LPC1518's phenomenon of abnormal about RS485

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LPC1518's phenomenon of abnormal about RS485

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shawn_ni
Contributor I

Hi guys;

    I have some questions of RS485, in using NXP's MCU(LPC1518). Specific as follows:

   

     We configured the RTS signal to provide an output enable signal to control an RS-485 transceiver(USART_CFG[20] = 1).

      But, we found the phenomenon of abnormal that DE would be Low at Sending Data Operation incidentally.It continue about 6us.

      Could you have some suggestions about it ?

      Thank you very much.

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FelipeGarcia
NXP TechSupport
NXP TechSupport

Hello Shawn,

 

You can modify the polarity of the output enable signal using the OEPOL bit in USART_CFG[21].

 

Have a great day,
Felipe

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shawn_ni
Contributor I

Hi Felipe;

      Thank you for your suggestion .

      However, the problem still exists(only Low become High).

      I uploaded the detailed waveform pictures for reference.

      Can you  help us analysis this problem again ?

Thank you very much,

ShawnRS485-DE_Wave.png

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FelipeGarcia
NXP TechSupport
NXP TechSupport

Hello Shawn,

 

I did some testing on my side using LPCXpresso1549 and I found the following behavior in the RTS signal when transmission is taking place, please see snippet below:

pastedImage_4.png

Could you please confirm if this is the same issue you found on LPC1518?

 

Best regards,

Felipe

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shawn_ni
Contributor I

Hi Felipe;

      Yes, that's the same issue.(only diffrent in OEPOL bit set)

      And we did the same testing on LPC1751.

     The phenomenon of DE(RTS) is always High When transmission is taking place .

     Waveform figure as below  for reference to you:

RS485_DE_20190712141412.png

   

     We are looking forward to your advice~

Best regards,

Shawn

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FelipeGarcia
NXP TechSupport
NXP TechSupport

Hi Shawn,

I am going to ask about this issue internally. This seems to be a design error but I will let you know as soon as I have a response.

Best regards,

Felipe

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