This chapter is meant to be a reference for some of the advanced features of Comedi.
When one or several digital inputs are used to modify an output value, either an accumulator or a single digital line or bit, a bitfield structure is typically used in the Comedi interface. The digital inputs have two properties, "sensitive" inputs and "modifier" inputs. Edge transitions on sensitive inputs cause changes in the output signal, whereas modifier inputs change the effect of edge transitions on sensitive inputs. Note that inputs can be both modifier inputs and sensitive inputs.
For simplification purposes, it is assumed that multiple digital inputs do not change simultaneously.
The combined state of the modifier inputs determine a modifier state. For each combination of modifier state and sensitive input, there is a set of bits that determine the effect on the output value due to positive or negative transitions of the sensitive input. For each transition direction, there are two bits defined as follows:
For example, a simple digital follower is specified by the bit pattern 01 10, because it sets the output on positive transitions of the input, and clears the output on negative transitions. A digital inverter is similarily 10 01. These systems have only one sensitive input.
As another example, a simple up counter, which increments on positive transitions of one input, is specified by 01 00. This system has only one sensitive input.
When multiple digital inputs are used, the inputs are divided into two types, inputs which cause changes in the accumulator, and those that only modify the meaning of transitions on other inputs. Modifier inputs do not require bitfields, but there needs to be a bitfield of length 4*(2^(N-1)) for each edge sensitive input, where N is the total number of inputs. Since N is usually 2 or 3, with only one edge sensitive input, the scaling issues are not significant.
Configuration instructions are used to access device and driver features that do not fit well into other parts of the Comedi interface. This includes changing the direction of configurable digital I/O lines, configuring complex triggering engines, and counter/timer configuration.
If a specified ID is not supported, the driver must return -EINVAL.
Status: Implemented
ID: COMEDI_INPUT, COMEDI_OUTPUT, COMEDI_OPENDRAIN
Length: 1
Chanspec: used to specify channel
These IDs are used to configure direction of digital I/O lines. Direction is chosen by the ID. On typical devices, multiple channels are grouped together in blocks for determining direction. Configuring one channel in a block configures the entire block.
There should also be a method to read the configuration.
Errors: Should return -EINVAL if the ID is not supported.
Status: design
ID: not assigned
Length:
Chanspec: used to specify channel
Description:
Some devices have the capability to add white noise (dithering) to analog input measurement. This additional noise can then be averaged out, to get a more accurate measurement of the input signal. It should not be assumed that channels can be separately configured. A simple design can use 1 bit to turn this feature on/off.
Some devices have the capability of changing the glitch characteristics of analog output subsytems. The default (off) case should be where the average settling time is lowest. A simple design can use 1 bit to turn this feature on/off.
Some devices have a configurable analog filters as part of the analog input stage. A simple designe can use 1 bit to enable/disable the filter. Default is disabled, i.e., the filter being bypassed, or if the choice is between two filters, the filter with the largest bandwidth.
Status: design
ID: not assigned
Length:
Chanspec: ignored
Some devices have the ability to cyclicly loop through samples kept in an on-board analog output FIFO. This config should allow the user to enable/disable this mode.
This config should allow the user to configure the number of samples to loop through. It may be necessary to configure the channels used.
Status: alpha
ID: not assigned
Chanspec: ignored
This section covers common information for all extended triggering configuration, and doesn't describe a particular type of extended trigger.
Extended triggering is used to configure triggering engines that do not fit into commands. In a typical programming sequence, the application will use configuration instructions to configure an extended trigger, and the issue a command, specifying TRIG_OTHER as one of the trigger sources.
Extended trigger configuration should be designed in such a way that the user can probe for valid parameters, similar to how command testing works. An extended trigger config instruction should not configure the hardware directly, rather, the configuration should be saved until the subsequent command is issued. This allows more flexibility for future interface changes.
It has not been decided whether the config stage should return a token that is then used as the trigger argument in the command. Using tokens is one method to satisfy the problem that extended trigger configurations may have subtle compatiblity issues with other trigger sources/arguments that can only be determined at command test time. Passing all stages of a command test should only be allowed with a properly configured extended trigger.
Extended triggers must use data[1] as flags. The upper 16 bits are reserved and used only for flags that are common to all extended triggers. The lower 16 bits may be defined by the particular type of extended trigger.
Various types of extended triggers must use data[1] to know which event the extended trigger will be assigned to in the command structure. The possible values are an OR'd mask of the following:
Status: alpha
ID: not assigned
Implementation: ni_mio_common
Chanspec: ignored
Analog triggering is described by a digital combining machine that has two sensitive digital inputs. The sensitive digital inputs are generated by configurable analog comparators. The analog comparators generate a digital 1 when the analog triggering signal is greater than the comparator level. The digital inputs are not modifier inputs. Note, however, there is an effective modifier due to the restriction that the primary analog comparator level must be less than the secondary analog comparator level.
If only one analog comparator signal is used, the combining machine for the secondary input should be set to ignored, and the secondary analog level should be set to 0.
The interpretation of the chanspec and voltage levels is device dependent, but should correspond to similar values of the analog input subdevice, if possible.
Notes: Reading range information is not addressed. This makes it difficult to convert comparator voltages to data values.
Possible extensions: A parameter that specifies the necessary time that the set condition has to be true before the trigger is generated. A parameter that specifies the necessary time that the reset condition has to be true before the state machine is reset.
Status: design
ID: not assigned
Chanspec: ignored
The pattern matching trigger issues a trigger when all of a specifed set of input lines match a specified pattern. If the device allows, the input lines should correspond to the input lines of a digital input subdevice, however, this will necessarily be device dependent. Each possible digital line that can be matched is assigned a bit in the mask and pattern. A bit set in the mask indicates that the input line must match the corresponding bit in the pattern. A bit cleared in the mask indicates that the input line is ignored.
Notes: This only allows 32 bits in the pattern/mask, which may be too few. Devices may support selecting different sets of lines from which to match a pattern.
Discovery: The number of bits can be discovered by setting the mask to all 1's. The driver must modify this value and return -EAGAIN.
Status: design
ID: not assigned
Chanspec: used to specify counter
Counters can be operated either in synchronous mode (using insn_read) or asynchronous mode (using commands), similar to analog input subdevices. The input signal for both modes is the accumulator. Commands on counter subdevices are almost always specified using scan_begin_src=TRIG_OTHER, with the counter configuration also serving as the extended configuration for the scan begin source.
Counters are made up of an accumulator and a combining machine that determines when the accumulator should be incremented or decremented based on the values of the input signals. The combining machine optionally determines when the accumulator should be latched and put into a buffer. This feature is used in asynchronous mode.
Notes: How to access multiple pieces of data acquired at each event?
Status: design
ID: not assigned
Chanspec: ?
data[1] is flags, including the flags for the command triggering configuration. If a command is not subsequently issued on the subdevice, the command triggering portion of the flags are ignored.
data[2] determines the mode of operation. The mode of operation is actually a bitfield that encodes what to do for various transitions of the source signals.
data[3] and data[4] determine the primary source for the counter, similar to _src and _arg used in commands.
Notes: How to specify which events cause a latch and push, and what should get latched.