Serial.read has garbled data after a Serial.write

Question

I'm using the Serial1 of the Arduino Leonardo to send and receive data with another MCU. The following code will always retrieve garbled data:

byte streamReadResponse;
Serial1.begin(115200);
Serial1.setTimeout(9000);
Serial1.print(cmd);
Serial1.print('\r');
Serial1.flush(); //wait for all the data to be sent to the serial
streamReadResponse = Serial1.readBytesUntil('>', data, dataLength);

But, if I add a delay(50) before the Serial1 read, I would get the expected output. The value 50 was found by trial and error.

What am I missing? Why is the flush() command not working as expected?

• drodri: Could you please tell more about "garbled data". You mean what it is inside your "data" variable? What is the final value of streamReadResponse after the call to readBytesUntil with and without the delay?
• imjosh: Try putting a ~2.2k pull-down resistor on the Arduino's RX pin (connect the resistor to ground and the pin).
• jfpoilpret: Which version of Arduino IDE are you using? It seems HardwareSerial.cpp has changed a lot along different versions, in particular the flush() method. In version 1.0.5 it should be OK normally.
• Chris Stratton: This question really is not answerable without more details, such as the nature of the garbled data as already requested. It appears to have been abandoned and should probably be closed.
• Roberto: @ChrisStratton you are right - I was not even sure on how to provide the details - I still hope my own answer can help other people.

Answer 1

I've finally found that when the MCU to which Arduino is talking to was initialized, it would send spurious data to the Serial - I guess it would start first with a lower baud rate, and then switch to the programmed baud rate of 115200.

To fix this, I had to flush the Arduino serial RX before sending any data. So:

byte streamReadResponse;
Serial1.begin(115200);
Serial1.setTimeout(9000);
while (Serial1.read() >= 0)
 ; // do nothing
Serial1.print(cmd);
Serial1.print('\r');
Serial1.flush(); //wait for all the data to be sent to the serial
streamReadResponse = Serial1.readBytesUntil('>', data, dataLength);

Answer 2

Usually, this kind of issue is the result of the several times by time unity of loop statements execution, with the baudrate defined for your serial i/o it's very possible that the flush are override by the new information input and the result are the "garbage" on that.

On my sketches, ever I put "idle function calls" in some points of the code to avoid this and works succesfully.

• Cybergibbons: I don't understand why this would be. Surely .flush() should do as documented unless Roberto has overridden it?
• JohnnyDeauville: I don't know why, but, for example, for prevent in a project the malfunction of a clock module, I must call to read it fewer than times, when the documentation not tell nothing about this.The fact is that many times the execution speed of the main loop can make some features of functions and/or response of devices will be wrong. As if you push data onto a funnel and arrives to the exit mistaken.
• imjosh: .flush() pauses execution until the output buffer is empty. It shouldn't have anything to do with garbage on your input. Try putting a ~2.2k+- pull-down resistor between serial-in and ground to make sure you aren't getting noise or cross talk in the input buffer.