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SCS152-IS Datasheet(PDF) 11 Page - Microchip Technology |
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SCS152-IS Datasheet(HTML) 11 Page - Microchip Technology |
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11 / 16 page  SCS152 © 1997 Microchip Technology Inc. Preliminary DS40150B-page 11 5.0 COMPATIBILITY WITH INDUSTRY STANDARD DEVICES The RESET and RDBIT commands meets the require- ments of ISO 7816-3 for the ‘Answer-to-Reset’ sequence for synchronous chip cards. The basic counter modification commands BITPROG and ERASE correspond with the WRITE and ERASE-WITH-CARRY commands of industry standard devices. However, industry standard devices use eight bits to represent each digit, and a WRITE command will program the current bit to ‘0’. The application can, therefore, decrement a digit by programming any of the eight bits to ‘0’, whereas with the SCS152, any BITPROG command will always decrement the current digit value. If the application decrements each digit as described above, the WRITE command will appear to program the current bit to ‘0’. Thus, while the SCS152 will accept and interpret the commands correctly, the changes to the memory map will only correspond to that of the other cards, if the first ‘1’ in a digit field is always programmed ‘0’ with a WRITE command. For example, if digit field D3 contains the value ‘6’, it will be read out as 00111111. If a BITPROG command is issued anywhere in D3, its value will be decremented to ‘5’, and it will always read out as 00011111, regardless of which address in the digit the BITPROG command was issued. The ISS is only an external view of the CFG field and cannot be changed. A BITPROG command at this address will actually decrement digit D4. It should also be noted that the address counter appears to wrap from 275 back to 0, which differs from other similar devices. 5.1 Fail Safe Counter Update The carry is performed by a BITPROG, followed by an ERASE. This process first deducts a number of tokens (the BITPROG command) and then adds some again (with the ERASE command), but never more than pre- viously deducted with the BITPROG command. If the device is pulled from the reader between the BITPROG and ERASE commands, the user may lose many tokens. This situation can be avoided by substituting the DECR-ERASE command whenever a sequence of BITPROG and ERASE commands are needed to update the token counter (Step 5c in Section 4.2). This will modify the contents of the EEPROM in one indivis- ible operation that will either complete or not modify the EEPROM in any way, if external power is removed early on during the EEPROM programming. |
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