button_asm.py

# (c) 2024 David Whale - 20/03/2024
# ARM thumb assembly language that reads button A

from microbit import *
from micropython import const

REG_GPIO_P0     = const(0x50000000)
REG_GPIO_IN     = const(REG_GPIO_P0 + 0x510)
BUTTON_A_PIN    = const(14) # P0_14 of the nRF5x GPIO block

@micropython.asm_thumb
def button_a_pressed():
    movwt(r0, REG_GPIO_IN)   # r0 = REG_GPIO_IN   GPIO input register address
    ldr(r0, [r0, 0])         # r0 = mem[r0]       read GPIO pin states
    movw(r1, BUTTON_A_PIN)   # r1 = BUTTON_A_PIN  bit number we are interested in
    lsr(r0, r1)              # r0 = r0 >> r1      right shift our bit down to bit 0
    mov(r1, 1)               # r1 = 1
    and_(r0, r1)             # r0 = r0 & r1       mask out all other bits except bit 0
    eor(r0,r1)               # r0 = r0 ^ r1       invert the bit, low=pressed, high=released
    # value in r0 at the end, is the value returned back to python
    
while True:
    if button_a_pressed():
        display.show(Image.YES)
    else:
        display.show(Image.NO)