aboutsummaryrefslogtreecommitdiffstats
path: root/Documentation/gpio.txt
diff options
context:
space:
mode:
authorDavid Brownell <david-b@pacbell.net>2007-04-12 06:28:42 (GMT)
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2007-04-12 22:31:42 (GMT)
commit1668be71ccae5a9610fc8a224bd80fbe852f93ae (patch)
tree4efb5472b8295a9b7572709f8d3903c5a83943da /Documentation/gpio.txt
parentfe20e581a72979917e35d5146458ceba79be391f (diff)
[PATCH] doc: gpio.txt describes open-drain emulation
Update the GPIO docs to describe the idiom whereby open drain signals are emulated by toggling the GPIO direction. Signed-off-by: David Brownell <dbrownell@users.sourceforge.net> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'Documentation/gpio.txt')
-rw-r--r--Documentation/gpio.txt31
1 files changed, 30 insertions, 1 deletions
diff --git a/Documentation/gpio.txt b/Documentation/gpio.txt
index 989f113..f8528db 100644
--- a/Documentation/gpio.txt
+++ b/Documentation/gpio.txt
@@ -27,7 +27,7 @@ The exact capabilities of GPIOs vary between systems. Common options:
- Output values are writable (high=1, low=0). Some chips also have
options about how that value is driven, so that for example only one
value might be driven ... supporting "wire-OR" and similar schemes
- for the other value.
+ for the other value (notably, "open drain" signaling).
- Input values are likewise readable (1, 0). Some chips support readback
of pins configured as "output", which is very useful in such "wire-OR"
@@ -247,6 +247,35 @@ with gpio_get_value(), for example to initialize or update driver state
when the IRQ is edge-triggered.
+Emulating Open Drain Signals
+----------------------------
+Sometimes shared signals need to use "open drain" signaling, where only the
+low signal level is actually driven. (That term applies to CMOS transistors;
+"open collector" is used for TTL.) A pullup resistor causes the high signal
+level. This is sometimes called a "wire-AND"; or more practically, from the
+negative logic (low=true) perspective this is a "wire-OR".
+
+One common example of an open drain signal is a shared active-low IRQ line.
+Also, bidirectional data bus signals sometimes use open drain signals.
+
+Some GPIO controllers directly support open drain outputs; many don't. When
+you need open drain signaling but your hardware doesn't directly support it,
+there's a common idiom you can use to emulate it with any GPIO pin that can
+be used as either an input or an output:
+
+ LOW: gpio_direction_output(gpio, 0) ... this drives the signal
+ and overrides the pullup.
+
+ HIGH: gpio_direction_input(gpio) ... this turns off the output,
+ so the pullup (or some other device) controls the signal.
+
+If you are "driving" the signal high but gpio_get_value(gpio) reports a low
+value (after the appropriate rise time passes), you know some other component
+is driving the shared signal low. That's not necessarily an error. As one
+common example, that's how I2C clocks are stretched: a slave that needs a
+slower clock delays the rising edge of SCK, and the I2C master adjusts its
+signaling rate accordingly.
+
What do these conventions omit?
===============================

Privacy Policy