AFC(Auto Flow Control):自動流控制。S3C44BoX中的UART用nRTS（傳送請求信號）和nCTS（清除傳送信號）來支持自動流控制，以此實現UART之間的互聯。
The S3C2440A's UART 0 and UART 1 support auto flow control with nRTS and nCTS signals. In case, it can be
connected to external UARTs. If users want to connect a UART to a Modem, disable auto flow control bit in
UMCONn register and control the signal of nRTS by software.
In AFC, nRTS depends on the condition of the receiver and nCTS signals control the operation of the transmitter.
The UART's transmitter transfers the data in FIFO only when nCTS signals are activated (in AFC, nCTS means
that other UART's FIFO is ready to receive data). Before the UART receives data, nRTS has to be activated when
its receive FIFO has a spare more than 32-byte and has to be inactivated when its receive FIFO has a spare under
32-byte (in AFC, nRTS means that its own receive FIFO is ready to receive data).
