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HCTEST0006 3.3V UART 
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Joined: Tue Aug 22, 2017 7:15 pm
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Post HCTEST0006 3.3V UART
Dear HobbyComponents,

I've just bought the above mentioned logic analyser from the Amazon, and I tried to check an UART communication.

On one end there is an Arduino and I see the bytes sent clearly, but I can't see anything on the RX side. The device on the other side is tolerates 5V on the UART, but it uses 3.3V.

If you could help with this, that would be great.

Thanks a lot,
Andras Csehi


Tue Aug 22, 2017 7:22 pm
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Posts: 675
Post Re: HCTEST0006 3.3V UART
Hi Andras,

The threshold voltage for the analyser is usually around 1.5 to 1.6V so it shouldn't have any problems detecting your 3.3V signal in terms of logic levels. Is your Arduino receiving the Rx signal both when the analyser is and isn't connected?

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Wed Aug 23, 2017 8:05 am
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Joined: Tue Aug 22, 2017 7:15 pm
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Post Re: HCTEST0006 3.3V UART
Hi Andrew,

Thank you for your answer.

The Arduino board can receive data on the RX pin no matter if the analyzer was connected or not.

Edit: I have done some experiment, and when I disconnected the RX from the Arduino (but left the TX connected that I could send the command) I saw the answer to the command on the Analyser, so it is working, thank you.

Now the problem is why I can't see it when it is connected to the Arduino?

Regards,
Andras


Wed Aug 23, 2017 5:16 pm
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Post Re: HCTEST0006 3.3V UART
So just to confirm, when you have everything connected your Arduino receives the data sent by the device, but the analyser doesn't see the data. However, if you disconnect the Rx on the Arduino the analyser then sees the data.

In both cases this shows that data is being sent by your device but the act of connecting the Arduino and analyser at the same time is causing an issue. My first thought would be connecting both devices is causing a loading effect. However the analyser will have a lower threshold voltage than your Arduino so if this was the case I would expect the Arduino to be the one not receiving the data. So at the moment I'm not sure what could cause this issue.

What Arduino are you using and can you confirm that you have all the grounds connected? I.e. there is a ground connection running from your device to the Arduino and the analyser and there is no possibility of a bed connection? Finally how are the device and the Arduino three being powered? Is the Arduino connected to USB ports on the same computer?

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Thu Aug 24, 2017 8:28 am
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Joined: Tue Aug 22, 2017 7:15 pm
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Post Re: HCTEST0006 3.3V UART
You are right, when I disconnected the TX from the device, the analyser started to pick up the signal.

analyser -- USB -- COMPUTER -- USB -- Arduino -- 5V - 12V converter -- Device.

I am not 100% sure that the ground on the device and the ground on the Arduino is galvanically connected, because of the step-up converter.

However I did not check the SoftwareSerial library what is the default status of the RX pin? Pull-up?

Regards,
Andras

P.S: Howvere I run through my code again peppered it with debug messages and I found why I can't see the incoming data. If you check the data validity before you read it, the result will be always invalid.


Thu Aug 24, 2017 5:02 pm
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Post Re: HCTEST0006 3.3V UART
Quote:
You are right, when I disconnected the TX from the device, the analyser started to pick up the signal.


I'm a little confused about this comment, maybe I've misunderstood something, but my understanding is that the issue is with the Tx connection from the device which sends data to the Rx pin of the Arduino. If this is the case and you are disconnecting the Tx pin on the device then no data can be sent to the Arduino or analyser?

Quote:
I am not 100% sure that the ground on the device and the ground on the Arduino is galvanically connected, because of the step-up converter.


By step-up converter do you mean you have a level shifter between the device and the Arduino to step between 3.3V and 5V levels?

Quote:
However I did not check the SoftwareSerial library what is the default status of the RX pin? Pull-up?


I can't really comment on the SoftwareSerial library as it's not one of ours other than to say that it is very likely the pullup will be enabled. The serial Tx pin should idle high anyway as the start of a new data transmission (start bit) is signaled by a high to low transition. So when data is not being transmitted you should see a logic high. This is also the default state of the analyser pins when nothing is connected to them.

If you're using a software serial interface instead of the Arduinos hardware interface then I would suspect the problem is more likely to be caused by a software issue rather than a hardware problem.


Quote:
Howvere I run through my code again peppered it with debug messages and I found why I can't see the incoming data. If you check the data validity before you read it, the result will be always invalid.


Again, I can't comment on the functionality of the softwareserial library but are you saying this was the cause of your problem or was it just something that was adding extra confusion to your reported problem?

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Fri Aug 25, 2017 8:48 am
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