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Author Topic: Measuring Arduino UNO Atmega8U2 throughput rate ?  (Read 1490 times)
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Hi all,

This post is, in some sense, a continuation of Measuring Arduino's FT232 throughput rate ?. I'm on Ubuntu Linux as described on that thread.

Well, I just tried the Uno, and had to post back...

I must say - I love that they replaced the FT232 with something more open - as now, in principle, one should be able to program the Atmega8U2 for different USB behaviors. Although, I do not particularly like that the part is so small, it doesn't look solderable by hand (maybe only with hot air gun - but even then, it will be a challenge to hold those ultrasmall pins in place).  

Unfortunately, it seems that the default edition - for now - can not support speeds up to 2 Mbps, not even close.

First of all, I'm using the exact same procedure/code as in Measuring Arduino's FT232 throughput rate ? Reply #28. Some of the differences between UNO and Duemillanove are:
  • Device is no longer /dev/ttyUSB0 - now it is /dev/ttyACM0
  • The driver in Linux is no longer ftdi-sio - now it is usbcore
  • Using Arduino 0021 to program the Uno
  • The Uno will not need an extra capacitor between RESET and GND just for the sake of it being programmed by the IDE; however any other sort of serial access by other programs (stty, writeread_bonus) again resets the Uno - so I again had to add the capacitor to run the loopback test.
Here is a code snippet which shows some data about the Uno, and then shows the result of running writeread_bonus:
Code:
$ lsusb # not much shown
Bus 005 Device 005: ID 2341:0001  

$ find /sys/devices/pci0000\:00/ -name '*tty*'
/sys/devices/pci0000:00/0000:00:1d.3/usb5/5-2/5-2:1.0/tty
/sys/devices/pci0000:00/0000:00:1d.3/usb5/5-2/5-2:1.0/tty/ttyACM0

$ for ix in $(find /sys/devices/pci0000:00/0000:00:1d.3/usb5/5-2 -type f); do echo "$(basename $ix):" ; cat $ix ; done
...
idVendor:
2341
idProduct:
0001
...
manufacturer:
Arduino (www.arduino.cc)
product:
Arduino Uno
serial:
649383233313513181C0
...
uevent:
MAJOR=166
MINOR=0
DEVNAME=ttyACM0

$ stty 2000000 inpck -ixon -icanon -hupcl -isig -iexten -echok -echoctl -echoke min 0 -crtscts </dev/ttyACM0

$ ./writeread_bonus /dev/ttyACM0 2000000 ./ttester 3>myout.txt
stdalt opened; Alternative file descriptor: 3
Opening port /dev/ttyACM0;
Got speed 2000000 (4107/0x100b);
Got file/string './ttester'; opened as file (102400).
write_thread_function spawned

   EAG: 0, WR: 1, WF: 0, read: 102400, FRD:    59, TOU: 14080, OBS: 18944, RB/s:   14622.06, WB/s:  13019.38    
+++DONE+++
Wrote: 102400 bytes; Read: 102400 bytes; Total: 204800 bytes.
Start: 1286748859 s -251915 us; End: 1286748861 s 708822 us; Delta: 2 s 960737 us.
2000000 baud for 8N1 is 200000 Bps (bytes/sec).
Measured: write 34585.98 Bps (17.29%), read 34585.98 Bps (17.29%), total 69171.97 Bps.

Notice that - in spite of write gauging (done in respect to 2 Mbps, though) - the writeread_bonus program still detected an output buffer size of 18944 bytes (this number can, however, change between runs); and the effective speed settled at some 34 KBytes/s (some 17% of requested 2 Mbps).

I'm not sure if this is due to:
  • hardware (whether Atmega8U2 can at all support USB->serial at 2Mbps purely electrically), or
  • a limitation of the current firmware on the Atmega8U2, or
  • a limitation of the usbcore driver
If this is a limitation due to firmware or driver, I believe that it opens a very interesting topic for discussion - as then it is in principle possible to achieve 2 Mbps also with the Atmega8U2, as with the FT232 smiley Which is why, I thought the case of the UNO/Atmega8U2 throughput merits its own thread smiley

Well, I'd love to hear any responses on this,

Thanks,
Cheers!

« Last Edit: October 10, 2010, 05:53:57 pm by smd » Logged

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Well, to start with the 8U2 is a 16MHz AVR, so its UART probably completely tops out at 2Mbps (and perhaps at the 1Mbps max of a non-U2X configuration.)  And then all of the arguments that people have made about how few cycles a 16Mhz AVR will have to run 2Mbps async will apply twice.  Or maybe 4 times.

There are lots of hits on "LUFA AVR Throughput."  Have fun :-)
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Hi westfw,

Quote
Well, to start with the 8U2 is a 16MHz AVR, so its UART probably completely tops out at 2Mbps (and perhaps at the 1Mbps max of a non-U2X configuration.)  And then all of the arguments that people have made about how few cycles a 16Mhz AVR will have to run 2Mbps async will apply twice.  Or maybe 4 times.
Thanks for that - will be good to have it in mind.. Heck, I think even if 1 Mbps can be demonstrated, its all good smiley

Quote
There are lots of hits on "LUFA AVR Throughput."  Have fun smiley

Thanks for the query - lots of new terms to keep track of, such as LUFA (Lightweight USB Framework for AVRs)...

In any case, I browsed through the ATmega8U2 datasheet and it claims: "USB 2.0 Full-speed Device Module with Interrupt on Transfer Completion ... 48 MHz PLL for Full-speed Bus Operation : data transfer rates at 12 Mbit/s" - so maybe it will be possible to transfer some of that to the ATmega as well smiley

In any case, now I have a good starting point in looking into this further, should the need arise...

Thanks,
Cheers!
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