说明：  C8051检测用户设定的波特率，然后初始化自己的波特率。
(This code attempts to automatically detect the user s baud rate and initialize the 8051 s built-in uart. More specifically, these steps are taken: 1.If the code was assembled with the correct baud rate, skip to step 5. 2.If we have previously detected the baud rate, skip to step 5. 3.Wait for the user to press Enter (aka Carriage Return) and count how long it takes for the bits to arrive. 4.Calculate the required timer1 reload value for the user s baud rate. 5.Store the result so that we can (hopefully) skip having to detect the baud rate if this code is called again. 6.Initialize the various special function registers. )