使用单片机STC89C51RC普通IO口模拟串口三种方法-// 1. get_rx_pin_status()

// Returns 0 or 1 dependent on whether the receive pin is high or low.

// 2. set_tx_pin_high()

// Sets the transmit pin to the high state.

// 3. set_tx_pin_low()

// Sets the transmit pin to the low state.

// 4. idle()

// Background functions to execute while waiting for input.

// 5. timer_set( BAUD_RATE )

// Sets the timer to 3 times the baud rate.

// Enables the timer interrupt.