DS1985 Maxim, DS1985 Datasheet - Page 13

• Manufacturer Part Number: DS1985
• Description: The DS1985 16Kb Add-Only iButton is a rugged read/write data carrier that identifies and stores relevant information about the product or person to which it is attached
• Manufacturer: Maxim

DS1985
WRITE MEMORY [0FH]
The Write Memory command is used to program the 16384–bit EPROM data field. The bus master will
follow the command byte with a two byte starting address (TA1=(T7:T0), TA2=(T15:T8)) and a byte of
data (D7:D0). A 16-bit CRC of the command byte, address bytes, and data byte is computed by the
DS1985 and read back by the bus master to confirm that the correct command word, starting address, and
data byte were received.
The highest starting address within the DS1985 is 07FFH. If the bus master sends a starting address
higher than this, the five most significant address bits are set to 0 by the internal circuitry of the chip. This
will result in a mismatch between the CRC calculated by the DS1985 and the CRC calculated by the bus
master, indicating an error condition.
If the CRC read by the bus master is incorrect, a Reset Pulse must be issued and the entire sequence must
be repeated. If the CRC received by the bus master is correct, a programming pulse (12 volts on the 1-
Wire bus for 480 s) is issued by the bus master. Prior to programming, the entire unprogrammed 16384-
bit EPROM data field will appear as logical 1s. For each bit in the data byte provided by the bus master
that is set to a logical 0, the corresponding bit in the selected byte of the 16384-bit EPROM will be
programmed to a logical 0 after the programming pulse has been applied at that byte location.
After the 480 s programming pulse is applied and the data line returns to the idle level, the bus master
issues eight read time slots to verify that the appropriate bits have been programmed. The DS1985
responds with the data from the selected EPROM address sent least significant bit first. This byte contains
the logical AND of all bytes written to this EPROM data address. If the EPROM data byte contains 1s in
bit positions where the byte issued by the master contained 0s, a Reset Pulse should be issued and the
current byte address should be programmed again. If the DS1985 EPROM data byte contains 0s in the
same bit positions as the data byte, the programming was successful and the DS1985 will automatically
increment its address counter to select the next byte in the 16384-bit EPROM data field. The new 2-byte
address will also be loaded into the 16-bit CRC generator as a starting value. The bus master will issue
the next byte of data using eight write time slots.
As the DS1985 receives this byte of data into the scratchpad, it also shifts the data into the CRC generator
that has been preloaded with the current address and the result is a 16-bit CRC of the new data byte and
the new address. After supplying the data byte, the bus master will read this 16-bit CRC from the DS1985
with 16 read time slots to confirm that the address incremented properly and the data byte was received
correctly. If the CRC is incorrect, a Reset Pulse must be issued and the Write Memory command
sequence must be restarted. If the CRC is correct, the bus master will issue a programming pulse and the
selected byte in memory will be programmed.
Note that the initial pass through the Write Memory flow chart will generate a 16-bit CRC value that is
the result of shifting the command byte into the CRC generator, followed by the two address bytes, and
finally the data byte. Subsequent passes through the Write Memory flow chart due to the DS1985
automatically incrementing its address counter will generate a 16-bit CRC that is the result of loading (not
shifting) the new (incremented) address into the CRC generator and then shifting in the new data byte.
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