NXP Semiconductors K22F series, Reference Manual, Page: 1285 / 1407

TXD pin driver. Both the transmitter and receiver must be enabled (C2[TE] = 1 and
C2[RE] = 1). When C7816[ISO_7816EN] is set, it is not required that both C2[TE] and
C2[RE] are set.
47.4.6 Loop operation
In loop operation, the transmitter output goes to the receiver input. The unsynchronized
receiver input signal is disconnected from the UART.
RXD
Tx pin output
RXINV
TXINV
TRANSMITTER
RECEIVER
Figure 47-25. Loop operation (C1[LOOPS] = 1, C1[RSRC] = 0)
Enable loop operation by setting C1[LOOPS] and clearing C1[RSRC]. Setting
C1[LOOPS] disables the path from the unsynchronized receiver input signal to the
receiver. Clearing C1[RSRC] connects the transmitter output to the receiver input. Both
the transmitter and receiver must be enabled (C2[TE] = 1 and C2[RE] = 1). When
C7816[ISO_7816EN] is set, it is not required that both C2[TE] and C2[RE] are set.
47.4.7 ISO-7816/smartcard support
The UART provides mechanisms to support the ISO-7816 protocol that is commonly
used to interface with smartcards. The ISO-7816 protocol is an NRZ, single wire, half-
duplex interface. The TxD pin is used in open-drain mode because the data signal is used
for both transmitting and receiving. There are multiple subprotocols within the ISO-7816
standard. The UART supports both T = 0 and T = 1 protocols. The module also provides
for automated initial character detection and configuration, which allows for support of
both direct convention and inverse convention data formats. A variety of interrupts
specific to 7816 are provided in addition to the general interrupts to assist software.
Additionally, the module is able to provide automated NACK responses and has
programmed automated retransmission of failed packets. An assortment of programmable
timeouts and guard band times are also supported.
The term elemental time unit (ETU) is frequently used in the context of ISO-7816. This
concept is used to relate the frequency that the system (UART) is running at and the
frequency that data is being transmitted and received. One ETU represents the time it
Chapter 47 Universal Asynchronous Receiver/Transmitter (UART)
K22F Sub-Family Reference Manual, Rev. 4, 08/2016
NXP Semiconductors 1285