update the device tree binding documentation for the GPIO matrix keypad
driver: mention the driver's selecting all columns at once, reword the
delay descriptions, add the missing active low GPIO pin level property
Signed-off-by: Gerhard Sittig <gsi@xxxxxxx>
---
.../bindings/input/gpio-matrix-keypad.txt | 42 +++++++++++++++++---
1 file changed, 36 insertions(+), 6 deletions(-)
diff --git a/Documentation/devicetree/bindings/input/gpio-matrix-keypad.txt b/Documentation/devicetree/bindings/input/gpio-matrix-keypad.txt
index ead641c..11d030e 100644
--- a/Documentation/devicetree/bindings/input/gpio-matrix-keypad.txt
+++ b/Documentation/devicetree/bindings/input/gpio-matrix-keypad.txt
@@ -2,9 +2,30 @@
GPIO driven matrix keypad is used to interface a SoC with a matrix keypad.
The matrix keypad supports multiple row and column lines, a key can be
-placed at each intersection of a unique row and a unique column. The matrix
-keypad can sense a key-press and key-release by means of GPIO lines and
-report the event using GPIO interrupts to the cpu.
+placed at each intersection of a unique row and a unique column.
+
+Sampling the keypad status is done by individually activating column
+lines (which thus are outputs) and reading back row lines (which are
+inputs). Each combination of row and column is checked separately to
+determine the status of a single key.
+
+To reduce load on the CPU, changes in the keys' status can get detected
+by means of GPIO interrupts, which get generated upon changes in the pin
+levels. This approach assumes that any change in the key press status
+will result in a GPIO pin level change interrupt when all columns are
+selected (active) at the same time. Which in turn assumes that all
+column lines can get activated at the same time, and no harm is done to
+the hardware when multiple keys are pressed simultaneously.
+
+Simple keypad matrix layouts just connect GPIO lines to mechanical
+switches, with no other active components involved. Although due to the
+driver's operation of activating all columns at the same time for most
+of the time, the use of current limiting resistors may be desirable, or
+column GPIO lines shall operate in open collector mode.
+
+More involved matrix layouts may include externally connected line
+drivers and might influence signal polarity. The driver supports low
+active signals, while high active line levels are assumed by default.
Required Properties:
- compatible: Should be "gpio-matrix-keypad"
@@ -20,9 +41,18 @@ Required Properties:
Optional Properties:
- linux,no-autorepeat: do no enable autorepeat feature.
- linux,wakeup: use any event on keypad as wakeup event.
-- debounce-delay-ms: debounce interval in milliseconds
-- col-scan-delay-us: delay, measured in microseconds, that is needed
- before we can scan keypad after activating column gpio
+- debounce-delay-ms: debounce interval in milliseconds, which needs
+ to pass between detecting a change in the key
+ press status and sampling the new status, e.g.
+ to support multi key presses without spurious
+ single key events, or to cope with bouncing
+ mechanical key switches
+- col-scan-delay-us: delay interval in microseconds between selecting
+ a column line and reading back its row status,
+ such that pin levels can settle after
+ propagating through the matrix and its
+ associated hardware components
+- gpio-activelow: row pins as well as column pins are active low
Example:
matrix-keypad {
--
1.7.10.4
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