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Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/getting_started/gpio.md | 44 |
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 |