DocID025202 Rev 7
868/1080
RM0365
Universal synchronous asynchronous receiver transmitter (USART)
901
Note:
The error checking code (LRC/CRC) must be computed/verified by software.
Direct and inverse convention
The Smartcard protocol defines two conventions: direct and inverse.
The direct convention is defined as: LSB first, logical bit value of 1 corresponds to a H state
of the line and parity is even. In order to use this convention, the following control bits must
be programmed: MSBFIRST=0, DATAINV=0 (default values).
The inverse convention is defined as: MSB first, logical bit value 1 corresponds to an L state
on the signal line and parity is even. In order to use this convention, the following control bits
must be programmed: MSBFIRST=1, DATAINV=1.
Note:
When logical data values are inverted (0=H, 1=L), the parity bit is also inverted in the same
way.
In order to recognize the card convention, the card sends the initial character, TS, as the
first character of the ATR (Answer To Reset) frame. The two possible patterns for the TS
are: LHHL LLL LLH and LHHL HHH LLH.
•
(H) LHHL LLL LLH sets up the inverse convention: state L encodes value 1 and
moment 2 conveys the most significant bit (MSB first). when decoded by inverse
convention, the conveyed byte is equal to '3F'.
•
(H) LHHL HHH LLH sets up the direct convention: state H encodes value 1 and
moment 2 conveys the least significant bit (LSB first). when decoded by direct
convention, the conveyed byte is equal to '3B'.
Character parity is correct when there is an even number of bits set to 1 in the nine
moments 2 to 10.
As the USART does not know which convention is used by the card, it needs to be able to
recognize either pattern and act accordingly. The pattern recognition is not done in
hardware, but through a software sequence. Moreover, supposing that the USART is
configured in direct convention (default) and the card answers with the inverse convention,
TS = LHHL LLL LLH => the USART received character will be ‘03’ and the parity will be odd.
Therefore, two methods are available for TS pattern recognition:
Method 1
The USART is programmed in standard Smartcard mode/direct convention. In this case, the
TS pattern reception generates a parity error interrupt and error signal to the card.
•
The parity error interrupt informs the software that the card didn’t answer correctly in
direct convention. Software then reprograms the USART for inverse convention
•
In response to the error signal, the card retries the same TS character, and it will be
correctly received this time, by the reprogrammed USART
Alternatively, in answer to the parity error interrupt, the software may decide to reprogram
the USART and to also generate a new reset command to the card, then wait again for the
TS.