On Mon, Jun 20, 2011 at 3:26 AM, Uwe Bonnes
<bon@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>>>>>> "Caleb" == Caleb Kemere <ckemere@xxxxxxxxx> writes:
> Uwe> Otherwise, how do you read on the PC side? Do yo do asynchronous
> Uwe> reads like in libftdi-1.0/src/ftdi_stream.c?
>
> Caleb> I did test the asynchronous mode, but now I think I've distilled
> Caleb> the essence of the test down to the following really simple
> Caleb> (blocking) code:
>
> Caleb> int transferred = 0; int bytes = 0; while (bytes <
> Caleb> MAXIMUM_TRANSFER) { last_result =
> Caleb> libusb_bulk_transfer(ftdi.usb_dev, ftdi.out_ep, buffer,
> Caleb> bufferSize, &transferred, 1000); if ((last_result > 0) |
> Caleb> (transferred == 2)) break; else bytes+= transferred; }
>
> Caleb> I've been messing around with different values for the
> Caleb> read-chunksize of the FT2232H and the bufferSize of the bulk
> Caleb> transfer. As a reminder, my data streams from the FPGA to the
> Caleb> FT2232H at 6.5 MB/s. With that, I can consistently read for about
> Caleb> a 30 s or so before I get a buffer overflow (at half speed, I can
> Caleb> go for about 3 times that long). It seems like the USB subsystem
> Caleb> should be able to handle that data rate for an arbitrary length
> Caleb> of time, so I'm wondering what I could be doing wrong...
>
Uwe> Otherwise, why do you stop when you read 2 bytes? The FTDI will
alway return
Uwe> 2 status bytes, so this is a legal value which might happen sometimes.
I my case, a two byte message indicates that the transfer buffer on
the FT2232H is empty, which happens when the FPGA stops transmitting
because it has detected a buffer overflow previously. Unfortunately,
for our application, we can't tolerate lost data (or rather, we need
something like 99.9%), hence trying to debug.
I have noticed something which I don't understand - on my laptop (Core
2 Duo, about 3 years old), I get the performance described above. On
my desktop (Core i5, about 2 years old), I can run for many minutes
without ever running into trouble. Is there some way of understanding
what the USB on the laptop is doing differently?
thanks!
– caleb
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