the baud rates or equations to calculate them. \$\endgroup\$ – pipe Nov 21 '17 at 8:34
  • 2
    \$\begingroup\$ Hacking a hardware UART to do this will probably fail. If you implement a software UART on the corresponding pin, you can get those baudrates. But I've not encountered a micro with such restrictions on the UART - care to share which one it is? \$\endgroup\$ – Arsenal Nov 21 '17 at 13:21
  • 2
    \$\begingroup\$ How about stating what board/microcontroller you are talking about. If possible, also state the frequency(frequencies) of it's oscillator(s). \$\endgroup\$ – Tut Nov 21 '17 at 13:25
  • 1
    \$\begingroup\$ I'd be highly surprised if it can do a custom rate of 3M but couldn't do custom rates of 150k or 300k or 600k. Your proposed idea of running 5 times faster forgets that there are start and stop bits in UART, so if your data doesn't happen to have a 1 where the stop bit needs to be and 0 where the start bit should be it won't work. Plus you can't guarantee that the MCU won't leave gaps between each packet. \$\endgroup\$ – Tom Carpenter Nov 21 '17 at 13:29
2
\$\begingroup\$

Serial data is just a series of 1's and 0's on the wire so it is possible to synthetically make one data stream out of the bits of another by mimicking bit patterns...

https://wp.josh.com/2014/09/03/inside-neouart-tricking-a-serial-port-into-being-a-signal-generator/

But it is usually not as simple as just doubling the sender's baud rate and sending each data bit twice because there usually other bits in the stream besides the data bits like the start, stop, and parity bits.

enter image description here

It is often possible to work around these limitations, but the strategies are specific to your sending hardware (can you change the bits? the framing? the parity? on the fly?) and you application (can you live with not being able to send some data values and instead changing them to nearby values that fit into the allowed patterns?).

If you give more info on your hardware and protocol we can better help brainstorm ideas to get there!

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.