I have been looking at using an LPC824 instead of an LPC812 to connect to an RS-485 communication line.
Specifically, I am now testing the functionality provided by the RS-485 output enable signal. Section 13.5 or the user manual states:
When RTS signal is configured as an RS-485 output enable, it is asserted at the
beginning of an transmitted character, and de-asserted either at the end of the character,
or after a one character delay (selected by software).
What I am seeing is basically as described, but with a very small pulse in the signal between the end of one character and the start of the next.
Here is a logic analyzer trace showing what I describe:
The top trace shows the six characters being transmitted.
The third trace shows the RS-485 output enable signal - With the pulses in question.
The second trace shows a signal I create with a GPIO - functionality exported from an LPC812 project.
I can possibly put something in the line to filter this glitch out, but I don't see that it should be necessary,
Hi David,
Thanks a lot for your project.
I have tested your code on my LPCXpress lpc824 board, this is my test wave, a little different with you:
But I think it is not the problem, as you have post, the user manual already say:
So, I think the RTS wave is correct. The glitch just means one byte is transmit, then the RTS is de-asserted. I think this glitch won't influence your usage, do you meet some problems with this glitch?
About LPC802, I still don't have that board, I will find a board and test it again.
From the LPC802 user manual description, it should also have that glitch.
Any updated information from my side, I will let you know!
Have a great day,
Kerry
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Hi Kerry,
Looks like this has gone quiet. I guess you've not managed to acquire an LPC802 or LPC845 for testing. They're easily available in the UK.
Probably sensible for me to provide a little more evidence for you. Here are waveforms using the same setup from an LPC802:
With the OETA bit clear.
With the OETA bit set
As can clearly be seen, there is no pulse in the Output Enable signal between bytes.
Both my capture and yours from the LPC824 showed pulses in the Output Enable (RTS) signal between bytes. You mentioned that the pulses were to be expected and you would expect to see them on the LPC845 and LPC802. Since no pulses are present on the LPC802, there is clearly some mismatch and therefore something is incorrect.
The action I see on the LPC802 makes much more sense and matches what I've seen on other devices in the past, so I still believe it is the action of the LPC824 that is incorrect.
Your comments on this would be appreciated.
Thanks.
Hi David Rose,
Thank you for you code, and thank you for your patience.
I have got all the LPC802, LPC824, LPC845 board on my side, today, I test it with your code in all the lpc802, LPC824, LPC845 board, the same uart configuration:
LPC_USART0->CFG = (UART_CFG_ENABLE) // Enable UART operation - Specify parameters
| (UART_CFG_DATALEN_8)
| (UART_CFG_PARITY_NONE)
| (UART_CFG_STOPLEN_1)
| (UART_CFG_OESEL |UART_CFG_OEPOL |UART_CFG_OETA);
and check the user manual again.
I highly agree with your point, the LPC824 RTS wave is really abnormal.
Because, when OESEL=1, OETA=1, should like this in the user manual:the Output Enable signal remains asserted for 1 character time after then end the last stop bit of a transmission.
But from this WAVE:
After stop bit, the RTS should still wait for 1 character, but in LPC845, the RTS is desserted.
Anyway, I will report this problem to our according department, and thanks a lot for your good question.
Here, I post the LPC802, lpc824, lpc845 test wave, just for the post completeness.
1. LPC802
3. LPC845
If I get any updated information, I will let you know.
Have a great day,
Kerry
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Hello David Rose,
Actually, from the LP824 user manual description, I think this phenomena is the normal result.
As you know,
It is a transmission, so it should one byte transfer, that's why you have a de-assert after each byte.
Now, whether this De-assert influence your normal 485 communication?
If your hardware still working OK, I think you don't need to mind it. Otherwise, just as the user manual said:
You can use the GPIO as the RTS direction pin.
Wish it helps you!
If you still have question about it, please let me know!
Have a great day,
Kerry
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