
15-4
MCF5249UM
MOTOROLA
Operation
15.2.3
REQUEST-TO-SEND
The Request-To-Send (RTS) pins RTS0/GPO30 and RTS1/GPO31 are multiplexed with general purpose
output pins. When programmed for RTS, this active-low output signal can be programmed to be
automatically negated and asserted by either the receiver or transmitter. When connected to the
clear-to-send (CTS) input of a transmitter, this signal controls serial data flow.
15.2.4
CLEAR-TO-SEND
The multiplexed signals CTS0/GPI30 and CTS1/GPI31 can be programmed as general purpose inputs or
Clear-To-Send inputs. When programmed as (CTS), this active-low input is the clear-to-send input and
can generate an interrupt on change-of-state.
15.3
OPERATION
The following sections describe the operation of the baud-rate generator, transmitter and receiver, and
other operating modes of the UART module.
15.3.1
BAUD-RATE GENERATOR/TIMER
The timer references made here relative to clocking the UART are different than the MCF5249 timer
module that is integrated on the bus of the ColdFire core. The UART has a baud generator based on an
internal baud-rate timer that is dedicated to the UART. The Clock Select Register (USCR) can be
programmed to enable the baud-rate timer or an external clock source from TIN to generate baud rates.
When the baud-rate timer is used, a prescaler supplies an asynchronous 32x clock source to the baud-rate
timer. The baud-rate timer register value is programmed with the UBG1 and UBG2 registers. See
for more information.
An external TIN clock source, when enabled in the USCR, can generate an x1 or x16 asynchronous or
synchronous clock to the UART receiver and transmitter.
Figure 15-3 shows the relationship of clocking
sources.
Figure 15-3 Baud-Rate Timer Generator Diagram
MCF5249 UART
MCF5249 TIMER
TIN
TOUT
TIN
SYSTEM CLOCK
x1
PRESCALAR
x16
PRESCALAR
TIMER
OUTPUT
INTERNAL
TIMER
x32
PRESCALAR
TIN
BAUD
RATE
BAUD RATE OUTPUT PROGRAMMED IN USCR
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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