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DS1867 Datasheet(PDF) 6 Page - Dallas Semiconductor

Part No. DS1867
Description  Dual Digital Potentiometer with EEPROM
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Maker  DALLAS [Dallas Semiconductor]
Homepage  http://www.dalsemi.com
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DS1867
6 of 14
102199
The Cout output of the DS1867 can be used to drive the DQ input of another DS1867. When connecting
multiple devices, the total number of bits sent is always 17 times the number of DS1867s in the daisy
chain.
An optional feedback resistor can be placed between the Cout terminal of the last device and the DQ input
of the first DS1867, thus allowing the controlling processor to read, as well as, write data or circularly
clock data through the daisy chain. The value of the feedback or isolation resistor should be in the range
from 2 to 10 kohms.
When reading data via the COUT pin and isolation resistor, the DQ line is left floating by the reading
device. When RST is driven high, bit 17 is present on the COUT pin, which is fed back to the input DQ pin
through the isolation resistor. When the CLK input transitions low to high, bit 17 is loaded into the first
position of the I/O shift register and bit 16 becomes present on COUT and DQ of the next device. After 17
bits (or 17 times the number of DS1867s in the daisy chain), the data has shifted completely around and
back to its original position. When RST transitions to the low state to end data transfer, the value (the
same as before the read occurred) is loaded into the wiper-0, wiper-1, and stack select bit I/O register.
CASCADING MULTIPLE DEVICES Figure 5
NONVOLATILE WIPER SETTINGS
The DS1867 maintains the position of the wiper in the absence of power. This feature is provided through
the use of EEPROM type memory cell arrays. During normal operation, the position of the wiper is
determined by the device multiplexers and stored in the shadow memory (EEPROM). The manner in
which an update occurs has been optimized for reliability, durability, and performance. Additionally, the
update operation is totally transparent to the user.
When power is applied to the DS1867, wiper settings will be the last recorded in the EEPROM memory
cells or shadow memory before the last power-down. Changes to the EEPROM memory cells occur
during a predefined power-down sequence. If the DS1867 detects a voltage transition to 4.5 volts or less,
on the power supply input, the part initiates an automatic wiper storage sequence. This storage sequence
will save in EEPROM memory the contents of the I/O shift register before a total power-shutdown;
provided specific power-down timing requirements are met. The minimum total power-down time is
specified at 4 milliseconds. Power-down timing requirements on VCC are shown in Figure 6.
The EEPROM memory cells are specified to accept greater than 25,000 writes before a wear-out
condition. If the EEPROM memory cells do reach a wear-out condition, the DS1867 will still function
properly while power is applied. A minimum time of 4 ms between 4.5V and 3V is required to perform
the proper position storage of the wiper.




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