122 lines
3.8 KiB
Plaintext
122 lines
3.8 KiB
Plaintext
rotary-encoder - a generic driver for GPIO connected devices
|
|
Daniel Mack <daniel@caiaq.de>, Feb 2009
|
|
|
|
0. Function
|
|
-----------
|
|
|
|
Rotary encoders are devices which are connected to the CPU or other
|
|
peripherals with two wires. The outputs are phase-shifted by 90 degrees
|
|
and by triggering on falling and rising edges, the turn direction can
|
|
be determined.
|
|
|
|
Some encoders have both outputs low in stable states, whereas others also have
|
|
a stable state with both outputs high (half-period mode).
|
|
|
|
The phase diagram of these two outputs look like this:
|
|
|
|
_____ _____ _____
|
|
| | | | | |
|
|
Channel A ____| |_____| |_____| |____
|
|
|
|
: : : : : : : : : : : :
|
|
__ _____ _____ _____
|
|
| | | | | | |
|
|
Channel B |_____| |_____| |_____| |__
|
|
|
|
: : : : : : : : : : : :
|
|
Event a b c d a b c d a b c d
|
|
|
|
|<-------->|
|
|
one step
|
|
|
|
|<-->|
|
|
one step (half-period mode)
|
|
|
|
For more information, please see
|
|
http://en.wikipedia.org/wiki/Rotary_encoder
|
|
|
|
|
|
1. Events / state machine
|
|
-------------------------
|
|
|
|
In half-period mode, state a) and c) above are used to determine the
|
|
rotational direction based on the last stable state. Events are reported in
|
|
states b) and d) given that the new stable state is different from the last
|
|
(i.e. the rotation was not reversed half-way).
|
|
|
|
Otherwise, the following apply:
|
|
|
|
a) Rising edge on channel A, channel B in low state
|
|
This state is used to recognize a clockwise turn
|
|
|
|
b) Rising edge on channel B, channel A in high state
|
|
When entering this state, the encoder is put into 'armed' state,
|
|
meaning that there it has seen half the way of a one-step transition.
|
|
|
|
c) Falling edge on channel A, channel B in high state
|
|
This state is used to recognize a counter-clockwise turn
|
|
|
|
d) Falling edge on channel B, channel A in low state
|
|
Parking position. If the encoder enters this state, a full transition
|
|
should have happened, unless it flipped back on half the way. The
|
|
'armed' state tells us about that.
|
|
|
|
2. Platform requirements
|
|
------------------------
|
|
|
|
As there is no hardware dependent call in this driver, the platform it is
|
|
used with must support gpiolib. Another requirement is that IRQs must be
|
|
able to fire on both edges.
|
|
|
|
|
|
3. Board integration
|
|
--------------------
|
|
|
|
To use this driver in your system, register a platform_device with the
|
|
name 'rotary-encoder' and associate the IRQs and some specific platform
|
|
data with it.
|
|
|
|
struct rotary_encoder_platform_data is declared in
|
|
include/linux/rotary-encoder.h and needs to be filled with the number of
|
|
steps the encoder has and can carry information about externally inverted
|
|
signals (because of an inverting buffer or other reasons). The encoder
|
|
can be set up to deliver input information as either an absolute or relative
|
|
axes. For relative axes the input event returns +/-1 for each step. For
|
|
absolute axes the position of the encoder can either roll over between zero
|
|
and the number of steps or will clamp at the maximum and zero depending on
|
|
the configuration.
|
|
|
|
Because GPIO to IRQ mapping is platform specific, this information must
|
|
be given in separately to the driver. See the example below.
|
|
|
|
---------<snip>---------
|
|
|
|
/* board support file example */
|
|
|
|
#include <linux/input.h>
|
|
#include <linux/rotary_encoder.h>
|
|
|
|
#define GPIO_ROTARY_A 1
|
|
#define GPIO_ROTARY_B 2
|
|
|
|
static struct rotary_encoder_platform_data my_rotary_encoder_info = {
|
|
.steps = 24,
|
|
.axis = ABS_X,
|
|
.relative_axis = false,
|
|
.rollover = false,
|
|
.gpio_a = GPIO_ROTARY_A,
|
|
.gpio_b = GPIO_ROTARY_B,
|
|
.inverted_a = 0,
|
|
.inverted_b = 0,
|
|
.half_period = false,
|
|
};
|
|
|
|
static struct platform_device rotary_encoder_device = {
|
|
.name = "rotary-encoder",
|
|
.id = 0,
|
|
.dev = {
|
|
.platform_data = &my_rotary_encoder_info,
|
|
}
|
|
};
|
|
|