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added documentation for comedi_data_read_delayed() and comedi_data_read_hint()

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because I'm sick of anwering questions about why comedi_data_read() returns
inaccurate values.
This commit is contained in:
Frank Mori Hess 2002-11-18 23:49:57 +00:00
parent 962c9d0a89
commit f0e92b1322
1 changed files with 57 additions and 2 deletions
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@ -464,22 +464,77 @@ Description:
For the A/D conversion (if appropriate),
the device is configured to use range specification
range and (if appropriate) analog reference type
aref. Analog reference types that are not supported
aref. Analog reference types that are not supported
by the device are silently ignored.
The function comedi_data_read() reads one data value from
the specified channel and places the data value in the
location pointed to by data.
WARNING: comedi_data_read() does not do any pausing to
allow multiplexed analog inputs to settle before
performing an analog to digital conversion. If you are
switching between different channels and need to allow
your analog input to settle for an accurate reading,
use comedi_data_read_delayed(), or set the
input channel at an earlier time with
comedi_data_read_hint().
On sucess, comedi_data_read() returns 0. If there is an
error, -1 is returned.
Data values returned by this function are unsigned integers
less than or equal to the maximum sample value of the channel,
which can be determined using the function comedi_get_maxdata().
Conversion of data values to physical units can be performed
by the function comedi_to_phys().
Function: comedi_data_read_delayed -- read single sample from channel after delaying for specified settling time
Retval: int
Param: comedi_t * device
Param: unsigned int subdevice
Param: unsigned int channel
Param: unsigned int range
Param: unsigned int aref
Param: lsampl_t * data
Param: unsigned int nanosec
Description:
Similar to comedi_data_read() except it will wait for the
specified number of nanoseconds between setting the input
channel and taking a sample. For analog inputs, most
boards have a single
analog to digital converter which is multiplexed to be
able to read multiple channels. If the input is not allowed
to settle after the multiplexer switches channels, the
reading will be inaccurate. This function is useful
for allowing a multiplexed analog input to settle
when switching channels.
Although the settling time is specified in nanoseconds, the
actual settling time will be rounded up to the nearest
microsecond.
Function: comedi_data_read_hint -- tell driver which channel/range/aref you are going to read from next
Retval: int
Param: comedi_t * device
Param: unsigned int subdevice
Param: unsigned int channel
Param: unsigned int range
Param: unsigned int aref
Description:
Used to prepare an analog input for a subsequent call to
comedi_data_read(). It is not necessary to use this
function, but it can be useful for eliminating inaccuaracies
caused by insufficient settling times when switching the
channel
or gain on an analog input. This function sets an analog input
to the channel, range, and aref specified but does not
perform an actual analog to digital conversion.
Alternatively, one can simply use comedi_data_read_delayed(),
which sets up the
input, pauses to allow settling, then performs a conversion.
Function: comedi_data_write -- write single sample to channel
Retval: int
Param: comedi_t * device