Docs: Improve external-interrupts.rst.

Make the language friendlier; point to GPIO port definition for an
explanation of the EXTI line definition.

1 files changed, 24 insertions, 15 deletions

diff --git a/source/external-interrupts.rst b/source/external-interrupts.rst
index ccb9afd..9089d77 100644
--- a/source/external-interrupts.rst
+++ b/source/external-interrupts.rst
@@ -5,8 +5,8 @@
 External Interrupts
 ===================
 
-External interrupts can be used to trigger routines to run in response
-to changes in voltage on a pin.  Each :ref:`GPIO pin <gpio>` can be
+External interrupts can be used to make a voltage change on a pin
+cause a function to be called.  Each :ref:`GPIO pin <gpio>` can be
 used to detect transitions, such as when the voltage goes from
 :ref:`LOW <lang-constants-low>` to :ref:`HIGH <lang-constants-high>`,
 or from ``HIGH`` to ``LOW``. This can be used to avoid checking for
@@ -20,22 +20,21 @@ Overview
 --------
 
 External interrupts are often used to detect when events happen
-outside of the microcontroller. These can be used to tell Maple when
-events happen, such as when a sensor has data ready to be read, or
-when a button has been pushed.  When such an event happens, an
-interrupt is raised, and the Maple can react to it with a preset
-*interrupt handler*, which is a function that gets called whenever the
-event occurs.
+outside of the Maple. These can be used to tell Maple when events
+happen, such as when a sensor has data ready to be read, or when a
+button has been pushed.  When such an event happens, an interrupt is
+raised, and the Maple stops whatever it was doing to react to it by
+calling a function (called an *interrupt handler*) which you specify
+using :ref:`lang-attachinterrupt`.
 
 .. _external-interrupts-exti-line:
 
-Every GPIO pin can generate an external interrupt, subject to certain
-constraints.  There can be a maximum of 16 different external
-interrupts set up at a time.  This is because the external interrupt
-lines on the STM32 are shared between GPIO ports. In effect, this
-means that every pin on the Maple connects to what is called an *EXTI
-line*, and within an EXTI line, only one of the pins that connects to
-it can be used to detect external interrupts at a time.
+Every pin can generate an external interrupt, but there are some
+restrictions.  At most 16 different external interrupts can be used at
+one time.  Further, you can't just pick any 16 pins to use.  This is
+because every pin on the Maple connects to what is called an *EXTI
+line*, and only one pin per EXTI line can be used for external
+interrupts at a time [#fextisports]_.
 
 .. TODO [0.0.12] Maple Native links
 
@@ -65,3 +64,13 @@ Recommended Reading
   <http://www.st.com/stonline/products/literature/rm/13902.pdf>`_
   (PDF), Chapter 9, "General-purpose and alternate-function I/Os", and
   Chapter 10, "Interrupts and Events".
+
+.. rubric:: Footnotes
+
+.. [#fextisports] The underlying reason for this restriction is that
+   the external interrupt lines on the STM32 are shared between
+   :ref:`GPIO ports <gpio-ports>`.  There can be only one external
+   interrupt on each GPIO bit, out of all of the ports.  That is, if
+   PA4 has an external interrupt on it, then PB4 can't have one, too.
+   Since the GPIO bit numbers only go from 0 to 15, there can only be
+   16 external interrupts at a time.