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.