Arduino Mega ADK Serial.print stale data issue

Hi!

I was using a sketch that sends some sensor data over Serial to the computer, and had a python script to read it. It was running on a Diecimila-equivalent board. Now I have an Arduino Mega ADK, and run the sketch on that one. The problem I have, that first when read the serial on the computer, all I receive is a lot of stale buffer, then the board resets, then starts to read correctly.

To illustrate it, one example output reading the data received in on the computer side:

#HotJunction(C),ColdJunction(C)
31.25,0.69,9046
31.25,30.69,9548
31.25,30.69,10050
31.25,30.69,10553
31.25,30.69,11056
31.25,30.69,11558
31.00,30.69,12061
25,30.69,9046
31.25,30.69,9548
31.25,30.69,10050
31.25,30.69,10553
31.25,30.69,11056
31.25,30.69,11558
31.00,30.69,12061
0.00,0.00,0
30.25,30.25,501
30.25,30.31,1004
30.50,30.37,1506
30.50,30.44,2009

The first line is the header, printed after I connected to the board, but haven't read anything yet. Every line then is 3 comma-separated values: sensor reading, another sensor reading, finally a timestep in milliseconds since bootup. E.g the data line 31.25,30.69,9548 would be Sensor1=31.25, Sensor2=30.69, timestamp=9.548s since bootup.

Looking at the data, what I actually received:

• 7 lines of stale data (lines 1-7)
• A broken repeat of that stale data (lines 8-14)
• Correct data after reboot (lines 15 up)

The amount of stale data is different run by run.

Where does this behaviour come from? I feel it's some buffering issue. Is there a way to avoid it? I guess it's a Mega ADK issue, because with the old board I haven't seen this. Might have to do something with the fact that there are so many serial lines on that board, and I guess the firmware control is different?

What I do now, is that I throw away whatver I read in the first 10ms of starting the computer-side script. It fixes things in a hackish way, but I'd love to understand what is actually going on there, and have a better way to do that.

Code?

Does the Mega ADK print out the header "#HotJunction(C),ColdJunction(C)" in your setup function? Yet it is not re-printing it on the reset?

The header is in the receiver code (so I can pipe it into a file just like that), and should signify the point where my PC is ready to receive the data.

The code on the Mega ADK is this one: sketchbook/Thermocouple5.ino at master · imrehg/sketchbook · GitHub

The receiver code is something like this in Python (with just the relevant parts shown):

dev = serial.Serial('/dev/ttyACM0', baud, timeout=1)
print "#HotJunction(C),ColdJunction(C)"
while True:
    print dev.readline()

I got around the problem a bit by throwing away the first 50ms data or so:

dev = serial.Serial('/dev/ttyACM0', baud, timeout=1)
delay = 0.05    # seconds
finish = time.time() + delay
while time.time() < finish:
    dev.readline()   # just reads but not prints
print "#HotJunction(C),ColdJunction(C)"
while True:
    print dev.readline()

In this case the buffer is received but not printed first, then later started to print the real (new) data.