DS1996 Maxim, DS1996 Datasheet - Page 9

• Manufacturer Part Number: DS1996
• Description: The DS1996 Memory iButton is a rugged read/write data carrier that acts as a localized database that can be easily accessed with minimal hardware
• Manufacturer: Maxim

DS1996
Copy Scratchpad [55H]
This command is used to copy data from the scratchpad to memory. After issuing the copy scratchpad
command, the master must provide a 3-byte authorization pattern which is obtained by reading the
scratchpad for verification. This pattern must exactly match the data contained in the three address
registers (TA1, TA2, E/S, in that order). If the pattern matches, the AA (Authorization Accepted) flag
will be set and the copy will begin. A logic 0 will be transmitted after the data has been copied until a
reset pulse is issued by the master. Any attempt to reset the part will be ignored while the copy is in
progress. Copy typically takes 30 µs.
The data to be copied is determined by the three address registers. The scratchpad data from the
beginning offset through the ending offset, will be copied to memory, starting at the target address.
Anywhere from 1 to 32 bytes may be copied to memory with this command. Whole bytes are copied
even if only partially written. The AA flag will be cleared only by executing a write scratchpad
command.
Read Memory [F0H]
The read memory command may be used to read the entire memory. After issuing the command, the
master must provide the 2-byte target address. After the 2 bytes, the master reads data beginning from the
target address and may continue until the end of memory, at which point logic 1’s will be read. It is
important to realize that the target address registers will contain the address provided. The ending
offset/data status byte is unaffected.
The hardware of the DS1996 provides a means to accomplish error-free writing to the memory section.
To safeguard reading data in the 1-Wire environment and to simultaneously speed up data transfers, it is
recommended to packetize data into data packets of the size of one memory page each. Such a packet
would typically store a 16-bit CRC 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 the
Book of DS19xx iButton Standards, Chapter 7 for the recommended file structure to be used with the 1-
Wire environment.)
1-WIRE BUS SYSTEM
The 1-Wire bus is a system which has a single bus master and one or more slaves. In all instances the
DS1996 is a slave device. The bus master is typically a microcontroller. The discussion of this bus
system is broken down into three topics: hardware configuration, transaction sequence, and 1-Wire
signaling (signal types and timing). A 1-Wire protocol defines bus transactions in terms of the bus state
during specified time slots that are initiated on the falling edge of sync pulses from the bus master. For a
more detailed protocol description, refer to Chapter 4 of the Book of DS19xx iButton Standards.
HARDWARE CONFIGURATION
The 1-Wire bus has only a single line by definition; it is important that each device on the bus be able to
drive it at the appropriate time. To facilitate this, each device attached to the 1-Wire bus must have open
drain connection or 3-state outputs. The 1-Wire port of the DS1996 is open drain with an internal circuit
equivalent to that shown in Figure 8. A multidrop bus consists of a 1-Wire bus with multiple slaves
attached. At regular speed the 1-Wire bus has a maximum data rate of 16.3 kbits per second. The speed
can be boosted to 142 kbits per second by activating the Overdrive Mode. The 1-Wire bus requires a
pullup resistor of approximately 5 kΩ.
The idle state for the 1-Wire bus is high. If for any reason a transaction needs to be suspended, the bus
MUST be left in the idle state if the transaction is to resume. If this does not occur and the bus is left low
for more than 16 µs (Overdrive Speed) or more than 120 µs (regular speed), one or more of the devices
on the bus may be reset.
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