|author||David Brownell <email@example.com>||2007-04-11 23:28:42 -0700|
|committer||Linus Torvalds <firstname.lastname@example.org>||2007-04-12 15:31:42 -0700|
[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 <email@example.com> Signed-off-by: Andrew Morton <firstname.lastname@example.org> Signed-off-by: Linus Torvalds <email@example.com>
1 files changed, 30 insertions, 1 deletions
diff --git a/Documentation/gpio.txt b/Documentation/gpio.txt
index 989f1130f4f3..f8528db967fa 100644
@@ -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?