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gpio: document open drain/source behaviour

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This has been a totally undocumented feature for years so add some
generic concepts and documentation about open drain/source, include
some facts on how we now support for hardware.

Cc: Michael Hennerich <michael.hennerich@analog.com>
Cc: Nicolas Saenz Julienne <nicolassaenzj@gmail.com>
Cc: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
This commit is contained in:
Linus Walleij 2016-04-05 16:49:57 +02:00
parent a0e637387a
commit 6b5029d3ec
1 changed files with 89 additions and 0 deletions
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89
Documentation/gpio/driver.txt
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@ -68,6 +68,95 @@ control callbacks) if it is expected to call GPIO APIs from atomic context
on -RT (inside hard IRQ handlers and similar contexts). Normally this should
not be required.
GPIOs with open drain/source support
------------------------------------
Open drain (CMOS) or open collector (TTL) means the line is not actively driven
high: instead you provide the drain/collector as output, so when the transistor
is not open, it will present a high-impedance (tristate) to the external rail.
CMOS CONFIGURATION TTL CONFIGURATION
||--- out +--- out
in ----|| |/
||--+ in ----|
| |\
GND GND
This configuration is normally used as a way to achieve one of two things:
- Level-shifting: to reach a logical level higher than that of the silicon
where the output resides.
- inverse wire-OR on an I/O line, for example a GPIO line, making it possible
for any driving stage on the line to drive it low even if any other output
to the same line is simultaneously driving it high. A special case of this
is driving the SCL and SCA lines of an I2C bus, which is by definition a
wire-OR bus.
Both usecases require that the line be equipped with a pull-up resistor. This
resistor will make the line tend to high level unless one of the transistors on
the rail actively pulls it down.
Integrated electronics often have an output driver stage in the form of a CMOS
"totem-pole" with one N-MOS and one P-MOS transistor where one of them drives
the line high and one of them drives the line low. This is called a push-pull
output. The "totem-pole" looks like so:
VDD
|
OD ||--+
+--/ ---o|| P-MOS-FET
| ||--+
in --+ +----- out
| ||--+
+--/ ----|| N-MOS-FET
OS ||--+
|
GND
You see the little "switches" named "OD" and "OS" that enable/disable the
P-MOS or N-MOS transistor right after the split of the input. As you can see,
either transistor will go totally numb if this switch is open. The totem-pole
is then halved and give high impedance instead of actively driving the line
high or low respectively. That is usually how software-controlled open
drain/source works.
Some GPIO hardware come in open drain / open source configuration. Some are
hard-wired lines that will only support open drain or open source no matter
what: there is only one transistor there. Some are software-configurable:
by flipping a bit in a register the output can be configured as open drain
or open source, by flicking open the switches labeled "OD" and "OS" in the
drawing above.
By disabling the P-MOS transistor, the output can be driven between GND and
high impedance (open drain), and by disabling the N-MOS transistor, the output
can be driven between VDD and high impedance (open source). In the first case,
a pull-up resistor is needed on the outgoing rail to complete the circuit, and
in the second case, a pull-down resistor is needed on the rail.
Hardware that supports open drain or open source or both, can implement a
special callback in the gpio_chip: .set_single_ended() that takes an enum flag
telling whether to configure the line as open drain, open source or push-pull.
This will happen in response to the GPIO_OPEN_DRAIN or GPIO_OPEN_SOURCE flag
set in the machine file, or coming from other hardware descriptions.
If this state can not be configured in hardware, i.e. if the GPIO hardware does
not support open drain/open source in hardware, the GPIO library will instead
use a trick: when a line is set as output, if the line is flagged as open
drain, and the output value is negative, it will be driven low as usual. But
if the output value is set to positive, it will instead *NOT* be driven high,
instead it will be switched to input, as input mode is high impedance, thus
achieveing an "open drain emulation" of sorts: electrically the behaviour will
be identical, with the exception of possible hardware glitches when switching
the mode of the line.
For open source configuration the same principle is used, just that instead
of actively driving the line low, it is set to input.
GPIO drivers providing IRQs
---------------------------
It is custom that GPIO drivers (GPIO chips) are also providing interrupts,