Documented previously undocumented adc module.

docs/en/modules/adc.md

@@ -0,0 +1,49 @@
+# ADC Module
+The ADC module provides access to the in-built ADC.
+
+On the ESP8266 there is only a single-channel, which is multiplexed with the
+battery voltage. Depending on the setting in the "esp init data" (byte 107)
+one can either use the ADC to read an external voltage, or to read the
+system voltage, but not both.
+
+The default setting in the NodeMCU firmware can be controlled via user_config.h at compile time, by defining one of ESP_INIT_DATA_ENABLE_READVDD33, ESP_INIT_DATA_ENABLE_READADC or ESP_INIT_DATA_FIXED_VDD33_VALUE. To change the setting
+at a later date, use Espressif's flash download tool to create a new init data block.
+
+## adc.read()
+
+Samples the ADC.
+
+####Syntax
+`adc.read(channel)`
+
+####Parameters
+  - `channel`: Always zero on the ESP8266
+
+####Returns
+number:the sampled value
+
+####Example
+```lua
+val = adc.read(0)
+```
+___
+## adc.readvdd33()
+
+Reads the system voltage.
+
+####Syntax
+`adc.readvdd33()`
+
+####Parameters
+`nil`
+
+####Returns
+The system voltage, in millivolts.
+
+If the ESP8266 has been configured to use the ADC for sampling the external pin, this function will always return 65535. This is a hardware and/or SDK limitation.
+
+####Example
+```lua
+mv = adc.readvdd33()
+```
+___

mkdocs.yml

@@ -29,6 +29,7 @@ pages:
     - FAQ: 'en/faq.md'
     - Support: 'en/support.md'
     - Modules:
+        - 'adc': 'en/modules/adc.md'
         - 'node': 'en/modules/node.md'
         - 'file': 'en/modules/file.md'
         - 'rtcmem': 'en/modules/rtcmem.md'