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doc: add information about using the National Instrument GPCT

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The counter interface might eventually be changed to a more generic one.
However, for now it's good to have something documented on how things work.
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Éric Piel 2015-05-27 18:42:01 +02:00 • committed by Ian Abbott
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doc/other.xml
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@ -2010,6 +2010,117 @@ should get latched?
</section>
<section id="GPCT">
<title>
National instruments General Purpose Counters/Timers (GPCT)
</title>
Counters/timers and pulse generators are fairly different in terms of
functionality, but they correspond to similar devices seen either as input
or output. When generalising, these devices are both referred to as
"counters". The NI boards provide a couple of such counters, under the name of
GPCT.
A counter is made of the following basic elements:
<variablelist spacing="compact">
<varlistentry>
<term>Input source</term>
<listitem>the signal measured or the clock of the pulse generation.</listitem>
</varlistentry>
<varlistentry>
<term>Gate</term>
<listitem>controls when the counting (or sampling) occurs.</listitem>
</varlistentry>
<varlistentry>
<term>Register</term>
<listitem>holds the current count.</listitem>
</varlistentry>
<varlistentry>
<term>Out</term>
<listitem>for the output counters (pulse generators), the output signal.</listitem>
</varlistentry>
</variablelist>
<para>
There are many different ways to count, time or generate pulses. All the modes
rely on the counter and some particular configuration. For example, in a typical
buffered counting mode, the source is the (digital) signal that is measured, the
counter is increased at every rising edge of the signal, the gate is the
(digital) signal indicating when to save the counter to memory. It is preferable
you get first familiarized with these various modes by reading your NI board
documentation before reading the following description on the mapping to the
comedi interface.
</para>
<para>
Each counter of the board is represented in comedi as a subdevice of type
<constant>COMEDI_SUBD_COUNTER</constant>. Each subdevice has a device file
associated (eg, <filename>/dev/comedi0_subd11</filename>) in order to read or
write buffered data from or to the counter.
Note that the comedi subdevice has three "channels". In most case, only channel
0 is to be used. Reading or writing on channel 0 corresponds to reading/writing
the counter value. The GPCT also has two registers named A and B, they can be
accessed respectively via channels 1 and 2.
</para>
<para>
To configure the behaviour of the counter with comedi, the
function <function>comedi_set_counter_mode</function> is used.
The possible mode values are to be found in the <type>ni_gpct_mode_bits</type>
enum constants. For instance, by default the counter is cumulative, even in
buffered counting. To reinitialise it after each sampling (ie, after an edge on
the gate signal), one can add the <constant>NI_GPCT_LOADING_ON_GATE_BIT</constant>
to the mode. In that case, the counter will be reset to the value of the A
register after each sampling.
</para>
<para>
To configure the signal to be used as the "source", one uses the <function>
comedi_set_clock_source</function> with one constant from the
<type>ni_gpct_clock_source_bits</type> enum. When the period of the signal is
fixed and known, it should be specified as the last parameter of the method, otherwise
<literal>0</literal> should be passed. Note that in comedi this is called "clock"
because in timer and pulse generator, this signal is used as the clock.
</para>
<para>
To configure the signal to be used as the "gate", one uses the <function>
comedi_set_gate_source</function> with one constant from the <type>
ni_gpct_gate_select</type> enum. When the gate signal is not be used,
<constant>NI_GPCT_DISABLED_GATE_SELECT</constant> should be specified. Some
NI boards have two gates, but the behaviour associated with the second gate is
usually unknown so it is recommended to disable it. Note that this is called
"gate" because in some modes, this signal is used to block/unblock the counter.
</para>
<para>
The function <function>comedi_reset</function> will stop and reset a counter.
After being configured, to start a counter, it should be "armed", which can be
done either with the <function>comedi_arm</function> function (for simple counting
mode), or with the <structfield>start_src</structfield> member of the command
(for buffered counting).
</para>
<para>
One side thing to mention is the signal routing of the NI card, which is done
via the PFIs (Programmable Function Inputs).
NI's naming is confusing because they use the same name for the
terminal (ie, physical input/output pins) and for the signal
(ie, the logical information that controls/indicates a specific event).
The routing allows to configure which signal goes to a PFI terminal.
This is done via <function>comedi_set_routing</function>, with subdevice being
the special DIO comedi subdevice (eg, 7 on M-series), the PFI terminal
number as channel, the signal that should be routed to it encoded as source with
one of the constants from the <type>ni_pfi_routing</type> enum.
The direction of the pin must also be correctly configured (ie, whether it is
used as input or output). This is done via <function>comedi_dio_config</function>
with the same subdevice and channel, and either <constant>COMEDI_INPUT</constant>
or <constant>COMEDI_OUTPUT</constant>.
</para>
</section>
<section id="RTSI">
<title>
National instruments RTSI trigger bus