Answer to reset

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Answer to Reset (ATR) describes the first communication between a chip card and a card reader, after the card reader has indicated a Reset of the Card.

To open a communication to the Card Reader the Card will send a couple of Parameters to let the Reader know how it can communicate with the Card.

The ATR proceeds in five steps. Initial Character, Format Character, Interface Characters, Historical Bytes and the Check Character.

1 Initial Character
2 Format Character
3 Interface Character
4 Historical Characters
5 Check Character

[edit] Initial Character

The Initial Character, also called TS, sets the Bit-Code. Logic „1“ will be transferred as Low- (L) or High- (H). It also tells the reader the which divider the card is using. The Reader needs this Information to figure the Bit length. The Start-Bit and the 3 following ones are always send after the same Order. (L) H H L x x x x x x (L = Low, H = High, x = don’t care) The Reader calculates the Time between the first and the last Bit and divides it by three. The Total is called an "Elementary Time Unit" (etu) and sets the length of a Bit.

The next tree Bits of the Sequence set the Bit-Code. Tree Bits on "High" mean that Logic 1 will be decoded as "High" (Direct Converting), tree Bits on "low" mean that Logic 0 will be decoded as Low (Indirect Converting).

The First Bit it the Start-Bit and the last the Parity-Bit

Direct Conversion - (L) H H L H H H L L (H) Indirect Conversion - (L) H H L L L L L L (H)

Note that every Converting the Bit order can Change. In a Direct Converting the "Least Significant Bit" will always be transferred first. While in an Indirect Converting the "Most Significant Bit" will be transferred first.

[edit] Format Character

The Format Character (also called TO) tells the Reader how the two following Characters will be transferred. The Most Signifiant Nibble (MSN) of T0 shows the Bit order of the next four Interface Characters. The Least Significant Nibble (LSN) will be shown as a Number and tells the amount of Historical Character (1 to max. 15).

[edit] Interface Character

The Setup of the Interface Characters (also called TD) will be transferred similar to the TO, in MSN (Most Significant Nibble). It tells which next Interface Characters (TX(i+1), X Î {A, B, C, D}, i > 1) will be transferred in the ATR

The Least Significant Nibble (LSN) of TD(i) tells the used protocol type. (T = 0, T = 1). Depending on the Cards supported protocol types, TD(i+1) can also have Information about other protocols and will be transferes by a protocol selector.

TA(1), TB(1), TC(1) and TB(2) are called global Interface Bytes. They are transferring Information about the user of Voltage and Power. The Protocol gives order how to interpret TA(i), TB(i), TC(i) (i > 2).

[edit] Historical Characters

There is no Norm for the Historical Character. It holds Information about the Card Builder, Type of Card (Size etc.), Version number and the State of the Card.

[edit] Check Character

The Check Character can be an additional Priority Check. It has a XOR Shortcut (Exclusive Order) of the Bytes used in the ATR from TO till the last Byte of TCK. It Criteria are depending on the type of protocol used.