通过时钟使普通的IO口模拟UART串口通讯(Use Timer_A CCR1 hardware output modes and CCR0 SCCI data latch
//　to implement UART function @ 2400 baud. Software does not directly read and
//　write to RX and TX pins, instead proper use of output modes and SCCI data
//　latch are demonstrated. Use of these hardware features eliminates ISR
//　latency effects as hardware insures that output and input bit latching and
//　timing are perfectly synchronised with Timer_A regardless of other
//　software activity. In the Mainloop the UART function readies the UART to
//　receive one character and waits in LPM3 with all activity interrupt driven.
//　After a character has been received, the UART receive function forces exit
//　from LPM3 in the Mainloop which echo's back the received character.
//　ACLK = TACLK = LFXT1 = 32768Hz, MCLK = SMCLK = default DCO)