samples/adc_polling: Update README.md to reference AV; use AV in sample

* Update main.py

Read and use ADC Reference value to calculate the voltage value

* Update README.md

* Update samples/adc_polling/README.md

improved item 2

Co-authored-by: Mike Wadsten <mikewadsten@gmail.com>

* Update samples/adc_polling/main.py

Co-authored-by: Mike Wadsten <mikewadsten@gmail.com>

Co-authored-by: Mike Wadsten <mikewadsten@gmail.com>

Author: SWoto

samples/adc_polling/README.md

@@ -23,7 +23,16 @@ Make sure the hardware is set up correctly:
 
 1. Plug the XBee3 radio module into the XBee adapter and connect it to your
    computer's USB port.
-2. Connect a voltage variable source to the pin configured as ADC (light
+2. If using XBee 3 Zigbee, DigiMesh, or 802.15.4, ensure that the **AV** command
+   (Analog Voltage Reference) is configured to an input range which matches your
+   voltage source:
+
+     * 0 = 1.25 V
+     * 1 = 2.5 V
+     * 2 = VDD
+
+   If using XBee Cellular or XBee 3 Cellular, the analog reference voltage is 2.5V.
+3. Connect a voltage variable source to the pin configured as ADC (light
    sensor, temperature sensor, etc). For testing purposes we recommend using a
    potentiometer. Follow these steps to connect it:
 
@@ -112,4 +121,4 @@ FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-SOFTWARE.
+SOFTWARE.

samples/adc_polling/main.py

@@ -19,20 +19,35 @@
 # SOFTWARE.
 
 from machine import ADC
+import xbee
 import time
 
 # Pin D3 (AD3/DIO3)
 ADC_PIN_ID = "D3"
 
+# ADC reference voltage
+AV_VALUES = {0: 1.25, 1: 2.5, 2: 3.3, None: 2.5}
 
 print(" +-------------------------------------+")
 print(" | XBee MicroPython ADC Polling Sample |")
 print(" +-------------------------------------+\n")
 
+# Read the module's Analog Digital Reference
+try:
+    av = xbee.atcmd("AV")
+except KeyError:
+    # Reference is set to 2.5 V on XBee 3 Cellular
+    av = None
+reference = AV_VALUES[av]
+print("Configured Analog Digital Reference: AV:{}, {} V".format(av, reference))
+
+
 # Create an ADC object for pin DIO0/AD0.
 adc_pin = ADC(ADC_PIN_ID)
 
 # Start reading the analog voltage value present at the pin.
 while True:
-    print("- ADC value:", adc_pin.read())
+    value = adc_pin.read()
+    print("- ADC value:", value)
+    print("- Analog voltage [V]:", value * reference / 4095)
     time.sleep(1)