10. Inline assembler

Here you will learn how to write inline assembler in MicroPython.

Note: this is an advanced tutorial, intended for those who already know a bit about microcontrollers and assembly language.

MicroPython includes an inline assembler. It allows you to write assembly routines as a Python function, and you can call them as you would a normal Python function.

10.1. Returning a value

Inline assembler functions are denoted by a special function decorator. Let’s start with the simplest example:

def fun():
    movw(r0, 42)

You can enter this in a script or at the REPL. This function takes no arguments and returns the number 42. r0 is a register, and the value in this register when the function returns is the value that is returned. MicroPython always interprets the r0 as an integer, and converts it to an integer object for the caller.

If you run print(fun()) you will see it print out 42.

10.2. Accessing peripherals

For something a bit more complicated, let’s turn on an LED:

def led_on():
    movwt(r0, stm.GPIOA)
    movw(r1, 1 << 13)
    strh(r1, [r0, stm.GPIO_BSRRL])

This code uses a few new concepts:

  • stm is a module which provides a set of constants for easy access to the registers of the pyboard’s microcontroller. Try running import stm and then help(stm) at the REPL. It will give you a list of all the available constants.
  • stm.GPIOA is the address in memory of the GPIOA peripheral. On the pyboard, the red LED is on port A, pin PA13.
  • movwt moves a 32-bit number into a register. It is a convenience function that turns into 2 thumb instructions: movw followed by movt. The movt also shifts the immediate value right by 16 bits.
  • strh stores a half-word (16 bits). The instruction above stores the lower 16-bits of r1 into the memory location r0 + stm.GPIO_BSRRL. This has the effect of setting high all those pins on port A for which the corresponding bit in r0 is set. In our example above, the 13th bit in r0 is set, so PA13 is pulled high. This turns on the red LED.

10.3. Accepting arguments

Inline assembler functions can accept up to 4 arguments. If they are used, they must be named r0, r1, r2 and r3 to reflect the registers and the calling conventions.

Here is a function that adds its arguments:

def asm_add(r0, r1):
    add(r0, r0, r1)

This performs the computation r0 = r0 + r1. Since the result is put in r0, that is what is returned. Try asm_add(1, 2), it should return 3.

10.4. Loops

We can assign labels with label(my_label), and branch to them using b(my_label), or a conditional branch like bgt(my_label).

The following example flashes the green LED. It flashes it r0 times.

def flash_led(r0):
    # get the GPIOA address in r1
    movwt(r1, stm.GPIOA)

    # get the bit mask for PA14 (the pin LED #2 is on)
    movw(r2, 1 << 14)



    # turn LED on
    strh(r2, [r1, stm.GPIO_BSRRL])

    # delay for a bit
    movwt(r4, 5599900)
    sub(r4, r4, 1)
    cmp(r4, 0)

    # turn LED off
    strh(r2, [r1, stm.GPIO_BSRRH])

    # delay for a bit
    movwt(r4, 5599900)
    sub(r4, r4, 1)
    cmp(r4, 0)

    # loop r0 times
    sub(r0, r0, 1)
    cmp(r0, 0)

10.5. Further reading

For further information about supported instructions of the inline assembler, see the reference documentation.