SEC1210 datasheet(57/218 Pages) MICROCHIP

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DS00001561C-page 57

SEC1110/SEC1210

The minimum character Guard Time (2 etu) on transmission will be ensured by the fact that T=0 Mode is selected in the

Protocol Mode Register. On transmission, the guard period will be monitored only for a Parity Error response from the

Smart Card, and not for any other form of interference.

10.9

T=0 Byte Filtering

There is a new consideration regarding FIFO space. The Smart Card may insert NULL characters at various points in

the communication, whose purpose is to reset the Timeout Timer (being used for WWT). Also, there are an unpredict-

able number of INS bytes, which signal when a card is prepared to transfer only one byte instead of the whole remaining

block. A pair of state machines are provided to filter out these extra bytes in a T=0 exchange, thus ensuring that no valid

exchange will ever overflow the SC_FIFO.

Both state machines filter only bytes that are being received from the card, but they are called Incoming and Outgoing

based on the nature of the command being executed. The direction is defined relative to the card, so that Outgoing

means reading data out of the card, and Incoming means writing data into the card.

The special procedure bytes are those bytes sent by the card that are not data. These are:

• NULL, encoded as 0x60, which is used as padding to reset the WWT timing monitor

• SW1, encoded as 0x61-0x6F and 0x90-0x9F. This is the first byte of status, which flags the end of a transfer. It is

always followed by one byte, SW2, which completes the status indication and is the last byte of the transaction.

• INS and INS are used as flags, and represent a true (INS) and complemented (INS) echo of the Instruction byte

(sent by the terminal) that is being executed by the card. The encodings of INS and INS are such that they can

never be confused with NULL or SW1.

10.9.1

T=0 OUTGOING BYTE FILTER

The first (outgoing) state machine is used when a command is being issued that reads data from the card. In this sce-

nario, the card responds on receipt of the command, and it does not stop transmitting until the entire requested block

of data has been transferred. The format of this response is variable depending on the card’s performance. The Outgo-

ing state machine, then, filters out the variable portions of this response, leaving only the outgoing data and status, which

will be of a predictable maximum size of 258 bytes (256 bytes of information data plus the status bytes SW1 and SW2).

If the firmware requires a maximum packet size greater than 258 bytes (CCID firmware needs 259), then firmware can

split the packet.

To operate this filter, software specifies in the register set the number of data bytes it intends to read from the Smart

Card, and the INS byte value that it intends to send. It then enables the state machine with the dedicated Enable bit

(OSME, in the Protocol Mode Register), and transmits its command. When the transmission is completed (as deter-

mined by the Message Length Register used for transmission), the state machine becomes active. As the card

responds, any NULL characters at appropriate places are detected and discarded, and all INS and INS procedure bytes

are discarded, leaving only the data bytes and the two status bytes (SW1 and SW2) to be placed into the SC_FIFO.

A typical sequence of events for a T=0 outgoing exchange is shown in the figure below.

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