35.2.2 UART Asynchronous Receiver

The Asynchronous mode is typically used in RS-232 systems. The receiver block diagram is shown in Figure 35-2. The data are received on the RX pin and drive the data recovery block. The data recovery block is actually a high-speed shifter operating at 4 or 16 times the baud rate, whereas the serial Receive Shift Register (RSR) operates at the bit rate. When all bits of the character have been shifted in, they are immediately transferred to a two-character First-In First-Out (FIFO) memory. The FIFO buffering allows reception of two complete characters and the start of a third character before software must begin servicing the UART receiver. The FIFO registers and RSR are not directly accessible by software. Access to the received data is made via the UxRXB register.