Micro:bit hangs at read_analog() - MicroPython

terminal_free_20ms_before.py

# Add your Python code here. E.g.
from microbit import *

uart.init(115200)

x = 0
pin0.write_analog(80)
sleep(1000)

while True:
    x = x + 1
    # enough time, OK!
    sleep(20)
    v = pin0.read_analog()
    print("x = {}, analog = {}".format(x, v))

terminal_hang_10ms.py

# Add your Python code here. E.g.
from microbit import *

uart.init(115200)

x = 0
pin0.write_analog(80)
sleep(1000)

while True:
    x = x + 1
    # there is not enough time to read analog, hangs at x = 177
    sleep(10)
    v = pin0.read_analog()
    print("x = {}, analog = {}".format(x, v))

terminal_hang_20ms_after.py

# Add your Python code here. E.g.
from microbit import *

uart.init(115200)

x = 0
pin0.write_analog(80)
sleep(1000)

while True:
    x = x + 1
    # hangs at x = 1000~1700
    v = pin0.read_analog()
    sleep(20)
    print("x = {}, analog = {}".format(x, v))