NXP Semiconductors K22F series, Reference Manual, Page: 1290 / 1407

47.4.7.5 ATR Duration Time Counter
The ISO-7816 specification defines a specific time (in etus) within which the terminal
must receive the ATR (Answer to Reset), failing which the terminal must abort the card
session by initiating the deactivation sequence.
UART supports this in hardware via the ATR Duration Time (ATD) Counter which can
be programmed using AP7816a_T0 and AP7816b_T0 registers. The value loaded into the
ADT (ATR Duration Time) counter is given by the concatenation of the register fields as
shown; ADT = {AP7816a_T0[ADTI_H], AP7816a_T0[ADTI_L]}. This counter begins
to count on detection of the TS character which is detected when IS7816[INITD] flag is
set. Once the ATR process is completed, the ATD Counter must be disabled by writing 0
to AP7816x_T0 registers, in order to prevent the false occurrence of the ATD Duration
Time interrupt IS7816[ATD]. Note that this feature is only supported in T = 0 mode.
NOTE
The ADT counter starts counting on detection of the complete
TS Character. It must be noted that by this time, exactly 10
ETUs have elapsed since the start bit of the TS character. The
user must take this into account while programming
AP7816a_T0 and AP7816b_T0 registers.
47.4.7.6 Baud rate generation
The value in WF7816[GTFD] does not impact the clock frequency. SBR and BRFD are
used to generate the clock frequency. This clock frequency is used by the UART only and
is not seen by the smartcard device. The transmitter clocks operates at 1/16 the frequency
of the receive clock so that the receiver is able to sample the received value 16 times
during the ETU.
47.4.7.7 UART restrictions in ISO-7816 operation
Due to the flexibility of the UART module, there are several features and interrupts that
are not supported while running in ISO-7816 mode. These restrictions are documented
within the register field definitions.
Functional description
K22F Sub-Family Reference Manual, Rev. 4, 08/2016
1290 NXP Semiconductors