Update gpio.md

pages/gpio.md

@@ -1,7 +1,66 @@
 # GPIO
-GPIO (general-purpose input/output) is a series of digital interfaces that can be used to connect relays, LEDs or sensors.
+GPIO (general-purpose input/output) is a series of digital interfaces that can be used to connect relays, LEDs, or sensors.
 Using GPIO on a Pi-KVM is a feature designed for advanced users, so make sure you understand exactly what it is.
 
 In the case of Pi-KVM, the general term GPIO refers not only to the [physical interface of the Raspberry Pi](https://www.raspberrypi.org/documentation/usage/gpio),
 but also to various plugins (for example, for [USB relays](http://vusb.wikidot.com/project:driver-less-usb-relays-hid-interface))
 that can also be used transparently by emulating abstract GPIO API.
+
+# Configuration
+Setting up GPIO is quite complex. The interface is divided into several layers for flexibility. All configuration is performed using a file `/etc/kvmd/override.yaml` which has the [YAML syntax](https://docs.ansible.com/ansible/latest/reference_appendices/YAMLSyntax.html).
+
+* **Drivers** is the first layer that reflects the hardware that represents the IO ports (standard GPIO of Raspberry Pi, USB relay, and so on). Each driver has a type (a plugin that implements the hardware support) and an unique name. Multiple drivers of the same type can be defined at the same time. For example, you can connect multiple relays and give each one its own name. By default, one driver is configured with the name `__gpio__`, representing the physical GPIO interface of the Raspberry Pi (type `gpio`).
+    ```yaml
+    kvmd:
+        gpio:
+            drivers:
+                # This example shows how the __gpio__ driver is defined. You don't need to write it in your configuration.
+                __gpio__: # Names that start and end with two underscores are reserved. You don't have to define similar names yourself.
+                    type: gpio
+                    state_poll: 0.1
+
+                # You can define another gpio driver with a different polling interval
+                my_gpio:
+                    type: gpio
+                    state_poll: 1.5
+                    
+                # We have HID relay connected to Pi-KVM
+                relay:
+                    type: hidrelay
+                    device: /dev/hidraw0
+    ```
+* **Scheme** is the second layer that reflects how the various driver ports are configured. Each port has a unique name, mode (**input** or **output**), pin number, and refers to the driver that provides it. Two interaction modes are available for outputs: **pulse** and **switch**. In pulse mode, the output quickly switches its state to logical 1 and back, and in switch mode, it saves the state that the user set. If no driver is specified for the port in the scheme, then `__gpio__` will be used. In addition, when KVMD starts and finishes, all output ports are reset to 0. This can be avoided using the `initial` parameter.
+    ```yaml
+    kvmd:
+        gpio:
+            scheme:
+                # A certain device sends signals to the RPi and we want the Pi-KVM to display this as an led
+                led1:
+                    pin: 19
+                    mode: input
+                led2:
+                    pin: 16
+                    mode: input
+
+                # Two outputs of RPi's GPIO
+                button1:
+                    pin: 26
+                    mode: output
+                    switch: false  # Disable switching, only pulse available
+                button2:
+                    pin: 20
+                    mode: output
+                    switch: false
+
+                relay1:  # Channel 1 of the relay /dev/hidraw0
+                    pin: 0  # Numerating starts from 0
+                    mode: output  # Relays can't be inputs
+                    initial: null  # Don't reset the state to 0 when initializing and terminating KVMD
+                relay2:  # Channel 2
+                    pin: 1
+                    mode: output
+                    initial: null
+                    pulse:
+                        delay: 2 # Default pulse value
+                        max_delay: 2  # The pulse interval can be between min_pulse (0.1 by default) and max_pulse=5
+    ```