Method for detecting that contactless CPU card leaves radio-frequency field

A method for detecting whether a contactless CPU card has left a radio frequency field comprising the steps of:

• Step a, a card reader is switched to an interruption enabling condition;
• Step b, the card reader receives an instruction sent by a master computer and determines whether the card reader received a valid instruction, if yes, determining a type of the valid instruction, if the type is an instruction on informing to seek a card, go to Step c;
• if the type is an APDU instruction, go to Step d;
• if the type is an extension instruction, go to Step e;
• otherwise, go to Step f;
• Step c, the card reader switches off the interruption enabling condition, sends a card seeking instruction to the card and determines whether the card reader received a card seeking response returned by the card, if yes, setting a flag in the card to a radio frequency field and switching on the interruption enabling condition, go to Step f;
• otherwise, resetting the flag in the card to the radio frequency field and switching on the interruption enabling condition, go to Step f;
• Step d, the card reader switches off the interruption enabling condition, sends the APDU instruction to the card and determines whether the card reader received a response returned by the card, if yes, setting the flag in the card to the radio frequency field, sending the received response to the master computer via a USB interrupting channel and switching on the interruption enabling condition, then go to Step f;
• otherwise, resetting the flag of the card in the radio frequency field and switching on the interruption enabling condition, go to Step f;
• Step e, the card reader switches off the interruption enabling condition, performs an operation according to the extension instruction, sends an operation result to the master computer via the USB interrupting channel and switches on the interruption enabling condition, go to Step f;
• Step f, the card reader determines whether the flag of the card in the radio frequency field is set, if yes, sending a response that the card is in the radio frequency field to the master computer via the USB interrupting channel, then go back to Step a;
• otherwise, sending the response that the card has left the radio frequency field to the master computer via the USB interrupting channel, then go back to Step a;
• when the card reader receives a triggering of the interrupting, entering a regular interrupting process, comprising
• Step g, the card reader switching off the interruption enabling condition and clearing any interrupting flag;
• Step h, the card reader determines whether the flag of the card in the radio frequency field is set, if yes, go to Step i;
• otherwise, sending the card seeking instruction to the card and determining whether a card seeking response is returned by the card, if yes, go to Step I;
• otherwise, go to Step m;
• Step i, the card reader sends a detecting instruction to the card and determines whether the card reader received a detecting response returned by the card, if yes, storing the detecting response and go to Step I;
• otherwise, go to Step m;
• Step l, the card reader switching on the interruption enabling condition, exiting the interrupting process;
• and
• Step m, the card reader resetting the flag of the card in the radio frequency field, switching on the interruption enabling condition, and exiting the regular interrupting process.