From 735e925643458ac51e845b721e76ce7f92c8fd8d Mon Sep 17 00:00:00 2001 From: Marti Bolivar Date: Fri, 27 May 2011 22:43:00 -0400 Subject: Docs: Improve external-interrupts.rst. Make the language friendlier; point to GPIO port definition for an explanation of the EXTI line definition. --- docs/source/external-interrupts.rst | 39 +++++++++++++++++++++++-------------- 1 file changed, 24 insertions(+), 15 deletions(-) (limited to 'docs/source') diff --git a/docs/source/external-interrupts.rst b/docs/source/external-interrupts.rst index ccb9afd..9089d77 100644 --- a/docs/source/external-interrupts.rst +++ b/docs/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 ` 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 ` can be used to detect transitions, such as when the voltage goes from :ref:`LOW ` to :ref:`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 `_ (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 `. 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. -- cgit v1.2.3