DS2432X Dallas Semiconducotr, DS2432X Datasheet - Page 6

• Manufacturer Part Number: DS2432X
• Description: 1k-Bit Protected 1-Wire EEPROM with SHA-1 Engine
• Manufacturer: Dallas Semiconducotr

PRELIMINARY
DS2432
number of data bits sent by the master is not an integer multiple of 8 or if the data in the scratchpad is not
valid due to a loss of power. A valid write to the scratchpad will clear the PF bit. Bits 3, 4 and 6 have no
function; they always read 1. The Partial Flag supports the master checking the data integrity after a
Write command. The highest valued bit of the E/S register, called AA or Authorization Accepted, acts as
a flag to indicate that the data stored in the scratchpad has already been copied to the target memory
address. Writing data to the scratchpad clears this flag.
WRITING WITH VERIFICATION
To write data to the DS2432, the scratchpad has to be used as intermediate storage. First the master issues
the Write Scratchpad command to specify the desired target address, followed by the data to be written to
the scratchpad. Note that writes to data memory must be performed on 8-byte boundaries with the
3 LSBs of the target address (T2..T0) equal to 000b. If T2..T0 are sent with non-zero values, the device
will set these bits to zero and will write to the modified address upon completion of the command
sequence.
In addition, the entire 8-byte scratchpad will be copied to memory when commanded,
therefore eight bytes of data should be written into the scratchpad to ensure that the data to be copied is
known. Under certain conditions (see Write Scratchpad command) the master will receive an inverted
CRC16 of the command, address (actual address sent) and data at the end of the write scratchpad
command sequence. Note that the CRC is calculated based on the actual target address sent and not the
modified address in the case of a non-zero T2..T0. Knowing this CRC value, the master can compare it
to the value it has calculated itself to decide if the communication was successful and proceed to the
Copy Scratchpad command. If the master could not receive the CRC16, it should send the Read
Scratchpad command to verify data integrity. As preamble to the scratchpad data, the DS2432 repeats the
target address TA1 and TA2 and sends the contents of the E/S register. If the PF flag is set, data did not
arrive correctly in the scratchpad or there was a loss of power since data was last written to the
scratchpad. The master does not need to continue reading; it can start a new trial to write data to the
scratchpad. Similarly, a set AA flag together with a cleared PF flag indicates that the device did not
recognize the Write command. If everything went correctly, both flags are cleared. Now the master can
continue reading and verifying every data byte. After the master has verified the data, it can send the
Copy Scratchpad command, for example. This command must be followed exactly by the data of the
three address registers TA1, TA2 and E/S. The master should obtain the contents of these registers by
reading the scratchpad.
MEMORY AND SHA FUNCTION COMMANDS
Due to its design as a secure device the DS2432 has to behave differently from other 1-Wire memory
devices. Although most of the memory of the DS2432 can be read the same way as any other 1-Wire
memory, attempts to read the secret will result in FFh-bytes rather than real data. The “Memory and SHA
Function Flow Chart” (Figure 7) describes the protocols necessary for accessing the memory and
operating the SHA engine. The communication between master and DS2432 takes place either at regular
speed (default, OD = 0) or at Overdrive Speed (OD = 1). If not explicitly set into the Overdrive Mode the
DS2432 assumes regular speed.
Write Scratchpad [0Fh]
The Write Scratchpad command applies to the data memory, the secret and the writable addresses in the
register page. If the bus master sends a target address higher than 90h, the command will not be executed.
After issuing the write scratchpad command, the master must first provide the 2-byte target address,
followed by the data to be written to the scratchpad. The data will be written to the scratchpad starting at
the beginning of the scratchpad. Note that the ending offset (E2..E0) will always be 111b regardless of
the number of bytes that the master has transmitted. For this reason the master should always send
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