K60P100M100SF2RM Datasheet, PDF (1564/1809 Pages) Freescale Semiconductor, Inc – K60 Sub-Family Reference Manual

Application information
51.8.7.2 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.
UART
TRANSMITTER
TXD
RTS_B
RS-485 TRANSCEIVER
DI
Y
DE
DRIVER
Z
RECEIVER
RXD
RO
RE_B
A
RECEIVER _B
Figure 51-215. Transceiver driver enable using RTS
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.
51.8.8 IrDA minimum pulse width
The IrDA specifies a minimum pulse width of 1.6 µs. The UART hardware does not
include a mechanism to restrict/force the pulse width to be greater than or equal to 1.6 µs.
However, configuring the baud rate to 115.2 kbit/s and the narrow pulse width to 3/16 of
a bit time results in a pulse width of 1.6 µs.
51.8.9 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:
1. IS7816[WT] is programmed to assert after 9600 cycles of unresponsiveness.
2. The 9600 cycles pass without a response resulting in the WT interrupt asserting.
3. The IS7816[WT] is cleared at cycle 9700 by the interrupt service routine.
1564
K60 Sub-Family Reference Manual, Rev. 6, Nov 2011
Freescale Semiconductor, Inc.