JTAG-SMT3-NC bad UART character

[jmcexx]

Hi,

  I'm using JTAG-SMT3-NC for JTAG and UART on a custom board.  One of the boards has a very strange behavior when sending UART serial to the board.  The character 'U' doesn't work, but all other keys work fine.

 Digging into it a bit further, I can see with a scope that the UART signal is incorrect for the letter 'U'.  Please see attached scope plot.  The upper (white) trace is when the 'U' key is pressed.  Please compare this to the lower (yellow) is the 'A' key.  Manually decoding, the 'A' waveform is correct (0x41), but the 'U' is incorrect (0xd5 instead of 0x55).

  A few other notes:

• Same behavior on two totally separate laptops (windows10)
• Same behavior at different baud rates.
• 115200 baud, 1N8 normally used.
• The 'U' key works as expected on ~10 other boards we used.  Seems like something "happened" with this one.
• All other letters appear correct, and have their MSB cleared.

  Could anyone think what might cause this behavior?

UART trace U vs A.bmp

 

[JColvin]

Hi @jmcexx,

I am a little confused on the interpretation of the two characters; I am seeing the waveform you attached showing for 'U' as 0x55 (0b01010101) which would be correct. I have attached a labeled picture of the bits that I am seeing.

Thanks,
JColvin

[jmcexx]

Thank you for the reply.  

I think you'll find your UART decoding is backwards.  First bit should be a Low start bit (S), then 8 bit byte LSB first, followed by a high stop bit (P).  My working example of 'A' was a very poor choice, since it is the same either way.  Sorry for that confusion.

Please see below with what I see for 'S', 'T', 'U', 'V', and 'W'.  I really don't get what is going on with 'U'.  

Very strange.  I'm quite interested to understand what could cause this.  

Thank you,

  Ryan

 

[JColvin]

Hi @jmcexx,

You are quite right on me reading it backwards; I'm not sure what could cause the character to be read incorrectly though, though I have asked another engineer for their thoughts on what might've occurred.

I imagine this will be the case, but in the interest of clarification, these 10 or so boards each have their own JTAG SMT3 NC loaded onto them and were all tested in the same way through via a serial terminal of some kind on a host computer?

Thanks,
JColvin

[JColvin]

Oh, as one additional clarification that I presumed and didn't check, you are scoping pins 5 and 6 (RXD and TXD) coming from the JTAG SMT3 itself rather than somewhere else, correct?

Thanks,
JColvin

[jmcexx]

Yes, exactly.  Each board has a separate module soldered to it.  This is the only one with strange behavior.  I'm interested to understand why before we move to producing larger volumes.

Thanks,  Ryan

 

    

 

[jmcexx]

Yes, the scope plots are of pin 6 (TXD) referenced to pin 1 (GND) of the JTAG-SMT3-NC module.

[JColvin]

Hi @jmcexx,

I had asked the hardware engineer who developed the module about this, but they have never seen this type of behavior before. I know you said that you observed this behavior at the 115200 baud; do you still observe this same behavior at a lower baud rate?

Thanks,
JColvin

[jmcexx]

Thanks.  Yes, we get exactly the same behavior at different baud rates.  I know we tried at least 57600 baud.

 

 

[JColvin]

Hi @jmcexx,

We're not sure what the issue could be then. If you wish, I can work with you for an RMA of this module and if it's possible have the module sent back to us for examination; I know that desoldering those boards can be difficult if solder was used underneath the pads.

Let me know what you would prefer to do.

Thanks,
JColvin

[jmcexx]

Thank you.  I would like to send the module to you to see what you can figure out.  I'll remove the module.