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Diffstat (limited to 'Documentation/gpio/driver.txt')
| -rw-r--r-- | Documentation/gpio/driver.txt | 89 |
1 files changed, 89 insertions, 0 deletions
diff --git a/Documentation/gpio/driver.txt b/Documentation/gpio/driver.txt index bbeec41..ae6e029 100644 --- a/Documentation/gpio/driver.txt +++ b/Documentation/gpio/driver.txt @@ -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, |