bl led_flicker

led_flicker:
        // 配置寄存器
        ldr r0, =0x56000050
        ldr r1, (1<<4*2)
        str r1, [r0]

        ldr r0, =0x56000054
        ldr r1, =0
        ldr r2, =(1<<4)

loop:
        // 循环点亮，熄灭
        str r1, [r0]
        bl  delay
        str r2, [r0]
        bl  delay

delay:
        ldr r3, =30000

1:
        sub r3, r3, #1
        cmp r3, #0
        bne 1b
        mov pc, =lr