Information

received, which is not a valid initial character, is ignored and all flags resulting from the
invalid data are blocked from asserting. If C7816[ANACK] is set, a NACK is returned
for invalid received initial characters and an RXT interrupt is generated as programmed.
45.4.8.2 Protocol T = 0
When T = 0 protocol is selected, a relatively complex error detection scheme is used.
Data characters are formatted as illustrated in the following figure. This scheme is also
used for answer to reset and Peripheral Pin Select (PPS) formats.
BIT 0 BIT 1 BIT 2 BIT 3 BIT 4 BIT 5 BIT 6 BIT 7 BIT
STOP
BIT
PARITY
NEXT
START
BIT
START
BIT
ISO 7816 FORMAT WITHOUT PARITY ERROR (T=0)
STOP
BIT
BIT 0 BIT 1 BIT 2 BIT 3 BIT 4 BIT 5 BIT 6 BIT 7
BIT
STOP
BIT
PARITY
NEXT
START
BIT
START
BIT
NACK
ERROR
ISO 7816 FORMAT WITH PARITY ERROR (T=0)
Figure 45-216. ISO-7816 T = 0 data format
As with other protocols supported by the UART, the data character includes a start bit.
However, in this case, there are two stop bits rather than the typical single stop bit. In
addition to a standard even parity check, the receiver has the ability to generate and return
a NACK during the second half of the first stop bit period. The NACK must be at least
one time period (ETU) in length and no more than two time periods (ETU) in length. The
transmitter must wait for at least two time units (ETU) after detection of the error signal
before attempting to retransmit the character.
It is assumed that the UART and the device (smartcard) know in advance which device is
receiving and which is transmitting. No special mechanism is supplied by the UART to
control receive and transmit in the mode other than C2[TE] and C2[RE]. Initial Character
Detect feature is also supported in this mode.
45.4.8.3 Protocol T = 1
When T = 1 protocol is selected, the NACK error detection scheme is not used. Rather,
the parity bit is used on a character basis and a CRC or LRC is used on the block basis,
that is, for each group of characters. In this mode, the data format allows for a single stop
bit although additional inactive bit periods may be present between the stop bit and the
next start bit. Data characters are formatted as illustrated in the following figure.
Functional description
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
1130 Freescale Semiconductor, Inc.