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This directory contains the device entries for the dual quadrature
decoder.

HARDWARE
The dual quadrature decoder provides two independent channels of
quadrature decoding.  The first channel appears on the BaseBoard
connector pins 2 and 4, or on pins 10 and 12 depending on the slot
number for the peripheral.  The second channel is on pins 6 and 8,
or on pins 14 and 16.  A positive step indicates that the lower
numbered input pin (2, 6, 10, 14) went high before the higher
numbered pin (4, 8, 12, 16).  That is, the lowered numbered pin
is leading the quadrature cycle.  The maximum input frequency for
the decoder is 1.25 Megahertz.

DEVICE NODES
The device interfaces to the dual quadrature decoder offer a
select()'able interface to the latest readings, and a configurable
update rate.

.../step0, .../step1
   The quadrature step count.  The value is a signed decimal
number terminated by a newline.  Use select() to automatically get 
readings, and use '%d' to convert the ASCII encoded value to a signed
integer.

.../update_period0, .../update_period1
   This specifies the update period for step0 (1) in milliseconds.
That is, a select() on step0 will will give a readable file
descriptor every update_period0 milliseconds.  Setting this to
zero turns off timed updates and causes an update whenever there
is activity on the quadrature inputs.  A value of zero is often
used for user input quadrature sensors and a non-zero value is
often used for motor and wheel sensors.   The value chosen for
update_period must be a multiple of 4 milliseconds.