I have a Full Speed device that specifies the max packet size as 256 bytes. This is not USB compliant since the maxiumum packet size for a Full Speed Device should be 64 bytes. I can read (ReadFile) and write (WriteFile) to the device just fine, but I'm wondering if there could be issues that could arise that I'm just not seeing other than maybe a performance hit from writing across multiple usb frames (1ms)? I'm not really a USB expert, so any advice will be appreciated.
This is whats called the "compliant by hope" strategy.
From experience I can tell you that your device will crash a wide range of embedded hosts and cause corruption on others. (buffer overflows on most controllers where expected packet size is 64 and poor software is used.
These include different setup boxes, phones, etc.
Also, hacks like these, that work with a Nec hcd, might not work with an Intel one.
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I am working with an Arducam OV2640 to capture images and transmit from one microcontroller to another. I am getting inconsistent images. Occasionally they turn out ok but a large portion of the time they have a 'bogus Huffman Table'.
I am familiar with Huffman tables which has me guessing that I am losing some bytes in transmission be it from the camera to the micro-controller or from the wireless link between the micro-controllers I am using.
The only thing that has me confused is that I tested several thousand packets over the communication link between the two micros and I have a BER of zero (16 byte packets with 16 bit CRC with packet rejection and re-transmission if errors occur).
The image is also fine when I transmit it from the camera micro to my computer through UART.
Is the camera occasionally having issues? I have seen it mentioned as a problem but have no idea how this might be resolved.
I've been given the task of getting ADC samples onto an embedded linux computer at the highest rate I can (up to about 300kSPS). I am playing with several different platforms (odroid, edison) but easrly on I realized the limitations of using the build in ADCs from within linux and timing (I am relativly new to this).
Right now I am reliably getting 150kSPS using a teensy 3.2 with a very basic swapping buffer, a PDB, and the USB connection. USB writes take 2.5usec no matter my buffer size so any faster and the ADC read interrupt collides with the USB and I get nothing.
My question is: Would using an external ADC chip enable faster speeds? I see chips on Digikey and Mouser advertising 600kSPS and higher with SPI and even parallel outputs... but I fell like the bottleneck is the teensy with USB writes. Even if it could (and I am sure it could) read values 600k times a second how do you get it onto the computer without falling behind?
also, it is for long term collection so I can't just store everything and write it once the collection is over. The edison has a built in microcontroller, but no SPI implemented yet.
Edit:
To clarify, my question is weather there is any way to get large amounts of data very fast into my embedded linux device programmatically or is there some layer between a fast SPI device and the comptuer that I don't know about. So far my mentors have suggested I 1) learn to write a device driver for the SPI device or 2) recompile an image with RT_PREEMPT.
I am a physicist, and I had a revelation a few weeks ago about how I might be able to use my personal computer to get much finer control over laboratory experiments than is typically the case. Before I ran off to try this out though, I wanted to check the feasibility with people who have more expertise than myself in such matters.
The idea is to use the i/o ports---VGA, ethernet, speaker jacks, etc.---on the computer to talk directly to the sensors and actuators in the experimental setup. E.g. cut open one side of an ethernet cable (with the other end attached to the computer) and send each line to a different device. I knew a postdoc who did something very similar using a BeagleBone. He wrote some assembly code that let him sync everything with the internal clock and used the GPIO pins to effectively give him a hybrid signal generator/scope that was completely programmable. It seems like the same thing should be possible with a laptop, and this would have the additional benefit that you can do data analysis from the same device.
The main potential difficulty that I foresee is that the hardware on a BeagleBone is designed with this sort of i/o in mind, whereas I expect the hardware on a laptop will probably be harder to control directly. I know for example (from some preliminary investigation, http://ask.metafilter.com/125812/Simple-USB-control-how-to-blink-an-LED-via-code) that USB ports will be difficult to access this way, and VGA is (according to VGA 15 pin port data read and write using Matlab) impossible. I haven't found anything about using other ports like ethernet or speaker jacks, though.
So the main question is: will this idea be feasible (without investing many months for each new variation of the hardware), and if so what type of i/o (ethernet, speaker jacks, etc.) is likely to be the best bet?
Auxiliary questions are:
Where can I find material to learn how I might go about executing this plan? I'm not even sure what keywords to plug in on Google.
Will the ease with which I can do this depend strongly on operating system or hardware brand?
The only cable I can think of for a pc that can get close to this would be a parallel printer cable which is pretty much gone away. It's a 25 wire cable that data is spread across so that it can send more data at the same time. I'm just not sure if you can target a specific line or if it's more of a left to right fill as data is sent.
To use one on a laptop today would definitely be difficult. You won't find any laptops with parallel ports. There are usb to parallel cables and serial to parallel cables but I would guess that the only control you would have it to the usb or serial interface and not the parallel.
As for Ethernet, you have 4 twisted pair with only 2 pair in use and 2 pair that are extra.
There's some hardware that available called Zwave that you might want to look into. Zwave will allow you to build a network of devices that communicate in a mesh. I'm not sure what kind of response time you need.
I actually just thought of something that might be a good solution. Check out security equipment. There's a lot of equipment available for pc's that monitor doors, windows, sensors, etc. That industry might what your looking for.
