Is it possible to use serial communication with Arduino thanks to a baudrate upper than 9600 ?

Good afternoon everyone, I have to send an important amount of data thanks to the serial communication, from Matlab to an Arduino mega. ( For your information : I have to send 500 bytes of data to refresh a 3D Cube. And this must be done at least 24 times per seconds for visual perception. This is the simple version. In fact, I would like to refresh the 3 colours of each Leds in my cube, namely I have to send 500*3*24 bytes per second.).

Thus, I need to send 500*24 = 12000 bytes per second ( if a data frame has 1 start bit and one stop bit, it means I need a baudrate = 500*(8+1+1)*24 =120000 bps.

So the issue is that i made some test with 'tic' and 'toc'. Before 9600 bps, everything is ok, the more the baudrate is, faster is my communication. But if I try an other usual bit transmit rate ( 14400 for instance), nothing changes : it cannot go faster than with 9600. And this is the case for each transmit rate upper than 9600.

I changed the transmit rate on The Arduino too.

Do you know why and how can I modify this "limit" ? Or do you know how can I send data as fast as I want to ?

[Walter Roberson]

Is it a real serial port, or is it serial over USB ?

[mathieu giraud]

No, I'm using the serial over USB on the Mega ( pin RX0 ). As an example, this is my matlab code :

 s=serial('com4','baudrate',9600);
fopen(s);

 tic
 for a=1:50
fwrite(s,'a','char');
 end
toc
fclose(s);
delete(s);
instrreset;

In both case ( BAUD = 9600 or 115200), I got the same result for tictoc : Elapsed time is 0.052218 seconds ( with some variations ).

Answer 1

[Walter Roberson]

Serial over USB does not pay attention to baud rate for the USB portion of the transmission. The USB portion carries the data as fast as USB can. If the data is never converted to a real serial port, then the baud rate is more or less decoration. The USB driver on the receiving end would need to buffer the data for a physical serial port at the destination.

However, serial over USB is defined by the standards to require buffering of data until either the USB packet is full (taking into account the couple of overhead bytes used to communicate the virtual pin status such as DTR), or until a timeout has occurred. As long as data is still being received, the timeout will keep getting postponed; if you come one byte of filling up the USB packet it will wait for the timeout before sending the packet. I have posted different figures for the timeout period in the past, and I seem to be having difficulty now in finding an official source. I believe you are seeing the effect of the timeout.

The USB protocol does define a "Push" request to trigger the driver to send the packet in the next available slot instead of waiting for the packet to fill. Unfortunately MATLAB does not provide any mechanism for requesting a push.

I know I read about all this before but right now I am having difficulty finding the reference information :(

If you are being affected by the wait for the buffer to fill, then there should be a magic value somewhere near 1000 bytes where the time required suddenly decreases, due to exactly filling the packet.

[mathieu giraud]

Thank you for your detailled answer. I will look after this magical transmit rate, but I don't think it will be enough for the rate I want.

So do you think is there a way to modify the depths of matlab ( or to create an add-on), and add this 'push' ? More over, how can the Arduino software send data faster with an increased baudrate? The cable is still the same. I'm a little surprised of the lack of information about this subject. As you said, I couldn't find relevant piece of information.

Finally, according to you, I will not have this issue if I use a real serial cable such as a RS232 ?

PS : Sorry for possible mistakes in my english.

[Walter Roberson]

True serial ports use on-chip queues of 2 to 8 bytes and empty the queue as soon as possible. They do not wait for buffers to fill up. You can have difficulty with the way you configure the serial port objects.

[mathieu giraud]

In any case, I have to tranmit at least 120kbps, so it seems to be the only solution. I have looked for other way to transmit data such Ethernet for instance, but it uses the MOSI pin, and I already need this one.

[Walter Roberson]

It is not a magic transmit rate, it is a magic data size being sent. Where you have

tic
for a=1:50
  fwrite(s,'a','char');
end
toc

there should be a magic size such as 510 or 512 or 998 or 1022 where suddenly the latency drops due to the packet being full and flushed immediately.

[mathieu giraud]

Finally, I've just succeeded to send my data fast enough. You can use Arduino at 512000bauds. The issue is that Matlab (or USB protocol as you said) waits around 1 ms before sending a fwrite . Insted of sending one uint8, wait 1 ms, sending an other; I send 64 bytes, wait a little, send 64 bytes ... etc.

So my communication is 64 times faster, which is enough. Thank you for your help !