TRANSMITTER
UART
RECEIVER
DRIVER
RS-485 TRANSCEIVER
RECEIVER
TXD
RTS_B
RXD
DI
DE
RO
RE_B
Y
Z
A
B
Figure 47-29. Transceiver driver enable using RTS
In the figure, the receiver enable signal is asserted. Another option for this 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.
47.8.8
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.
47.8.9
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. In most
cases, if the condition that caused the interrupt to trigger still exists when the interrupt is
cleared, then 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.
4. After the WT interrupt has been cleared, the smartcard remains unresponsive. At
cycle 9701 the WT interrupt will be reasserted.
Application information
K22F Sub-Family Reference Manual, Rev. 4, 08/2016
1300 NXP Semiconductors
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