BIT 0 BIT 1 BIT 2 BIT 3 BIT 4 BIT 5 BIT 6 BIT 7 BIT STOPBITPARITY NEXTSTARTBITSTARTBITISO 7816 FORMAT WITHOUT PARITY ERROR (T=0)STOPBITBIT 0 BIT 1 BIT 2 BIT 3 BIT 4 BIT 5 BIT 6 BIT 7 BIT STOPBITPARITY NEXTSTARTBITSTARTBITNACKERRORISO 7816 FORMAT WITH PARITY ERROR (T=0)Figure 38-24. ISO-7816 T = 0 data formatAs 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. Inaddition to a standard even parity check, the receiver has the ability to generate and returna NACK during the second half of the first stop bit period. The NACK must be at leastone time period (ETU) in length and no more than two time periods (ETU) in length. Thetransmitter must wait for at least two time units (ETU) after detection of the error signalbefore attempting to retransmit the character.It is assumed that the UART and the device (smartcard) know in advance which device isreceiving and which is transmitting. No special mechanism is supplied by the UART tocontrol receive and transmit in the mode other than C2[TE] and C2[RE]. Initial CharacterDetect feature is also supported in this mode.38.5.7.3 Protocol T = 1When 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 stopbit although additional inactive bit periods may be present between the stop bit and thenext start bit. Data characters are formatted as illustrated in the following figure.BIT 0 BIT 1 BIT 2 BIT 3 BIT 4 BIT 5 BIT 6 BIT 7 BIT STOPBITPARITY NEXTSTARTBITSTARTBITISO 7816 FORMAT (T=1)Figure 38-25. ISO 7816 T=1 data formatThe smallest data unit that is transferred is a block. A block is made up of several datacharacters and may vary in size depending on the block type. The UART does notprovide a mechanism to decode the block type. As part of the block, an LRC or CRC isincluded. The UART does not calculate the CRC or LRC for transmitted blocks, nor doesit verify the validity of the CRC or LRC for received blocks. The 7816 protocol requiresthat the initiator and the smartcard (device) takes alternate turns in transmitting andreceiving blocks. When the UART detects that the last character in a block has beentransmitted it will automatically clear C2[TE], C3[TXDIR] and enter receive mode.Functional descriptionKL27 Sub-Family Reference Manual , Rev. 5, 01/2016732 Freescale Semiconductor, Inc.
