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UART Modules
Freescale Semiconductor
32-18
Therefore, UBG1
n
equals 0x01 and UBG2
n
equals 0xB0.
32.4.1.2.2
External Clock
An external source clock (DT
n
IN) passes through a divide-by-1 or 16 prescaler. If f
extc
is the external clock
frequency, baud rate can be described with this equation:
Eqn. 32-2
32.4.2
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. For detailed descriptions, refer
to
Section 32.3, “Memory Map/Register Definition
.”
Figure 32-18. Transmitter and Receiver Functional Diagram
32.4.2.1
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
Baudrate
f
extc
(16 or 1)
---------------------
=
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
Transmit Buffer
(UTB
n
)
(2 Registers)
FIFO
U
n
RXD
U
n
TXD
Содержание MCF54455
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Страница 145: ...Enhanced Multiply Accumulate Unit EMAC 5 21 Freescale Semiconductor ...
Страница 173: ...Cache 6 28 Freescale Semiconductor ...
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Страница 323: ...Reset Controller Module 13 8 Freescale Semiconductor ...
Страница 389: ...Pin Multiplexing and Control 16 44 Freescale Semiconductor ...
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