按照这里的教程我用了TTL-USB，出现了上面我说的问题。所以TTL-USB不能用吗？
以及，我是需要用openmv给esp8266传输数据，也需要扩展版吗？

我用IDE把程序烧录到openmv之后，串口调试助手显示“串口打开失败”

需要自己拔一次数据线再重新连接，串口会显示已打开，但是无法输出数据
且过几秒之后IDE会这样

代码就是最简单的让openmv输出helloworld

# UART Control
#
# This example shows how to use the serial port on your OpenMV Cam. Attach pin
# P4 to the serial input of a serial LCD screen to see "Hello World!" printed
# on the serial LCD display.

import time
from pyb import UART

# Always pass UART 3 for the UART number for your OpenMV Cam.
# The second argument is the UART baud rate. For a more advanced UART control
# example see the BLE-Shield driver.
uart = UART(3, 19200)

while(True):
    uart.write("Hello World!\r")
    time.sleep_ms(1000)