NXP Semiconductors K22F series, Reference Manual, Page: 1288 / 1407

The smallest data unit that is transferred is a block. A block is made up of several data
characters and may vary in size depending on the block type. The UART does not
provide a mechanism to decode the block type. As part of the block, an LRC or CRC is
included. The UART does not calculate the CRC or LRC for transmitted blocks, nor does
it verify the validity of the CRC or LRC for received blocks. The 7816 protocol requires
that the initiator and the smartcard (device) takes alternate turns in transmitting and
receiving blocks. When the UART detects that the last character in a block has been
transmitted it will automatically clear C2[TE], C3[TXDIR] and enter receive mode.
Therefore, the software must program the transmit buffer with the next data to be
transmitted and then enable C2[TE] and set C3[TXDIR], once the software has
determined that the last character of the received block has been received. The UART
detects that the last character of the transmit block has been sent when TL7816[TLEN] =
0 and four additional characters have been sent. The four additional characters are made
up of three prior to TL7816[TLEN] decrementing (prologue) and one after
TL7816[TLEN] = 0, the final character of the epilogue.
47.4.7.4 Wait time and guard time parameters
The ISO-7816 specification defines several wait time and guard time parameters. The
UART allows for flexible configuration and violation detection of these settings. On
reset, the wait time (IS7816[WT]) defaults to 9600 ETUs and guard time (GT) to 12
ETUs. These values are controlled by parameters in the WP7816, WN7816, and WF7816
registers. Additionally, the value of C7816[TTYPE] also factors into the calculation. The
formulae used to calculate the number ETUs for each wait time and guard time value are
shown in
Table 47-13 .
Wait time (WT) is defined as the maximum allowable time between the leading edge of a
character transmitted by the smartcard device and the leading edge of the previous
character that was transmitted by the UART or the device. Similarly, character wait time
(CWT) is defined as the maximum allowable time between the leading edge of two
characters within the same block. Block wait time (BWT) is defined as the maximum
time between the leading edge character of the last block received by the smartcard
device and the leading edge of the first character transmitted by the smartcard device.
Guard time (GT) is defined as the minimum allowable time between the leading edge of
two consecutive characters. Character guard time (CGT) is the minimum allowable time
between the leading edges of two consecutive characters in the same direction, that is,
transmission or reception. Block guard time (BGT) is the minimum allowable time
between the leading edges of two consecutive characters in opposite directions, that is,
transmission then reception or reception then transmission.
Functional description
K22F Sub-Family Reference Manual, Rev. 4, 08/2016
1288 NXP Semiconductors