UART Modules
MCF5213 ColdFire® Integrated Microcontroller Reference Manual, Rev. 3
21-18
Freescale Semiconductor
Figure 21-18. Transmitter and Receiver Functional Diagram
21.4.2.1
Transmitter
The transmitter is enabled through the UART command register (UCR
n
). When it is ready to accept a
character, UART sets USR
n
[TXRDY]. The transmitter converts parallel data from the CPU to a serial bit
stream on U
n
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 U
n
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 transmit buffer
(UTB
n
). If the transmitter receives a disable command, it continues until any character in the transmitter
shift register is completely sent.
If the transmitter is reset through a software command, operation stops immediately (see
“UART Command Registers (UCRn)
”). The transmitter is reenabled through the UCR
n
to resume
operation after a disable or software reset.
If the clear-to-send operation is enabled, U
n
CTS must be asserted for the character to be transmitted. If
U
n
CTS is negated in the middle of a transmission, the character in the shift register is sent and U
n
TXD
remains in mark state until U
n
CTS is reasserted. If transmitter is forced to send a continuous low condition
by issuing a
SEND
BREAK
command, transmitter ignores the state of U
n
CTS.
Receiver Shift Register
UART Command Register (UCR
n
)
W
UART Status Register (USR
n
)
R
Transmitter Shift Register
UART Mode Register 1 (UMR1
n
)
R/W
UART Mode Register 2 (UMR2
n
)
R/W
Transmitter Holding Register
W
Receiver Holding Register 3
Receiver Holding Register 2
Receiver Holding Register 1
R
UART Receive
UART
Buffer (URB
n
)
(4 Registers)
UART
n
External
Interface
U
n
RXD
U
n
TXD
Transmit Buffer
(UTB
n
)
(2 Registers)
FIFO