ATSHA204A datasheet(15/93 Pages) MICROCHIP

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1.2.

SN<2:7>

The values of these bits are programmed by Microchip during the manufacturing process

and are different for every die. These 6-bytes (48-bits) are optionally included in some

SHA-256 computations made by the ATSHA204A

•

RevNum

Four bytes of information that are used by Microchip to provide manufacturing revision information.

These bytes can be freely read as RevNum<0:3>, but should never be used by system software,

because they may change due to a silicon revision.

2.1.3

One Time Programmable (OTP) Zone

The OTP zone of 64 bytes (512 bits) is part of the EEPROM array and can be used for read-only storage.

Prior to locking the configuration section using Lock(LockConfig), the OTP zone is inaccessible and

can be neither read nor written. After configuration locking, but prior to locking of the OTP zone using

Lock(LockValue), the entire OTP zone can be written using the Write command. If desired, the data

to be written can be encrypted. When unlocked the OTP zone cannot be read.

Once the OTP zone is locked, the OTP mode byte in the Configuration zone controls the permissions of

this zone, as follows:

•

Read-only Mode

The data cannot be modified and would be used to store fixed model numbers, calibration

information, manufacturing history and/or other data that should never change. The Write

command always returns an error and leaves the memory unmodified. All 64-bytes within the OTP

section are always available for reading using either 4-byte or 32-byte reads.

•

Consumption Mode

The bits function as one-way fuses and can be used to track consumption or usage of the item to

which the ATSHA204A is attached. For examples, in a battery, they might be used to track charging

cycles or use time; in a printer ink cartridge, they might track the quantity of material consumed; in

a medical device, they might track the number of permitted uses for a limited use item. In this

mode, the Write command can only cause bits to transition from a one to a zero. Logically, this

means the data value in the input parameter list is AND'ed with the current value in the word(s) and

the result written back to memory. As an example, writing a value of 0xFF results in no change to

the byte and writing a value of 0x00 causes the byte in memory to go to zero, regardless of the

previous value. Once a bit has transitioned to a zero, it can never transition back to a one.

•

Legacy Mode

The operation of the OTP zone is consistent with the fuse array on the Microchip(Formerly Atmel)

ATSA102S. Reads of words zero and one are always prohibited, while reads of the remaining 14

words are always permitted. Only 4-byte (32-bit) reads are permitted and any attempt to execute a

32-byte (256-bit) read results in an error return code. All Write operations to the OTP zone are

prohibited. See Section 9. Compatibility for more of the Microchip ATSA102S compatibility details.

All OTP zone bits have a value of one on shipment from the Microchip factory.

Table 2-8. OTP Zone

Word (HEX)

Address (HEX)

Default

0x00

0xFFFFFFFF

0x01

0x04

0xFFFFFFFF

0x02

0x08

0xFFFFFFFF

ATSHA204A

Device Organization

DS40002025A-page 15

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