Comedi

11 / 148

}

return 0;

}

The source code ﬁle for the above program can be found in the Comedilib source at demo/tut2.c and if installed as a package

usually at /usr/share/doc/libcomedi-dev/demo/ with all the other tutorial/demo ﬁles.

3.4 Asynchronous acquisition

Of special importance is the so called "asynchronous data acquisition" where Comedi is sampling in the background at a given

sample rate. The user can retrieve the data whenever it is convenient. Comedi stores the data in a ring-buffer so that programs can

perform other tasks in the foreground, for example plotting data or interacting with the user. This technique is used in programs

such as ktimetrace or comedirecord.

There are two different ways how a sequence of channels is measured during asynchronous acquisition (see also the Figure in

the introduction):

• The channels are measured with the help of a multiplexer which switches to the next channel after each measurement. This

means that the sampling rate is divided by the number of channels.

• The channels are all measured at the same time, for example when every channel has its own converter. In this case the

sampling rate need not to be divided by the number of channels.

How your Comedi device handles the asynchronous acquisition can be found out with the command comedi_board_info -v.

The program demo/tut3.c demonstrates the asynchronous acquisition. The general strategy is always the same: ﬁrst, we tell

Comedi all sampling parameters such as the sampling rate, the number of channels and anything it needs to know so that it can

run independently in the background. Then Comedi checks our request and it might modify it. For example we might want to

have a sampling rate of 16kHz but we only get 1kHz. Finally we can start the asynchronous acquisition. Once it has been started

we need to check periodically if data is available and request it from Comedi so that its internal buffer won’t overrun.

In summary the asynchonous acquisition is performed in the following way:

• Create a command structure of type comedi_cmd

• Call the function comedi_get_cmd_generic_timed() to ﬁll the command structure with your comedi device, subdevice,

sampling rate and number of channels.

• Create a channel-list and store it in the command structure. This tells comedi which channels should be sampled in the

background.

• Call comedi_command_test() with your command structure. Comedi might modify your requested sampling rate and

channels.

• Call comedi_command_test() again which now should return zero for success.

• Call comedi_command() to start the asynchronous acquisition. From now on the kernel ringbuffer will be ﬁlled at the

speciﬁed sampling rate.

• Call periodically the standard function read() and receive the data. The result should always be non zero as long as the

acquisition is running.

• Convert the received data either into lsampl_t or sampl_t depending on the subdevice ﬂag SDF_LSAMPL.

• Poll for data with read() as long as it returns a positive result or until the program terminates.

The program below is a stripped down version of the program cmd.c in the demo directory. To compile it run:

gcc tut3.c -lcomedi -lm -o tut3

1 2 ... 17 18 19 20 21 22 23 24 25 26 27 ... 158 159

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