DS2423

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WRITING WITH VERIFICATION

To write data to the DS2423, 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. Under certain conditions (see Write Scratchpad command) the master will receive an

inverted CRC16 of the command, address 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 has to send the Read Scratchpad command to read back the scratchpad to

verify data integrity. As preamble to the scratchpad data, the DS2423 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. 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 indicates that the Write command was not recognized by the device.

If everything went correctly, both flags are cleared and the ending offset indicates the address of the last

byte written to the scratchpad. Now the master can continue reading and verifying every data byte. After

the master has verified the data, it has to send the Copy Scratchpad command. This command must be

followed exactly by the data of the three address registers TA1, TA2 and E/S. The master may obtain the

contents of these registers by reading the scratchpad or derive it from the target address and the amount of

data to be written. As soon as the DS2423 has received these bytes correctly, it will copy the data to the

requested location beginning at the target address.

MEMORY FUNCTION COMMANDS

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

An example follows the flowchart. The communication between master and DS2423 takes place either at

regular speed (default, OD = 0) or at Overdrive speed (OD = 1). If not explicitly set into the Overdrive

mode the DS2423 assumes regular speed.

Write Scratchpad Command [0FH]

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 byte offset (T4:T0). The ending offset (E4: E0) will be the byte offset at which the master stops

writing data. Only full data bytes are accepted. If the last data byte is incomplete its content will be

ignored and the partial byte flag PF will be set.

When executing the Write Scratchpad command the CRC generator inside the DS2423 (see Figure 12)

calculates a CRC over the entire data stream, starting at the command code and ending at the last data

byte 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 as supplied by the master and all the data bytes. The master may end the Write

Scratchpad command at any time. However, if the ending offset is 11111b, the master may send 16 read

time slots and will receive the CRC generated by the DS2423.

The memory address range of the DS2423 is 0000H to 01FFH. If the bus master sends a target address

higher than this, the internal circuitry of the chip will set seven most significant address bits to 0 as they

are shifted into the internal address register. The Read Scratchpad command will reveal the target address

as it will be used by the DS2423. The master will identify such address modifications by comparing the

target address read back to the target address transmitted. If the master does not read the scratchpad, a

subsequent Copy Scratchpad command will not work since the most significant bits of the target address

the master sends will not match the value the DS2423 expects.