I want to read the data generated by USB sound card connected to my RaspberryPi using a C code. The samples should be stored in an array or are written to a csv file.
I am using ALSA library through a function "snd_pcm_readi". Can someone explain how to access the data read by "snd_pcm_readi"?
Or is there a better alternative?
Look at the libusb library, https://libusb.info/
This library gives you simple C functions to find and open the device, and then send and receive data. You may want to do some reading about USB devices.
You may also want to look at udev - you can write a udev rule to symbolically link the desired device to a known filename.
You may need to know the vendor_id and product_id. At the command line, enter lsusb to see the usb devices.
Related
Let's say I have a webcam, and I installed the device driver for this webcam in my Linux OS, now a device file will be created for the device driver (for example: /dev/video0).
Now say I want to create a program in C that wants to access this webcam. How can my program access the device driver for the webcam, should my program use the device file (/dev/video0) to access the device driver, or is there another way?
You asked a general question, and then gave a specific example. I'll try to address both.
When you load a driver, the way to communicate with it from user space is by whatever means this driver defined. Typically, this is through a /dev device created for the driver. If that's the case, yes, that's the only way to communicate with it.
This is not universally true. Many drivers also have entries under the /sys sysfs pseudo file system, and some aspects can be modified through there. In fact, there are whole classes of drivers that are only accessible through the /sys fs. Prominent examples are GPIO and Led devices, that can be turned on and off via access to /sys/class/gpio and similar paths.
Another option, considered deprecated but still sometimes used, is to use the /proc pseudo file system. Again, this is up to the driver to define its communication method. As the user, you will have to follow whatever protocol the driver defined.
Also, some drivers don't have any file system presence at all. The most obvious standard example are network interfaces. The only way to communicate with them is via the networking system calls.
In the particular example you provided, you talked about a video camera that appears as /dev/video0. Such a camera is, usually, a Video4Linux (or v4l) camera, and those are accessed via their character devices.
With that said, the protocol for communicating with the camera might have wrappers that makes life easier. If you open the actual device, you might have to implement a rather complicated handshake with it. Instead, you can use the v4l library to wrap the details of the access.
Make no mistake. You're still talking to the character device in /dev. It's just that it's not your code that does it, but the library's.
I'm working with a USB device in Linux and have written a library to control this device.
Without going in to TOO many details, the device uses a standard UART protocol, so all I have to do is open a serial connection with open, configure the relevant parameters like baud rate, stop bit, parity, etc, etc, and start bit-banging registers.
The library works fine, however, it its hard coded to assume that this device is /dev/ttyUSB0. That is, this is what I pass to open. If open fails, I exit.
What I would like to do is detect that this device is present, regardless if it's /dev/ttyUSB0, /dev/ttyUSB1, etc. And even detect if there are multiple of these devices connected.
I can write code to poll certain registers on the device that will return serial number, product ID, etc, so I can detect that what is on the other end of the USB is indeed my device... but how can I find a list of connected USB devices, again, in native C?
OR is there a more elegant way of doing this, such as interfacing with it's kernel module, or something? I can provide the USB driver it actually uses, but I'm sort of lost when looking through the code.
Thanks for any insight.
The elegant method is to use udev to create a descriptive symlink for your device when it is connected. Add a line something like this to /etc/udev/rules.d
SUBSYSTEM=="tty",ENV{ID_MODEL}=="My_FlowMeter_Model",ENV{ID_USB_INTERFACE_NUM}=="00",SYMLINK+="flowmeter",RUN+="/bin/su pi -c /home/pi/record-flowmeter.sh
That's a very slightly modified version of an actual udev rule my research group uses to collect data from USB devices connected to battery-powered Raspberry Pi boxes. It also runs a script automatically, which has commands like
stty -F /dev/flowmeter 500000 -ixon -echo -icanon
If you want to know the "real" device filename, you could do readlink /dev/flowmeter. But for most uses you can just use the link: fd = open("/dev/flowmeter"); (or pass it as an argument to your program)
Naturally you should replace flowmeter with a short name for your own device, as well as updating the ID_MODEL based on the output from lsusb.
Multiple devices are a bit more complicated, but there are plenty of examples of udev rules out there.
On Linux, the information you are looking for is in the /sys filesystem, specifically under /sys/bus/usb/devices. From there you will need to search the filesystem to find your device.
For example, I just plugged a USB-serial dongle into my Linux (kernel version 2.6.35) and the device appeared under /sys/bus/usb/devices/2.1-8. Here, I am able to find that this is my device by vendorId:deviceId by checking the files idVendor and idProduct. Here, there is a directory named 2.1-8:1:0 which contains a directory named ttyUSB0.
Obviously, to find your device you will need code (or a shell script using find) to scan the directory tree, looking for the right entries.
How can I communicate with an mbed LPC1768 using C without using a terminal emulator like minicom or teraterm?
For example if I send an integer from my C code than that led should be turned on.
How can I do this?
Without reading user manual or datasheet you can't do any thing to your board. So just go through it then follow below link for demo application LPC1768 board
1.NXP LPC1768 ARM Cortex-M3 Red Suite Demo
2.LPC1768 ARM Cortex-M3 CrossWorks Demo
3.mbed
4.At github
If you are using linux or osx check out the following answer:
How do I read data from serial port in Linux using C?
You can check your /dev folder and look for a file that is created when you plug your mBed. The file name would be something like /dev/ttyS0. That file represents the communication port, you can read/write that file to receive/send data over serial port.
If you want a cross platform solution try RxTx library for Java.
You can use this library
what it does, is that it initialize UART port which is connected to the debugger on your board and use it as serial input output. so with any hyper terminal of your choice, you can write code to do something if you send certain string of characters or just use original printf when something happens. and it will be printed on your PC screen.
My project consists of a part that has to send sms via GSM modem using AT commands. I have learnt using these commands via hyper terminal and putty. What I want is that how can I use these AT commands in my "C" code built on eclipse.
Please guide me!
It's going to depend on your OS. I'm assuming the modem is connected to the computer via serial or USB or something like that. On most Unix it would be something like this:
int modem_fd = open("/path/to/modem", O_ASYNC);
char modem_command[] = "ATDT5551234"
write(modem_fd, modem_command, sizeof(modem_command));
You can also get similar functionality with fopen() and fwrite(), which are part of stdio.h.
Consider the device as a file and do I/O operations. We ca send AT commands similar to write data to file.But data must be in mentioned form by the device manufactures.
I am writing a program to read data from a Bluetooth USB dongle. I am using Linux, so I suspect there may be a POSIX library to read from it, or perhaps there is a predefined file descriptor for each USB drive. How do I read a stream of data from a USB port in C?
The most common way of interacting with random USB device is libusb. This provides low-level access to the device, so if you want something more complex (for example, if it's an actual USB drive with a filesystem on it), you might want to use some existing driver for the device rather than trying to interact with it directly.
OK, given the answer above: the PS3 controller is, almost certainly, a HID device. You pair with it like you do any other Bluetooth HID device. It will appear as a joystick (or mouse, or keyboard, as appropriate) automatically once paired, with no software required on your part. This site seems to have a guide, though obviously I haven't tested it: http://www.ydl.net/support/solutions/ydl_6.x/ps3_bluetooth_sixaxis.shtml