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UART Modules
MCF5272 ColdFire
®
Integrated Microprocessor User’s Manual, Rev. 3
16-22
Freescale Semiconductor
Once enabled, the autobaud detector can calculate the transmission rate only once. If a new calculation is
required, UCR[ENAB] must first be cleared and then set again. Thus, a new UART connection cannot be
hot-plugged into an active UART connection if that connection is operating at a different rate without first
reinitializing the autobaud detector.
16.5.2
Transmitter and Receiver Operating Modes
is a functional block diagram of the transmitter and receiver showing the command and
operating registers, which are described generally in the following sections and described in detail in
Section 16.3, “Register Descriptions
.”
Figure 16-24. Transmitter and Receiver Functional Diagram
16.5.2.1
Transmitting
The transmitter is enabled through the UART command register (UCR
n
). When it is ready to accept a
character, the UART sets USR
n
[TxRDY]. The transmitter converts parallel data from the CPU to a serial
bit stream on TxD. It automatically sends a start bit followed by the programmed number of data bits, an
optional parity bit, and the programmed number of stop bits. The lsb is sent first. Data is shifted from the
transmitter output on the falling edge of the clock source.
After the stop bits are sent, if no new character is in the transmitter holding register, the TxD output
remains high (mark condition) and the transmitter empty bit, USR
n
[TxEMP], is set. Transmission resumes
and TxEMP is cleared when the CPU loads a new character into the UART transmitter buffer (UTB
n
). If
the transmitter receives a disable command, it continues until any character in the transmitter shift register
is completely sent.
UART Command Register (UCR0)
W
UART Status Register (USR0)
R
Transmitter Shift Register
UART Mode Register 1 (UMR1)
R/W
UART Mode Register 2 (UMR2)
R/W
Transmitter FIFO
W
Receiver Shift Register
Receiver FIFO
R
UART
External
Interface
RXD
TXD
UART Transmit
FIFO (UTB)
(24 Bytes)
UART Receive
FIFO (URB)
(24 Bytes)