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:
@micropython.asm_thumb 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:
@micropython.asm_thumb def led_on(): movwt(r0, stm.GPIOA) movw(r1, 1 << 13) strh(r1, [r0, stm.GPIO_BSRRL])
This code uses a few new concepts:
stmis a module which provides a set of constants for easy access to the registers of the pyboard’s microcontroller. Try running
import stmand then
help(stm)at the REPL. It will give you a list of all the available constants.
stm.GPIOAis the address in memory of the GPIOA peripheral. On the pyboard, the red LED is on port A, pin PA13.
movwtmoves a 32-bit number into a register. It is a convenience function that turns into 2 thumb instructions:
movtalso shifts the immediate value right by 16 bits.
strhstores a half-word (16 bits). The instruction above stores the lower 16-bits of
r1into 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
r0is set. In our example above, the 13th bit in
r0is set, so PA13 is pulled high. This turns on the red LED.
10.3. Accepting arguments¶
Inline assembler functions can accept up to 3 arguments. If they are
used, they must be named
r2 to reflect the registers
and the calling conventions.
Here is a function that adds its arguments:
@micropython.asm_thumb def asm_add(r0, r1): add(r0, r0, r1)
This performs the computation
r0 = r0 + r1. Since the result is put
r0, that is what is returned. Try
asm_add(1, 2), it should return
We can assign labels with
label(my_label), and branch to them using
b(my_label), or a conditional branch like
The following example flashes the green LED. It flashes it
@micropython.asm_thumb 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) b(loop_entry) label(loop1) # turn LED on strh(r2, [r1, stm.GPIO_BSRRL]) # delay for a bit movwt(r4, 5599900) label(delay_on) sub(r4, r4, 1) cmp(r4, 0) bgt(delay_on) # turn LED off strh(r2, [r1, stm.GPIO_BSRRH]) # delay for a bit movwt(r4, 5599900) label(delay_off) sub(r4, r4, 1) cmp(r4, 0) bgt(delay_off) # loop r0 times sub(r0, r0, 1) label(loop_entry) cmp(r0, 0) bgt(loop1)
10.5. Further reading¶
For further information about supported instructions of the inline assembler, see the reference documentation.