DS1972: 1024-Bit EEPROM iButton

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WRITING WITH VERIFICATION

To write data to the DS1972, 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 Copy Scratchpad commands must be performed on 8-byte boundaries, i. e., the 3 LSBs of

the target address (T2..T0) must be equal to 000b. If T2..T0 are sent with non-zero values, the copy function will be

blocked. 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.

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 a preamble to the scratchpad

data, the DS1972 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 FUNCTION COMMANDS

The Memory Function Flow Chart (Figure 7) describes the protocols necessary for accessing the memory of the

DS1972. An example on how to use these functions to write to and read from the device is included at the end of

this document. The communication between master and DS1972 takes place either at regular speed (default, OD =

0) or at Overdrive Speed (OD = 1). If not explicitly set into the Overdrive mode, the DS1972 assumes regular

speed.

WRITE SCRATCHPAD COMMAND [0Fh]

The Write Scratchpad command applies to the data memory, and the writable addresses in the register page. In

order for the scratchpad data to be valid for copying to the array, the user must perform a Write Scratchpad

command of 8 bytes starting at a valid row boundary. The Write Scratchpad command accepts invalid addresses,

and partial rows, but subsequent Copy Scratchpad commands are blocked.

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 is written to the scratchpad starting at the byte offset of T2:T0.

The E/S bits E2:E0 are loaded with the starting byte offset, and increment with each susequent byte. Effectively,

E2:E0 is the byte offset of the last full byte written to the scratchpad. Only full data bytes are accepted.

When executing the Write Scratchpad command, the CRC generator inside the DS1972 (Figure 13) calculates a

CRC of the entire data stream, starting at the command code and ending at the last data byte as sent by the

master. This CRC is generated using the CRC16 polynomial by first clearing the CRC generator and then shifting

in the command code (0Fh) of the Write Scratchpad command, the Target Addresses (TA1 and TA2), and all the

data bytes. Note that the CRC16 calculation is performed with the actual TA1 and TA2 and data sent by the

master. The master may end the Write Scratchpad command at any time. However, if the end of the scratchpad is

reached (E2:E0 = 111b), the master may send 16 read-time slots and receive the CRC generated by the DS1972.

If a Write Scratchpad is attempted to a write-protected location, the scratchpad is loaded with the data already in

memory, rather than the data transmitted. Similarly, if the target address page is in EPROM mode, the scratchpad

is loaded with the bitwise logical AND of the transmitted data and data already in memory.