From 1088b37e70e192d2a279bbfc55d33238f090d72e Mon Sep 17 00:00:00 2001
From: Marti Bolivar <mbolivar@mit.edu>
Date: Mon, 11 Oct 2010 20:02:04 -0400
Subject: more docs

---
 docs/source/adc.rst | 70 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 69 insertions(+), 1 deletion(-)

(limited to 'docs/source/adc.rst')

diff --git a/docs/source/adc.rst b/docs/source/adc.rst
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--- a/docs/source/adc.rst
+++ b/docs/source/adc.rst
@@ -4,4 +4,72 @@
  ADC
 =====
 
-Stub.
+Analog-Digital Conversion is the process of reading a physical voltage
+as a number. The Maple has a large number of pins which are capable of
+taking 12-bit ADC measurements, which means that voltages from ground
+to +3.3v are read as numbers from 0 to 4095; this corresponds to a
+theoretical sensitivity of just under 1 millivolt. In reality, a
+number of factors introduce noise and bias into this reading and a
+number of techniques must be used to get good precision and accuracy.
+
+.. compound::
+
+   The header pins with ADC functionality (marked as "AIN" on the
+   silkscreen) are:
+
+     D0, D1, D2, D3, D10, D11, D12, D13, D15, D16, D17, D18, D19, D20, D27, D28
+
+.. contents:: Contents
+   :local:
+
+.. _adc-noise-bias:
+
+Noise and Bias
+--------------
+
+The biggest issues with analog-digital conversion are noise and bias.
+With the Maple, we have tried to isolate the ADC pins and traces from
+strong noise sources but there are always trade--offs between noise,
+additional functionality, cost, and package size.
+
+The 6 ADC pins in a bank (D15--D20) generally have the least
+noise and should be used for fine measurements.  If the 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.
+
+An important factor when taking a voltage reading is the reference
+voltages that the sample is being compared against. In the case of the
+Maple, the high reference is |vcc| and the low reference is ground.
+This means that noise or fluctuations on either |vcc| or ground will
+affect the measurement. It also means that the voltage you are trying
+to sample must be between ground and 3.3V. In the case of a variable
+reading, it is best if the voltage varies over the entire range of
+0--3.3V; otherwise, only a fraction of the sensitivity is being
+leveraged.  Resistor dividers and constant voltage diodes are basic
+tools which can help bring a given voltage signal into the appropriate
+range; opamps and other powered components can also be used.
+
+.. _adc-function-reference:
+
+Function Reference
+------------------
+
+.. TODO function reference (figure out Doxygen first)
+
+.. _adc-recommended-reading:
+
+Recommended Reading
+-------------------
+
+* `Wikipedia article on Analog-to-digital converter <http://en.wikipedia.org/wiki/Analog-to-digital_converter>`_
+* `Arduino Analog Input Tutorial <http://arduino.cc/en/Tutorial/AnalogInputPins>`_
+* STMicro documentation for STM32F103RB microcontroller:
+
+  * `All <http://www.st.com/mcu/devicedocs-STM32F103RB-110.html>`_
+  * `Datasheet (pdf) <http://www.st.com/stonline/products/literature/ds/13587.pdf>`_
+  * `Reference Manual (pdf) <http://www.st.com/stonline/products/literature/rm/13902.pdf>`_
+  * `Application Note on ADC Modes (pdf) <http://www.st.com/stonline/products/literature/an/16840.pdf>`_
+  * `Application Note on ADC Oversampling (pdf) <http://www.st.com/stonline/products/literature/an/14183.pdf>`_
+
+
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