I think the easiest way would be to use the USB port as a Human Interface Device (HID) and using a custom built PIC program and a PIC that includes the USB functionality to encode the data to be sent to the computer and in that way be able to program it independently from the OS due to the fact that all mayor OS have the HID USB functionality.
Anyways if you used your MIC/VGA/HDMI whatever other port you still need a device to encode the data or transmit it, and another program inside the computer to decode that data being sent.
And remember that different hardware has different software (drivers) that might decode the raw data in other odd ways rendering your IO hardware dependent.
Hope this helps, but thats why the USB was invented in the first place to make it hardware and os independent.
We have a system based around an Atom Z510/Intel SCH US15W Q7 card (running Debian Linux.) We need to transfer blocks of data from a device on the Low Pin Count Bus. As far as I know this chipset does not provide DMA facilities, meaning the processor has to read the data out a byte at a time in a software loop. (The device driver actually implements this using the "rep insb" x86 instructions so the loop is actually implemented by the CPU if I understand correctly.)
This is far from optimal, but it should be possible to hit a transfer rate of 14Mb/s. Instead we can barely manage 4Mb/s with transactions on the bus no closer than 2us apart even though each read to the slave device is is done in 560ns. I don't believe other traffic on the bus is to blame, but am still investigating.
My question is:
Does any one know if there are any configuration registers on the SCH that could affect the LPC bus timing?
I cannot find any useful information on the device on the Intel website, nor have I spotted anything in the Linux Kernel code that appears to be fiddling with any such registers (but I'm a noob when it come to Linux Kernel stuff.)
I'm not an x86 expert so any other factors that might come into play or any other 'war stories' relating to this device would be good to know about too.
Edit: I have found the datasheet. I've not seen anything in it that explains this behaviour, but I am investigating the possibility of mapping our device as a firmware device as the firmware bus cycles don't seem to suffer the same delays.
For the record, the solution was to modify the FPGA firmware such that the chip's data in/out register was mapped to four adjacent addresses and the driver modified to do 32 bit inb/outb instructions. Although the SCH does not implement 32 bit LPC read/write operations, the result is 4 back-to-back 8 bit operations followed by the same dead time as I was getting previously with a single byte, meaning it averages about 1us per byte. Not ideal, but still a doubling in throughput.
It transpires the firmware cycles were quicker because the SCH transfers 64 bytes at a time from the firmware flash - after 64 bytes there is the same 1.4us gap, indicating this is the per-transaction latency of the device. Exploiting this may have been slightly quicker than the above solution however the trade-off is that it is limited to 64 bytes chunks and each byte takes longer (680ns IIRC) due to the additional cycles required to do a firmware read.
I'm looking into connecting multiple low resolution USB webcams to a single computer. What implications might this have on performance? How does, for example, four 320x240 cameras fare against a single 640x480 camera? I'm not well versed in the architecture of the USB interface, what are the performance caveats? By performance I mean how would it affect the time to read the image data from multiple cameras compared to a single one.
Each webcam is connected to a different USB port? If so, its good.
Even if its just 1 port with 4 connected webcams. I dont think 4 320x240 will have any problem either. USB 2.0 = 320Mbps. Streaming a 320x240 video wouldn't be over 1mbps. Worst case scenario, putting a 320x240 at 2mbps + 1mb of other data. That would be 12mbps bandwidth between your usb port and the device.
So from the above, the 1 USB port can handle 4 webcams connected by a splitter just as fast as 1 640x480 webcam.,
Processing these images depends on your computer speed and how you write your algorithm.
The maximum data rate of USB is way higher than what you will actually get.
Webcams will probably use isochronous transfer, which under USB 2 can only get about 40% (if I recall correctly) of the bus time, and this also has a good bit of overhead.
I don't know for sure, but I suspect that this is why usb webcam resolutions and data rates seem to have hit a ceiling several years ago. They may start to increase again with the use of USB 3.
I'd suggest that you attach each of your cameras to it's own USB 2 port, as the 40% is shared among all isochronous connections.
One of those connections sharing bandwidth with a keyboard or even a usb mass storage device should be ok, because they would only use parts of the remainder of the bandwidth.
Wrong.
First, USB 2.0 is 480mbps theoretical, and you should be able to get up to about 80% of that with a direct connection.
Second, to calculate the bandwidth used by a camera, image bit depth must be taken into account, therefore:
BW = hresolution() * vresolution() * imagebitdepth(bit) * framerate(frame/s) (in bit/sec)
imagebitdepth can be, for webcams, 8, 16, 24, or 32 bits (ranging from Y800 monochrome to RGBA/RGBT colour full, check spec)
Therefore, a typical webcam # 640*480 resolution, 30fps, 16bit RGB bit-masked RGBA image bit depth will require 147.456 Mbps, and consequently, one of similar spec but # 320*240 resolution would require 36.864 Mbps, as opposed to the major BS stated by Shawn above with his 1mbps which then is also inconsistent with just about all of his other, also wrong data.
Simulatenous operation is nevertheless largely driver dependent, it is up to the manufacturer to take the otherwise minimal effort and expose unique device IDs to DirectShow.