Re: [libusb] less data length in async bulk transfer with 2 cameras

[Tim R. <ti...@pr...>]

zheng rong wrote:
>
>     Are you using a Cypress FX2?
>
> Exactly, I am using FX2LP CY7C68013 with a CMOS camera.

Why are you using libusb, instead of writing a video4linux driver that 
could work with all applications?


> I am using Ubuntu OS on a DELL laptop workstation.
> How did you fix the problem finally?

We addressed it in three ways.  First, my client's camera produced UYVY 
data, which has 2 bytes per pixel.  They wanted 720x564 data, but at 720 
width, the FIFOs would overflow on all but the most well-behaved 
systems.  So, we added hardware to limit the output to 640 width.  I ran 
a Python simulation at the microsecond level that showed that this was 
safe, in the absence of pathogenic behavior.

Second, we added a bulk endpoint.  A bulk pipe has more available 
bandwidth than an isoch pipe, but it is also more dependent on bus 
load.  Note that you can do this on the fly by using a different 
alternate setting.

Third -- the real solution -- was to spin the camera board to add a 32k 
external FIFO chip.


> The camera in may design has resolution of 752x480.
> When pixel clock is 26.6MHz, the line-valid interval A= 28.2us, 
> line-blank interval Q = 3.52us, shown as the above figure.
> So a 125us micro-frame will cover 3.94 *(A+Q), and the device will get 
> 752*4=3008 bytes. It looks that this is a little dangerous, right?

It's close, but it should fit.


> Also, as you mentioned, if it miss one micro frame, the data will be 
> lost because the device buffer is not very much (4KB) to keep the data 
> until the next micro frame. right?

It's a danger, but a small one.  In the real world, isoch devices don't 
drop packets, unless your USB cables are poor quality.  I've run cameras 
for days at a time and never dropped a packet.


> Back to the original problem, for bulk transfer mode, increasing the 
> buffer size on device side can guarantee smooth data transfer.

I've now forgotten your original problem.  If you set up the FX2 to use 
quad-buffering on the bulk endpoint, you should not overrun the FIFOs 
unless the bus is busy.


> What is the appropriate usage of isoc transfer mode? also require 
> increasing the device buffer?

I don't understand the question.  For bulk, you'd set EP2CFG to 0xE0 for 
quad 512-byte buffers.  For isoch, you'd set EP2CFG to 0xD8, for quad 
1024-byte buffers.

-- 
Tim Roberts, ti...@pr...
Providenza & Boekelheide, Inc.