mcf51jf128 Freescale Semiconductor, Inc, mcf51jf128 Datasheet - Page 1134

• Manufacturer Part Number: mcf51jf128
• Description: Mcf51jf128 Reference Manual
• Manufacturer: Freescale Semiconductor, Inc

Application information
The transmitter's CTS signal can be used for hardware flow control whether its RTS
signal is used for hardware flow control, transceiver driver enable, or not at all.
43.8.7.2 Transceiver driver enable using RTS
RS-485 is a multiple drop communication protocol in which the UART transceiver's
driver is 3-stated unless that UART is driving. The RTS signal can be used by the
transmitter to enable the driver of a transceiver. The polarity of RTS can be matched to
the polarity of the transceiver's driver enable signal. Refer to the following figure.
In the figure, the receiver enable signal is asserted. Another option for that connection is
to connect RTS_B to both DE and RE_B. The transceiver's receiver is disabled while
driving. A pullup can pull RXD to a non-floating value during this time. This option can
be refined further by operating the UART in single-wire mode, freeing the RXD pin for
other uses.
43.8.8 Clearing 7816 wait timer (WT, BWT, CWT) interrupts
The 7816 wait timer interrupts associated with IS7816[WT], IS7816[BWT] and
IS7816[CWT] will automatically reassert if they are cleared and the wait time is still
violated. This behavior is similar to most of the other interrupts on the UART as in most
cases if the condition that caused the interrupt to trigger still exists when the interrupt is
cleared, than the interrupt will reassert. For example, consider the following scenario:
1134
Figure 43-119. Transceiver driver enable using RTS
TRANSMITTER
MCF51JF128 Reference Manual, Rev. 2, 03/2011
RECEIVER
UART
TRANSMITTER
RECEIVER
Figure 43-118. Ready-to-receive
UART
RTS_B
TXD
RXD
RTS_B
CTS_B
TXD
RXD
Preliminary
DE
RE_B
DI
RO
RS-485 TRANSCEIVER
RTS_B
CTS_B
TXD
RXD
RECEIVER
DRIVER
TRANSMITTER
RECEIVER
UART
Y
Z
A
B
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