@@ -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
Make the text clearer, remove reference to confusing "positive" and "negative" and elaborate a bit. Signed-off-by: Linus Walleij <linus.walleij@linaro.org> --- Documentation/gpio/driver.txt | 20 ++++++++++++++------ 1 file changed, 14 insertions(+), 6 deletions(-)