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10-2 Intel® PXA255 Processor Developer’s Manual
UARTs
10.2 Overview
Each serial port contains a UART and a slow infrared transmit encoder and receive decoder that
conforms to the IRDA Serial Infrared (SIR) Physical Layer Link Specification.
Each UART performs serial-to-parallel conversion on data characters received from a peripheral
device or a modem and parallel-to-serial conversion on data characters received from the
processor. The processor can read a UART’s complete status during functional operation. Status
information includes the type and condition of transfer operations and error conditions (parity,
overrun, framing, or break interrupt) associated with the UART.
Each serial port operates in FIFO or non-FIFO mode. In FIFO mode, a 64-byte Transmit FIFO
holds data from the processor until it is transmitted on the serial link and a 64-byte Receive FIFO
buffers data from the serial link until it is read by the processor. In non-FIFO mode, the transmit
and Receive FIFOs are bypassed.
Each UART includes a programmable baud rate generator that can divide the input clock by 1 to
(2
16
–1). This produces a 16X clock that can be used to drive the internal transmitter and receiver
logic. Software can program interrupts to meet its requirements. This minimizes the number of
computations required to handle the communications link. Each UART operates in an environment
that is controlled by software and can be polled or is interrupt driven.
10.2.1 Full Function UART
The FFUART supports modem control capability. The maximum tested baud rate on this UART is
230.4 kbps. The divisor programmed in the divisor latch registers must be equal to or greater than
four.
10.2.2 Bluetooth UART
The BTUART is a high speed UART that supports baud rates up to 921.6 kbps and can be
connected to a Bluetooth module. It supports the functions in the feature list, (see Section 10.1) but
only supports two modem control pins (nCTS, nRTS).
10.2.3 Standard UART
The STUART supports all functions in the feature list (see Section 10.1), but does not support
modem control capability. The UART’s maximum tested baud rate is 230.4 kbps. The divisors
programmed in divisor latch registers must be equal to or greater than four.
10.2.4 Compatibility with 16550
The processor UARTs are functionally compatible with the 16550 industry standard. Each UART
supports all of the 16550’s functions as well as the following features:
• DMA requests for transmit and receive data services
• Slow infrared asynchronous interface
• Non-Return-to-Zero (NRZ) encoding/decoding function