kopia lustrzana https://github.com/micropython/micropython
132 wiersze
3.9 KiB
ReStructuredText
132 wiersze
3.9 KiB
ReStructuredText
.. _pyboard_tutorial_assembler:
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Inline assembler
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================
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Here you will learn how to write inline assembler in MicroPython.
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**Note**: this is an advanced tutorial, intended for those who already
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know a bit about microcontrollers and assembly language.
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MicroPython includes an inline assembler. It allows you to write
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assembly routines as a Python function, and you can call them as you would
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a normal Python function.
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Returning a value
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-----------------
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Inline assembler functions are denoted by a special function decorator.
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Let's start with the simplest example::
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@micropython.asm_thumb
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def fun():
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movw(r0, 42)
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You can enter this in a script or at the REPL. This function takes no
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arguments and returns the number 42. ``r0`` is a register, and the value
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in this register when the function returns is the value that is returned.
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MicroPython always interprets the ``r0`` as an integer, and converts it to an
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integer object for the caller.
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If you run ``print(fun())`` you will see it print out 42.
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Accessing peripherals
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---------------------
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For something a bit more complicated, let's turn on an LED::
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@micropython.asm_thumb
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def led_on():
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movwt(r0, stm.GPIOA)
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movw(r1, 1 << 13)
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strh(r1, [r0, stm.GPIO_BSRRL])
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This code uses a few new concepts:
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- ``stm`` is a module which provides a set of constants for easy
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access to the registers of the pyboard's microcontroller. Try
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running ``import stm`` and then ``help(stm)`` at the REPL. It will
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give you a list of all the available constants.
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- ``stm.GPIOA`` is the address in memory of the GPIOA peripheral.
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On the pyboard, the red LED is on port A, pin PA13.
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- ``movwt`` moves a 32-bit number into a register. It is a convenience
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function that turns into 2 thumb instructions: ``movw`` followed by ``movt``.
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The ``movt`` also shifts the immediate value right by 16 bits.
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- ``strh`` stores a half-word (16 bits). The instruction above stores
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the lower 16-bits of ``r1`` into the memory location ``r0 + stm.GPIO_BSRRL``.
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This has the effect of setting high all those pins on port A for which
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the corresponding bit in ``r0`` is set. In our example above, the 13th
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bit in ``r0`` is set, so PA13 is pulled high. This turns on the red LED.
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Accepting arguments
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-------------------
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Inline assembler functions can accept up to 4 arguments. If they are
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used, they must be named ``r0``, ``r1``, ``r2`` and ``r3`` to reflect the registers
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and the calling conventions.
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Here is a function that adds its arguments::
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@micropython.asm_thumb
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def asm_add(r0, r1):
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add(r0, r0, r1)
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This performs the computation ``r0 = r0 + r1``. Since the result is put
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in ``r0``, that is what is returned. Try ``asm_add(1, 2)``, it should return
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3.
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Loops
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-----
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We can assign labels with ``label(my_label)``, and branch to them using
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``b(my_label)``, or a conditional branch like ``bgt(my_label)``.
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The following example flashes the green LED. It flashes it ``r0`` times. ::
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@micropython.asm_thumb
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def flash_led(r0):
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# get the GPIOA address in r1
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movwt(r1, stm.GPIOA)
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# get the bit mask for PA14 (the pin LED #2 is on)
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movw(r2, 1 << 14)
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b(loop_entry)
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label(loop1)
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# turn LED on
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strh(r2, [r1, stm.GPIO_BSRRL])
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# delay for a bit
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movwt(r4, 5599900)
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label(delay_on)
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sub(r4, r4, 1)
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cmp(r4, 0)
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bgt(delay_on)
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# turn LED off
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strh(r2, [r1, stm.GPIO_BSRRH])
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# delay for a bit
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movwt(r4, 5599900)
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label(delay_off)
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sub(r4, r4, 1)
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cmp(r4, 0)
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bgt(delay_off)
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# loop r0 times
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sub(r0, r0, 1)
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label(loop_entry)
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cmp(r0, 0)
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bgt(loop1)
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Further reading
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---------------
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For further information about supported instructions of the inline assembler,
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see the :ref:`reference documentation <asm_thumb2_index>`.
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