An Answer To Reset (ATR) is a message output by a contact Smart Card conforming to ISO/IEC 7816 standards, following electrical reset of the card's chip by a card reader. The ATR conveys information about the communication parameters proposed by the card, and the card's nature and state.
By extension, ATR often refers to a message obtained from a Smart Card in an early communication stage; or from the card reader used to access that card, which may transform the card's message into an ATR-like format (this occurs e.g. for some PC/SC card readers when accessing an ISO/IEC 14443 Smart Card).
The presence of an ATR is often used as a first indication that a Smart Card appears operative, and its content examined as a first test that it is of the appropriate kind for a given usage.
Contact Smart Cards communicate over a signal named Input/Output (I/O) either synchronously (data bits are sent and received at the rhythm of one per period of the clock supplied to the card on its CLK signal) or asynchronously (data bits are exchanged over I/O with another mechanism for bit delimitation, similar to traditional asynchronous serial communication). The two modes are exclusive in a given communication session, and most cards are built with support for a single mode. Microprocessor-based contact Smart Cards are mostly of the asynchronous variety, used for all Subscriber Identity Modules (SIM) for mobile phones, all bank cards with contacts conforming to EMV specifications, all contact Java Cards, and Smart Cards for pay television. Memory-only cards are generally of the synchronous variety.
ATR under asynchronous and synchronous transmission have entirely different form and content. The ATR in asynchronous transmission is extensively normalized, and relatively complex to parse.
Some Smart Cards (mostly of the asynchronous variety) have different ATR depending on if the reset is the first since power-up (Cold ATR) or not (Warm ATR).
Read more about Answer To Reset: ATR in Asynchronous Transmission, ATR in Synchronous Transmission
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