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UART Modules
MCF5213 ColdFire® Integrated Microcontroller Reference Manual, Rev. 3
Freescale Semiconductor
21-17
21.4.1.1
Programmable Divider
n
transmitter and receiver can use the following clock sources:
•
An external clock signal on the DT
n
IN pin.
When not divided,
DT
n
IN
provides a synchronous
clock; when divided by 16, it is asynchronous.
•
The internal bus clock supplies an asynchronous clock source divided by 32 and then divided by
the 16-bit value programmed in UBG1
n
and UBG2
n
. See
Section 21.3.11, “UART Baud Rate
Generator Registers (UBG1n/UBG2n)
The choice of DTIN or internal bus clock is programmed in the UCSR.
Figure 21-17. Clocking Source Diagram
NOTE
If DT
n
IN is a clocking source for the timer or UART, that timer module
cannot use DT
n
IN for timer input capture.
21.4.1.2
Calculating Baud Rates
The following sections describe how to calculate baud rates.
21.4.1.2.1
Internal Bus Clock Baud Rates
When the internal bus clock is the UART clocking source, it goes through a divide-by-32 prescaler and
then passes through the 16-bit divider of the concatenated UBG1
n
and UBG2
n
registers. The baud-rate
calculation is:
Eqn. 21-1
Using a 66-MHz internal bus clock and letting baud rate equal 9600, then
Eqn. 21-2
Therefore, UBG1
n
equals 0x00 and UBG2
n
equals 0xD6.
21.4.1.2.2
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. 21-3
21.4.2
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 21.3, “Memory Map/Register Definition
.”
Baudrate
f
sys
32 x Divider
[
]
------------------------------------
=
Divider
66
MHz
32 x 9600
[
]
-------------------------------
215
decimal
(
)
0x00D6
hexadecimal
(
)
=
=
=
Baudrate
f
extc
(16 or 1)
---------------------
=
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