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| -rw-r--r-- | source/external-interrupts.rst | 39 |
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. |