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Is there a problem with the RS-485 output enable signal on the LPC824?

Question asked by David Rose on Jun 27, 2017
Latest reply on May 17, 2018 by Kerry Zhou

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:


Image of transmission with output control signal


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,