difference between these 2 UART driver:
1) boot uart driver no interrupt support
2) boot uart driver all function run in RAM. no ICACHE_FLASH_ATTR.
baudrate at 230400: send are 0xC0, 0x16, 0x06, 0x40, 0x00, 0x00, 0xxx, 0xxx, 0xC0, receive are 0x90, 0x8a, 0x40, 0xd9, 0xf0
could someone tell me why the uart_div_modify() failed?