DS1855E-050 Maxim Integrated Products, DS1855E-050 Datasheet - Page 10

• Manufacturer Part Number: DS1855E-050
• Description: Digital Potentiometer ICs
• Manufacturer: Maxim Integrated Products

Specifications of DS1855E-050

• Number Of Pots: Dual
• Taps Per Pot: 100, 256
• Resistance: 10 KOhms, 50 KOhms
• Wiper Memory: Non Volatile
• Digital Interface: Serial (2-Wire)
• Operating Supply Voltage: 3 V, 5 V
• Supply Current: 500 uA
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Mounting Style: SMD/SMT
• Supply Voltage (max): 5.5 V
• Supply Voltage (min): 2.7 V
• Package / Case: TSSOP-14
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

DS1855
LOCKING AND UNLOCKING EEPROM
In addition to the WP pin, it is possible to write-protect, or lock, certain portions of the EEPROM through
software control. The DS1855 256-byte EEPROM can be visualized as three blocks, or partitions. The
lower block is from 00h to 7Fh. The upper block is 80h to F7h. And the upper page is from F8 to FFh.
The lower and upper blocks are user EEPROM. The upper page is EEPROM that contains the pot
settings, as well as the lock registers.
Locking the EEPROM is a two-step process. First, the software lock configuration byte (FAh) is used to
choose which portion(s) of EEPROM are to be locked. The three least significant bits of FAh are B2, B1,
and B0. B2 selects the upper page (F8–FFh). B1 selects the upper block (80–F7h). The LSBit, B0, selects
the lower block (00–7Fh). The user may lock one, two, or all three partitions at once. The second step
required to turn on the lock is to write the password into the lock bytes (FBh and FCh). The password to
lock is 56h, 25h (FBh and FCh, respectively). Once the EEPROM is locked, the user may still read data
out of the locked portions, but performing a write will not write to EEPROM.
Unlocking the EEPROM consists of entering the password into bytes FBh and FCh. The password to
unlock is 67h, 36h (FBh and FCh, respectively). However, when attempting to unlock the upper page,
which contains the lock bytes (FBh and FCh), the two-byte password must be written in one write cycle.
If a 2-wire STOP command is sent between the write to FBh and FCh, the upper page will remain locked.
In order to modify the Software Lock Configuration Byte (FAh), the upper page must be unlocked. In
other words, the upper page must be unlocked in order to make changes to the locking of the upper and
lower blocks.
READING AND WRITING THE POTENTIOMETER VALUES
Reading from and writing to the potentiometers consists of a standard read or write to EEPROM memory
at the addresses F8h and F9h. The 8-bit value at address F9h controls the wiper setting for potentiometer
0, which has 100 positions. The 8-bit value at address F8h controls the wiper setting of potentiometer 1,
which has 256 positions. Potentiometer 1 may be set to any value between 00h and FFh. 00h sets the
wiper of potentiometer 1 to its lowest value and FFh sets the wiper to its highest. Potentiometer 0 may be
set to any value between 00h and 63h. A value of 00h sets the wiper of potentiometer 0 to its lowest
position and 63h sets the wiper to its highest position. Any hexadecimal value is a valid address. Setting a
value greater than the upper limit of the potentiometer’s range, 64h or greater for potentiometer 0, will
result in setting the wiper to its highest position, but the MSB will be ignored. The memory locations F8h
and F9h, which control the potentiometers’ settings, are programmed to FFh when shipped from the
factory. All other memory locations are initially programmed to 00h.
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