Rainbow Electronics AT88SA100S - Manual (Page 2)

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AT88SA100S [Preliminary]
8558A–SMEM–03/09
1.1. Memory Resources
Sram 256 bits of SRAM that are used for storage of a key. The LoadSram command provides a
mechanism to securely initialize this block during personalization. This memory will retain its value
when the chip is put/goes to sleep, so long as a supply voltage in excess of V
RETAIN
is still supplied to
the chip.
MemValid A single bit that tells whether or not Sram contains valid data. It’s cleared when power is lost and set
when the SRAM is loaded with a secret key.
Fuse Block of 128 fuse bits that can be read and written through the 1 wire interface. The first 8 bits are
lock bits that control burn ability on 16 bit words of the array. Fuse[88-95] are manufacturing ID
values fixed by Atmel. Fuse[96-127] are programmed by Atmel with a serial number that (in
combination with MaskSN) is guaranteed to be unique and which includes lot/wafer info.
ROM Metal mask programmed memory. Unrestricted reads are permitted on the first 64 bits of this array.
The physical ROM will be larger and will contain other information that cannot be read.
ROM MfrID 2 bytes of ROM that, when combined with Fuse[88:95] specifies the 24 bit manufacturing
ID code (MfrID). This value is assigned by Atmel and is always the same for all chips of a
particular model number. For the AT88SA100S, this value is 0xFF FF FF. The MfrID can
be read by accessing ROM bytes 0 & 1 of Address 0. The 24 bit MfrID value can always
be read by the system and is always included in the message digested by the MAC
command.
ROM SN 2 bytes of ROM that can be used to identify chips among others on the wafer. These bits
reduce the number of fuses necessary to construct a unique serial number. The ROM SN
is read by accessing ROM bytes 2 & 3 of Address 0. The complete serial number is
extended with an additional 32 fuses to form the 48 bit serial number. The serial number
can always be read by the system and is optionally included in the message digested by
the MAC command.
RevNum 4 bytes of ROM that are used by Atmel to identify the design revision of the AT88SA100S
chip. These bytes can be freely read as the four bytes returned from ROM address 1,
however system code should not depend on this value as it may change from time to time.
1.2. Fuse Map
The AT88SA100S chip incorporates 128 one-time fuses within the chip. Once burned, there is no way to reset the
value of a fuse. Fuses, with the exception of the manufacturing ID and serial number bits, initialized by Atmel, have a
value of 1 when shipped from the Atmel factory and transition to a 0 when they are burned.
Table 1. The 128 fuses in the AT88SA100S chip are arranged in the following manner:
Fuse # Name Description
0 – 7 Fuse Lock Bits Each bit locks the current value of the corresponding 16 bit block of the fuse array,
see below for more details.
8 Æ 87 Status Fuses These fuses can be written with the BurnFuse command and can always be read
with the Read command.
88 Æ 95 MfrID See Section 1.1. Set by Atmel, can’t be modified in the field
96 Æ 127 Serial Number See Section 1.1. Set by Atmel, can’t be modified in the field
Fuse Lock Bits These 8 fuses can be used to prevent further writing of the status fuses. Bit 0, when burned, locks
Fuse[0-15] from being modified, Bit[1] locks Fuse[16-31] and so on up through bit 5, which locks
Fuse[80-87]. Fuse[88-127] can never be modified with the BurnFuse command. Note that burning bit
0 has the effect of preventing any changes to the current value of the lock bits.