PRELIMINARY

DS2432

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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