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diff --git a/docs/source/adc.rst b/docs/source/adc.rst new file mode 100644 index 0000000..937b178 --- /dev/null +++ b/docs/source/adc.rst @@ -0,0 +1,104 @@ +.. _adc: + +===== + ADC +===== + +Analog to digital conversion is the process of reading a physical +voltage as a number. Maple can convert voltages between 0 and 3.3V to +numbers between 0 and 4095. + +.. contents:: Contents + :local: + +ADC On Maple +------------ + +Doing analog-to-digital conversion on the Maple is simple. +:ref:`Maple IDE <ide>` contains a basic example. To see it, choose +Analog > AnalogInSerial from the :ref:`examples menu <ide-examples>`. + +In order to set up your board for conversion, first connect the wire +(potentiometer, etc.) with the voltage you want to measure to a +:ref:`pin <gpio>` which can perform ADC. Each pin which can do ADC +has "AIN" (or "ain") written next to the the pin number. Then, as in +the example program, set the chosen pin's :ref:`pin mode +<lang-pinmode>` to ``INPUT_ANALOG`` by calling ``pinMode(<your_pin>, +INPUT_ANALOG)``. You will usually do this in your :ref:`lang-setup` +function. Now you can use :ref:`lang-analogread` to perform an ADC +reading. + +.. _adc-function-reference: + +Function Reference +------------------ + +* :ref:`lang-analogread` +* :ref:`lang-pinmode` +* :ref:`libmaple-adc` (low-level ADC support) + +.. _adc-noise-bias: + +Noise and Bias +-------------- + +Maple has a large number of pins which are capable of taking 12-bit +ADC measurements, which means that voltages from 0 to 3.3V are read as +numbers from 0 to 4095. In theory, this means that a change in +voltage of about 1 millivolt should change the numeric voltage reading +by 1. In reality, however, a number of issues introduce noise and +bias into this reading, and a number of techniques must be used to get +good precision and accuracy. + +In order to allow for good readings, LeafLabs has tried to isolate at +least some of each board's ADC pins and traces from strong noise +sources. However, there are always trade-offs between noise, +additional functionality, cost, and package size. More information on +these isolated pins is available in each board's hardware +documentation: + +* :ref:`Maple <maple-adc-bank>` +* :ref:`Maple RET6 Edition <maple-ret6-adc-bank>` +* :ref:`Maple Mini <maple-mini-adc-bank>` +* :ref:`Maple Native Beta <maple-native-b-adc-bank>` + +That said, there are a number of more general things you can do to try +to get good readings. If your input voltage changes relatively +slowly, a number of samples can be taken in succession and averaged +together, or the same voltage can even be sampled by multiple ADC pins +at the same time. + +Another important factor when taking a voltage reading is the +reference voltages that the sample is being compared against. For +Maple, the high reference is |vdda| and the low reference is ground. +This means that noise or fluctuations on either |vdda| or ground will +affect the measurement. It also means that the voltage you are trying +to sample must be between ground and 3.3 V. + +.. _adc-range: + +In the case of a variable reading, it is best if the voltage varies +over the entire range of 0 through 3.3 V; otherwise, only a fraction +of the sensitivity is being used. Some basic tools to accomplish this +are `resistor dividers +<http://en.wikipedia.org/wiki/Voltage_divider>`_ and `Zener diodes +<http://en.wikipedia.org/wiki/Zener_diode>`_\ +. However, `operational amplifiers +<http://en.wikipedia.org/wiki/Operational_amplifier>`_ and other +powered components can also be used if greater precision is required. + +.. _adc-recommended-reading: + +Recommended Reading +------------------- + +* `Wikipedia: Analog-to-Digital Converter + <http://en.wikipedia.org/wiki/Analog-to-digital_converter>`_ +* `Arduino Analog Input Tutorial + <http://arduino.cc/en/Tutorial/AnalogInputPins>`_ +* ST documentation: + + * `Application Note on ADC Modes + <http://www.st.com/stonline/products/literature/an/16840.pdf>`_ (PDF) + * `Application Note on ADC Oversampling + <http://www.st.com/stonline/products/literature/an/14183.pdf>`_ (PDF) |