按住boot再按rst后，系统提示无法识别USB的设备（设备描述符请求失败）。
（rv_debugger_plus进入boot后可以正确识别串口）
若进入boot后接串口转usb模块烧录，则会握手失败，报错如下
[10:47:59.355] - ========= load eflash_loader.bin =========
[10:47:59.355] - Load eflash_loader.bin via uart
[10:47:59.355] - ========= image load =========
[10:47:59.840] - tx rx and power off, press the machine!
[10:47:59.840] - cutoff time is 0.05
[10:47:59.902] - power on tx and rx
[10:48:00.903] - reset cnt: 0, reset hold: 0.05, shake hand delay: 0.1
[10:48:00.903] - clean buf
[10:48:01.027] - send sync
[10:48:01.137] - ack is b''
[10:48:01.137] - retry
[10:48:01.403] - tx rx and power off, press the machine!
[10:48:01.403] - cutoff time is 0.05
[10:48:01.465] - power on tx and rx
[10:48:02.459] - reset cnt: 0, reset hold: 0.05, shake hand delay: 0.1
[10:48:02.459] - clean buf
[10:48:02.584] - send sync
[10:48:02.694] - ack is b''
[10:48:02.694] - retry
[10:48:02.959] - tx rx and power off, press the machine!
[10:48:02.959] - cutoff time is 0.05
[10:48:03.022] - power on tx and rx
[10:48:04.027] - reset cnt: 0, reset hold: 0.05, shake hand delay: 0.1
[10:48:04.027] - clean buf
[10:48:04.152] - send sync
[10:48:04.262] - ack is b''
[10:48:04.262] - retry
[10:48:04.324] - shake hand fail
[10:48:04.324] - ########################################################################
[10:48:04.324] - 请按照以下描述排查问题：
[10:48:04.324] - GPIO28是否上拉到板子自身的3.3V，而不是外部的3.3V
[10:48:04.324] - GPIO15(RX)是否连接到USB转串口的TX引脚
[10:48:04.324] - GPIO14(TX)是否连接到USB转串口的RX引脚
[10:48:04.324] - 在使用烧录软件进行烧录前，是否在GPIO28拉高的情况下，使用Reset/Chip_En复位了芯片
[10:48:04.324] - 烧录软件所选择的COM口，是否是连接芯片的串口
[10:48:04.324] - 烧录软件上选择的波特率是否是USB转串口支持的波特率
[10:48:04.324] - 3.3V供电是否正常
[10:48:04.324] - 板子供电电流是否正常(烧录模式下，芯片耗电电流5-7mA)
[10:48:04.324] - ########################################################################
[10:48:04.324] - shake hand fail
[10:48:04.340] - Load helper bin time cost(ms): 4984.44580078125
[10:48:04.340] - {"ErrorCode": "0003","ErrorMsg":"BFLB LOAD HELP BIN FAIL"}
[10:48:04.340] - Burn Retry
[10:48:04.340] - 0
[10:48:04.340] - Burn return with retry fail
