DS1982 Maxim, DS1982 Datasheet - Page 11

• Manufacturer Part Number: DS1982
• Description: The DS1982 1Kb 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

DS1982
READ MEMORY [F0h]
The Read Memory command is used to read data from the 1024-bit EPROM data field. The bus master
follows the command byte with a 2-byte address (TA1=(T7:T0), TA2=(T15:T8)) that indicates a starting
byte location within the data field. An 8–bit CRC of the command byte and address bytes is computed by
the DS1982 and read back by the bus master to confirm that the correct command word and starting
address were received. 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, the bus master
issues read time slots and receives data from the DS1982 starting at the initial address and continuing
until the end of the 1024-bit data field is reached or until a Reset Pulse is issued. If reading occurs
through the end of memory space, the bus master may issue eight additional read time slots and the
DS1982 will respond with an 8-bit CRC of all data bytes read from the initial starting byte through the
last byte of memory. After the CRC is received by the bus master, any subsequent read time slots will
appear as logical 1s until a Reset Pulse is issued. Any reads ended by a Reset Pulse prior to reaching the
end of memory will not have the 8-bit CRC available.
Typically a 16-bit CRC would be stored with each page of data to ensure rapid, error-free data transfers
that eliminate having to read a page multiple times to determine if the received data is correct or not. (See
Book of DS19xx iButton Standards, Chapter 7 for the recommended file structure to be used with the
1-Wire environment.) If CRC values are imbedded within the data, a Reset Pulse may be issued at the end
of memory space during a Read Memory command.
READ STATUS [AAh]
The Read Status command is used to read data from the EPROM Status data field. The bus master
follows the command byte with a two-byte address (TA1=(T7:T0), TA2=(T15:T8)) that indicates a
starting byte location within the data field. An 8–bit CRC of the command byte and address bytes is
computed by the DS1982 and read back by the bus master to confirm that the correct command word and
starting address were received. 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, the bus
master issues read time slots and receives data from the DS1982 starting at the supplied address and
continuing until the end of the EPROM Status data field is reached. At that point the bus master will
receive an 8-bit CRC that is the result of shifting into the CRC generator all of the data bytes from the
initial starting byte through the final factory-programmed byte that contains the 00h value.
This feature is provided since the EPROM Status information may change over time making it impossible
to program the data once and include an accompanying CRC that will always be valid. Therefore, the
Read Status command supplies an 8-bit CRC that is based on and always is consistent with the current
data stored in the EPROM Status data field.
After the 8-bit CRC is read, the bus master will receive logical 1s from the DS1982 until a Reset Pulse is
issued. The Read Status command sequence can be exited at any point by issuing a Reset Pulse.
READ DATA/GENERATE 8-BIT CRC [C3h]
The Read Data/Generate 8-bit CRC command is used to read data from the 1024-bit EPROM memory
field. The bus master follows the command byte with a two-byte address (TA1=(T7:T0), TA2=(T15:T8))
that indicates a starting byte location within the data field. An 8-bit CRC of the command byte and
address bytes is computed by the DS1982 and read back by the bus master to confirm that the correct
command word and starting address were received. 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, the bus master issues read time slots and receives data from the DS1982 starting at the
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