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-rw-r--r--Documentation/getting_started/gpio.md44
1 files changed, 38 insertions, 6 deletions
diff --git a/Documentation/getting_started/gpio.md b/Documentation/getting_started/gpio.md
index d4e8ca7df4..19e74eec11 100644
--- a/Documentation/getting_started/gpio.md
+++ b/Documentation/getting_started/gpio.md
@@ -115,6 +115,44 @@ variant's override table.
This configuration is often hooked into the mainboard's `enable_dev` callback,
defined in its `struct chip_operations`.
+## Unconnected and unused pads
+
+In digital electronics, it is generally recommended to tie unconnected GPIOs to
+a defined signal like VCC or GND by setting their direction to output, adding an
+external pull resistor or configuring an internal pull resistor. This is done to
+prevent floating of the pin state, which can cause various issues like EMI,
+higher power usage due to continuously switching logic, etc.
+
+On Intel PCHs from Sunrise Point onwards, termination of unconnected GPIOs is
+explicitly not required, when the input buffer is disabled by setting the bit
+`GPIORXDIS` which effectively disconnects the pad from the internal logic. All
+pads defaulting to GPIO mode have this bit set. However, in the mainboard's
+GPIO configuration the macro `PAD_NC(pad, NONE)` can be used to explicitly
+configure a pad as unconnected.
+
+In case there are no schematics available for a board and the vendor sets a pad
+to something like `GPIORXDIS=1`, `GPIOTXDIS=1` with an internal pull resistor,
+an unconnected or otherwise unused pad can be assumed. In this case it is
+recommended to keep the pull resistor, because the external circuit might rely
+on it.
+
+Unconnected pads defaulting to a native function (input and output) usually
+don't need to be configured as GPIO with the `GPIORXDIS` bit set. For clarity
+and documentation purpose the macro may be used as well for them.
+
+Some pads configured as native input function explicitly require external
+pull-ups when unused, according to the PDGs:
+- eDP_HPD
+- SMBCLK/SMBDATA
+- SML0CLK/SML0DATA/SML0ALERT
+- SATAGP*
+
+If the board was designed correctly, nothing needs to be done for them
+explicitly, while using `PAD_NC(pad, NONE)` can act as documentation. If such a
+pad is missing the external pull resistor due to bad board design, the pad
+should be configured with `PAD_NC(pad, NONE)` anyway to disconnect it
+internally.
+
## Potential issues (gotchas!)
There are a couple of configurations that you need to especially careful about,
@@ -124,12 +162,6 @@ The first is configuring a pin as an output, when it was designed to be an
input. There is a real risk in this case of short-circuiting a component which
could cause catastrophic failures, up to and including your mainboard!
-The other configuration option to watch out for deals with unconnected GPIOs.
-If no pullup or pulldown is declared with these, they may end up "floating",
-i.e., not at logical high or logical low. This can cause problems such as
-unwanted power consumption or not reading the pin correctly, if it was intended
-to be strapped.
-
## Pad-related known issues and workarounds
### LPC_CLKRUNB blocks S0ix states when board uses eSPI