MicroPython • DMA produces jibberish after ~14000 floating point operations

Code:

from machine import Pinfrom rp2 import PIO, StateMachine, asm_pio, DMAfrom array import arrayfrom uctypes import addressofimport timePIO0_TXF0       = 0x50200010PIO0_SM0_CLKDIV = 0x502000c8DMA0_Read  = 0x50000000; DMA1_Read  = 0x50000040DMA0_Write = 0x50000004; DMA1_Write = 0x50000044DMA0_Count = 0x50000008; DMA1_Count = 0x50000048DMA0_Trig  = 0x5000000C; DMA1_Trig  = 0x5000004CDMA0_Ctrl  = 0x50000010; DMA1_Ctrl  = 0x50000050BASEPIN = 9N=512buffer = bytearray(N)FREQUENCY = 440           # global, used by stop()# Get free memoryimport gcdef get_free_memory():    return gc.mem_free()    print(str(get_free_memory()) + " bytes free")def out_zero(bipolar = 1):    Pin(BASEPIN, Pin.OUT).value(0)@asm_pio(out_init=PIO.OUT_LOW, out_shiftdir=PIO.SHIFT_RIGHT,         autopull=True, pull_thresh=32)def parallel():    out(pins,8)sm = StateMachine(0, parallel, freq=100_000_000, out_base=Pin(BASEPIN))dma0 = DMA()dma1 = DMA()def DMA_Stop():    dma1.ctrl = 0    dma0.ctrl = 0    def DMA_Start(words):    # dma0: From buffer to port (to state machine)    # size = 2 -> 32 bit transfer   (1 -> B2 signal)    control0 = dma0.pack_ctrl(inc_read=True,                              inc_write=False,                              size=2,                              treq_sel=0,                              enable = 1,                              high_pri= 1,                              chain_to = 1)    dma0.config(read=buffer,                write=PIO0_TXF0,                count=words,                ctrl=control0)    # dma1: chain dma0 for next package of data    control1 = dma1.pack_ctrl(inc_read=False,                              inc_write=False,                              size=2,                              treq_sel=63,                              high_pri= 1,                              enable = 1,                              chain_to = 0)    dma1.config(read=addressof(array('i',[addressof(buffer)])),                write=DMA0_Read,                count=1,                ctrl=control1,                trigger = 1)#------------------------------------------------------------------def set_N(n):    global N, buffer    N = n    buffer = bytearray(N)    def set_buffer(yvalues):    global buffer    n = len(yvalues)    i = 0    for y in yvalues:        buffer[i] = y        i+=1def rect():    global buffer    for i in range(N):        if i<= N/2:            buffer[i] = 0         else:            buffer[i] = 255def start(f):    global sm, FREQUENCY    FREQUENCY = f    stop()        sm=StateMachine(0, parallel, freq= int(f*N), out_base=Pin(BASEPIN))    DMA_Start(int(N/4))    sm.active(1)    print("f = ", int(f*N)/N)def stop():    global sm    DMA_Stop()    if FREQUENCY < 40:        time.sleep(0.1)      # for very low frequencies allow DMA transfer to terminate    sm.active(0)    time.sleep(0.1)    out_zero(bipolar = True)def mainloop():    rect()    start(440)    v = 0    while(True):        v = 0.1 * 2if __name__ == "__main__":    mainloop()