T30, WCE7 and C# - Bytes sent after closing COM port

Technical Support
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1

I am currently testing a T30 with an Iris board, image 1.4 of CE7, using C# and your UART demo application slightly modified.

Everything works well until I close the port. If data was sent, sometimes there is an additionnal 8 bytes sent after closing the port.

I rooted down the problem to the DiscardOutBuffer() method. If I call it after sending data I get the problem.

comPort1 = new SerialPort();
...
opened_port = comPort1;
opened_port.Write(txtSend.Text.ToCharArray(), 0, txtSend.Text.Length);
opened_port.DiscardOutBuffer();

If I send the string “testing 1-2-3” and monitor the output on a PC using Tera Term, I get “testing 1-2-33-2-3-2-”. If I send “1234567” I get “123456756745674”. However if I send a multiple of 8 bytes everything is OK.

It looks like a bug since DiscardOutBuffer() should silently flush the buffer and not output 8 bytes from the sent buffer, which should alredy been flushed after sending the data…

This occurs on all UART, baud rate and parity setting. Any idea?

2

This issue is not shown so far. Currently a lot of the engineers are at busy at the embedded world. We will have a look on this as soon as possible.

3

Looks as if you have found a bug in the Tegra 3. We tracked this down and found that this actually happens when the reset register of the UART A (COM 1) is written. We will check if we find anything in the Erata of the Tegra chip and eventually will provide a solution for this.

As soon as it is confirmed this issue will be listed as Issue 16114 on the Tegra Roadmap.

4

The description of the problem is not completely correct:

Description: When you call PurgeCom
(or DiscardOutBuffer in .NET), some
data which already have been sent out
is sent again.

Workaround: Do not use
these functions for now.

The fact is that DiscadOutBuffer is automatically called by .NET when closing a port, which is also done automatically when exiting an application. The workaround in this case would be to never exit the application… This can become a problem if the garbage data gets interpreted as a valid command by the receiver.