samd: Add a vref=num option to the ADC and DAC constructor.

ADC: The argument of vref=num is an integer. Values for num are:

    SAMD21:
    0  INT1V   1.0V voltage reference
    1  INTVCC0 1/1.48 Analog voltage supply
    2  INTVCC1 1/2 Analog voltage supply (only for VDDANA > 2.0V)
    3  VREFA   External reference
    4  VREFB   External reference

    SAMD51:
    0  INTREF  internal bandgap reference
    1  INTVCC1 Analog voltage supply
    2  INTVCC0 1/2 Analog voltage supply (only for VDDANA > 2.0v)
    3  AREFA   External reference A
    4  AREFB   External reference B
    5  AREFC   External reference C (ADC1 only)

DAC: The argument of vref=num is an integer. Suitable values:

    SAMD21:
    0  INT1V   Internal voltage reference
    1  VDDANA  Analog voltage supply
    2  VREFA   External reference

    SAMD51:
    0  INTREF Internal bandgap reference
    1  VDDANA Analog voltage supply
    2  VREFAU Unbuffered external voltage reference (not buffered in DAC)
    4  VREFAB Buffered external voltage reference (buffered in DAC).

1 files changed, 53 insertions, 5 deletions

diff --git a/docs/samd/quickref.rst b/docs/samd/quickref.rst
index 5e8298d1b..aa3d563f7 100644
--- a/docs/samd/quickref.rst
+++ b/docs/samd/quickref.rst
@@ -254,16 +254,38 @@ an external ADC.
 ADC Constructor
 ```````````````
 
-.. class:: ADC(dest, *, average=16)
+.. class:: ADC(dest, *, average=16, vref=n)
   :noindex:
 
-    Construct and return a new ADC object using the following parameters:
+Construct and return a new ADC object using the following parameters:
 
-      - *dest* is the Pin object on which the ADC is output.
+  - *dest* is the Pin object on which the ADC is output.
 
-    Keyword arguments:
+Keyword arguments:
+
+  - *average* is used to reduce the noise. With a value of 16 the LSB noise is about 1 digit.
+  - *vref* sets the reference voltage for the ADC.
+
+    The default setting is for 3.3V. Other values are:
 
-      - *average* is used to reduce the noise. With a value of 16 the LSB noise is about 1 digit.
+    ==== ==============================  ===============================
+    vref SAMD21                          SAMD51
+    ==== ==============================  ===============================
+    0    1.0V voltage reference          internal bandgap reference (1V)
+    1    1/1.48 Analogue voltage supply  Analogue voltage supply
+    2    1/2 Analogue voltage supply     1/2 Analogue voltage supply
+    3    External reference A            External reference A
+    4    External reference B            External reference B
+    5    -                               External reference C
+    ==== ==============================  ===============================
+
+ADC Methods
+```````````
+
+.. method:: read_u16()
+
+Read a single ADC value as unsigned 16 bit quantity. The voltage range is defined
+by the vref option of the constructor, the resolutions by the bits option.
 
 DAC (digital to analog conversion)
 ----------------------------------
@@ -280,6 +302,32 @@ The DAC class provides a fast digital to analog conversion. Usage example::
 The resolution of the DAC is 12 bit for SAMD51 and 10 bit for SAMD21. SAMD21 devices
 have 1 DAC channel at GPIO PA02, SAMD51 devices have 2 DAC channels at GPIO PA02 and PA05.
 
+DAC Constructor
+```````````````
+
+.. class:: DAC(id, *, vref=3)
+  :noindex:
+
+The vref arguments defines the output voltage range, the callback option is used for
+dac_timed(). Suitable values for vref are:
+
+==== ============================  ================================
+vref SAMD21                        SAMD51
+==== ============================  ================================
+0    Internal voltage reference    Internal bandgap reference (~1V)
+1    Analogue voltage supply       Analogue voltage supply
+2    External reference            Unbuffered external reference
+3    -                             Buffered external reference
+==== ============================  ================================
+
+DAC Methods
+```````````
+
+.. method:: write(value)
+
+Write a single value to the selected DAC output. The value range is 0-1023 for
+SAMD21 and 0-4095 for SAMD51. The voltage range depends on the vref setting.
+
 Software SPI bus
 ----------------