diff options
| -rw-r--r-- | Documentation/gpio/driver.txt | 20 |
1 files changed, 14 insertions, 6 deletions
diff --git a/Documentation/gpio/driver.txt b/Documentation/gpio/driver.txt index ae6e0299b16c..6cb35a78eff4 100644 --- a/Documentation/gpio/driver.txt +++ b/Documentation/gpio/driver.txt @@ -100,6 +100,10 @@ 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. +The level on the line will go as high as the VDD on the pull-up resistor, which +may be higher than the level supported by the transistor, achieveing a +level-shift to the higher VDD. + 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 @@ -110,14 +114,18 @@ output. The "totem-pole" looks like so: OD ||--+ +--/ ---o|| P-MOS-FET | ||--+ -in --+ +----- out +IN --+ +----- out | ||--+ +--/ ----|| N-MOS-FET OS ||--+ | GND -You see the little "switches" named "OD" and "OS" that enable/disable the +The desired output signal (e.g. coming directly from some GPIO output register) +arrives at IN. The switches named "OD" and "OS" are normally closed, creating +a push-pull circuit. + +Consider 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 @@ -128,8 +136,8 @@ 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. +or open source, in practice 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 @@ -146,8 +154,8 @@ 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, +drain, and the IN output value is low, it will be driven low as usual. But +if the IN output value is set to high, 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 |