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和335x一样会打印CCCC，请看下面TRM手册里的说明。 5.4.7.1 UART Bootloader Operation Once the ROM Code configures the UART, it sends the UART pings for few seconds, which can be seen in the host. The pings consist of an ASCII capital C character. The UART boot mode supports only the CRC mode of XMODEM and does not support CHECKSUM mode. Both 128 and 1024 byte block sizes are supported. https://www.ti.com/lit/ug/spruiv7a/spruiv7a.pdf
