A counter is an input device, "counting" the number of triggers
that occur while the gate is enabled.  A timer is an output
device.

counter "command"

{
	unsigned int	mode;

	unsigned int	src1_src;
	unsigned int	src1_arg;

	unsigned int	src2_src;
	unsigned int	src2_arg;

	unsigned int	gate_src;
	unsigned int	gate_arg;
}

Counter examples:

  (Unless otherwise specified, _src's are TRIG_NONE.)

  Counting up at a rate of 1/ms:

    mode = UP_COUNTER;
    src1_src = TRIG_TIMER;
    src1_arg = 1000000;

  Counting down at a rate of 1/ms:

    mode = DOWN_COUNTER;
    arg1_src = TRIG_TIMER;
    arg1_arg = 1000000;

  Counting up at each rising transition of digital input 4:

    mode = UP_COUNTER;
    src1_src = TRIG_EXT;
    src1_arg = 4 | TRIG_EXT_RISING;

  Quadrature counting on di 0,1:
  
    mode = QUADRATURE;
    src1_src = TRIG_EXT;
    src1_arg = 0;
    src2_src = TRIG_EXT;
    src1_arg = 1;

  Interval timing of digital input 4:

    Counter setup:

      mode = UP_COUNTER;
      src1_src = TRIG_TIMER;
      gate_src = TRIG_EXT;
      gate_arg = 4 | LEVEL_1;
      event_src = TRIG_EXT;
      event_arg = 4 | FALLING_EDGE;

      flags = COUNT_RESET; /* counter resets at every event */

    Command setup:

      start_src = TRIG_NOW;
      scan_begin_src = TRIG_NOW;
      convert_src = TRIG_FOLLOW;
      scan_end_src = TRIG_COUNT;
      scan_end_arg = 1;
      stop_src = whatever;
  

Timer examples:




Counters are based on a number of digital inputs, typically
2 or 3, and the count is changed due to the transition of
one line and the status of other lines.

In the most generic sense, the digital inputs define a
particular state, and transitions between different states
are mapped to either increment, decrement, or not change
the counter.  A fourth option is to trigger an exception.
For example, in the case of 3 digital inputs, the transition
000->010 might be mapped to +1, to increment a counter.  The
other possible transitions (there are (2^3)*(2^3)=64 total)
would be similarily mapped.

An extension to this is to define that some transitions cause
the current counter value to be pushed into a FIFO, to be
read like asynchronous input.  This would be in parallel to
other functions.

Most hardware does not allow this kind of programming
flexibility.  A typical hardware device will only allow
transitions on one channel to change the counter, although
other channels may affect the amount that the counter
is changed.  



One idea:

Counters are based on 1,2, or 3 digital inputs, and the count
is based on transitions between the inputs.  A particular
counting mode can be specified by linking transitions to a
value to be added to the counter.

For the case of 1 input, you essentially have two possible
(non-trivial) counters:

Up counter on rising edge:
  0->1	+1
  1->0	0

Down counter on rising edge:
  0->1	-1
  1->0	0

(and the trailing edge counterparts)


2 inputs:

possible transitions:

  00->01
  00->10
  00->11
  01->00
  01->10
  01->11
  10->00
  10->01
  10->11
  11->00
  11->01
  11->10

if we limit ourselves to 1-bit transitions, we only have:

  00->01
  00->10
  01->00
  01->11
  10->00
  10->11
  11->01
  11->10

Example: an up counter on rising edge of x1 with enable on 1x:

  00->01	0
  00->10	0
  01->00	0
  01->11	0
  10->00	0
  10->11	1
  11->01	0
  11->10	0

Example: up/down counter on rising edge, 1x is up, 0x is down

  00->01	0
  00->10	-1
  01->00	0
  01->11	0
  10->00	0
  10->11	1
  11->01	0
  11->10	0

Example: quadrature counter, up is CCW

    01  00
    11  10

  00->01	1
  00->10	-1
  01->00	-1
  01->11	1
  10->00	1
  10->11	-1
  11->01	-1
  11->10	1

Additionally, we might have an additional bit that means "latch and
add to fifo" and/or interrupt.

Probably need to be able to specify input routing.

Need a way to specify invalid transitions.

Possibly specify reset bit (?)

Examples from STC spec:
  Simple event counting - sync, ok
  Simple gated-event counting - sync, ok
  buffered non-cumulative event counting - async, ok
  buffered cumulative event counting - async, ok
  relative position sensing (up/down) - sync, ok
  single period measurement - 
  single pulse-width measurement -
  buffered period measurement -
  buffered semi-period measurement -
  buffered pulse-width measurement -