uart设计，发送模块，无校验位。先输出一个低电平的起始位，然后从低到高输出8个数据位，接着是可选的奇偶校验位（这里没有），最后是高电平的停止位。-uart design, transmit module, no parity. First output of the start bit of a low level, and　low to high output 8 data bits, then the optional parity bit (there is no), the last stop bit is high.