Calculated UART Baud Rate Does not Match the Baud Rate Expected by the Receiver

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I am trying to decode a software UART from a microcontroller on my oscilloscope. To do this, I am probing the Tx line with reference to ground, and using the decode feature on my oscilloscope (Siglent SDS 1104X-E). To configure the UART on the oscilloscope side, I have to choose a custom baud rate, as the baud rate of my software UART isn’t fine tuned properly. To do this, I measure the period of a bit in the UART data. Then, using the period I simply divide 1bit by the period to get a value in bps.

For example, in some UART data that I had sent, I measured the length of 1 bit to be 0.2208ms, so

(1 bit) / (0.2208 * 10^(-3) s) ~= 4529 bps

I then set the custom baud rate on my oscilloscope to this value, and press the decode button expecting to see a nicely decoded signal; however, the receiver appears to have received garbage as seen below

What confuses me is that if I dial down the baud rate to around 4200 Baud, the signal is decoded perfectly as seen below

This value is so far off from the actual baud rate that I calculated from the signal, so what is going on here? The data is accurate and clean, but the baud rate required for decoding it is so far off.

I would also like to note that I don’t believe that this is an issue with the oscilloscope, because when I send the data to my PC through an FTDI breakout, and monitor it with

screen /dev/ttyUSB0 4529

I receive garbage data. The characters received are unknown and result as the unknown ascii character symbol; however what’s different in that scenario is that even if I change around the baud rate for screen, it is still unable to decode the data no matter what I change it to. What’s even weirder with that is that I am receiving the correct number of characters for whatever I send, its just the characters themselves are for some reason unrecognizable, even though the oscilloscope can decode them fine. So I am very perplexed as to what is going on here.

EDIT 1:

I have since gotten the baud rate up to about 4735bps which is within 2% of the standard 4800 bps, and I tried it with the oscilloscope, and it was still unable to decode it. It wasn’t until I manually dropped the reciever baud rate to about 4400bps on the oscilloscope, that it was able to decode the data.

Furthermore, I am convinced that it is not an issue with the oscilloscope, as I used a terminal program (picoterm) and tried reading the data that way. I connected with 4800 baud set, and it was unable to read the data, yielding only the unknown ascii character. I then dropped it to 4400 baud in the program, and it was able to receive the data just fine. So there is something with this stream of data which makes its baudrate less than the baudrate which it is calculated to be…microcontrolleruartcommunicationoscilloscopebit-bangShareCiteEditFollowFlagedited 5 hours agoasked 7 hours agoKalcifer18199 bronze badges New contributor

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Answer

I usually use my scope, Rigol 1054Z, to display perhaps 10 characters or more of say 0x5a, 0xa5 etc, then zoom in to two or one characters. One character is 10 bits, say, so I can easily calculate the baud rate.

Update 1

If for standard baud rates, all is well, it is only custom baud rates that cause trouble. Perhaps the scope needs more longer time, or say more than 1,000 bytes to do the sync/auto-bauding job.

By the way, are you sure your software created?, bit banging? cpu master clock dividing?, fiddling, custom baud rate of 4k2bd is precise and steady?

References

(1) SDS1000X User Manual – SIGLENT

Appendices

Appendix A – Serial Decode Setup

Appendix B – Decoding Test Results

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