Datasheet

MAX5477/MAX5478/MAX5479
Dual, 256-Tap, Nonvolatile, I
2
C-Interface,
Digital Potentiometers
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Digital Interface
The MAX5477/MAX5478/MAX5479 feature an internal,
nonvolatile EEPROM that stores the wiper state for ini-
tialization during power-up. The shift register decodes
the command and address bytes, routing the data to
the proper memory registers. Data written to a volatile
memory register immediately updates the wiper posi-
tion, or writes data to a nonvolatile register for storage
(see Table 3).
The volatile register retains data as long as the device
is powered. Removing power clears the volatile regis-
ter. The nonvolatile register retains data even after
power is removed. Upon power-up, the power-on reset
circuitry controls the transfer of data from the non-
volatile register to the volatile register.
Write Protect (WP)
A write-protect feature prevents accidental overwriting of
the EEPROM. Connect WP to V
DD
or leave unconnected
to prevent any EEPROM write cycles. Writing to the
volatile register (VREG) while WP = 1 updates the wiper
position with the protected data stored in the nonvolatile
register (NVREG). Connect WP to GND to allow write
commands to the EEPROM and to update the wiper
position from either the value in the EEPROM or directly
from the I
2
C interface (Table 2). Connecting WP to GND
increases the supply current by 19.6µA (max).
To ensure a fail-safe, write-protect feature, write the
data to be protected to both the nonvolatile and volatile
registers before pulling WP high. Releasing WP (WP =
0) and sending partial or invalid I
2
C commands (such
as single-byte address polling) can load the volatile
ADDRESS BYTE COMMAND BYTE DATA BYTE
1 2 3 4 5 6 7 8 9 10111213 14151617 18 1920212223242526 27
SCL CYCLE
NUMBER
START
(S)
A6 A5 A4 A3 A2 A1 A0
ACK
(A)
TX NV V R3 R2 R1 R0
ACK
(A)
D7 D6 D5 D4 D3 D2 D1 D0
ACK
(A)
STOP
(P)
NOTES
VREG 0101A2A1A00 00010001 D7D6D5D4D3D2D1D0
NVREG 0101A2A1A00 00100001 D7D6D5D4D3D2D1D0
NVREGxVREG 0101A2A1A00 01100001 D7D6D5D4D3D2D1D0
VREGxNVREG 0101A2A1A00 01010001 D7D6D5D4D3D2D1D0
WIPER A
ONLY
VREG 0101A2A1A00 00010010 D7D6D5D4D3D2D1D0
NVREG 0101A2A1A00 00100010 D7D6D5D4D3D2D1D0
NVREGxVREG 0101A2A1A00 01100010 D7D6D5D4D3D2D1D0
VREGxNVREG 0101A2A1A00 01010010 D7D6D5D4D3D2D1D0
WIPER B
ONLY
VREG 0101A2A1A00 00010011 D7D6D5D4D3D2D1D0
NVREG 0101A2A1A00 00100011 D7D6D5D4D3D2D1D0
NVREGxVREG 0101A2A1A00 01100011 D7D6D5D4D3D2D1D0
VREGxNVREG 0101A2A1A00 01010011 D7D6D5D4D3D2D1D0
WIPERS
A AND B
Table 3. Command Byte Summary
COMMAND WP = 0 WP = 1
Write to VREG
I
2
C data is written to VREG.
Wiper position updates with I
2
C data.
No change to NVREG.
Copy NVREG to VREG.
Wiper position updates with NVREG data.
No change to NVREG.
Write to NVREG
No change to VREG or wiper position.
I
2
C data is written to NVREG.
No change to VREG or wiper position.
No change to NVREG.
Copy NVREG to VREG
Copy NVREG to VREG.
Wiper position updates with NVREG data.
No change to NVREG.
Copy NVREG to VREG.
Wiper position updates with NVREG data.
No change to NVREG.
Copy VREG to NVREG
Copy VREG to NVREG.
No change to VREG or wiper position.
No change to VREG or wiper position.
No change to NVREG.
Table 2. Write-Protect Behavior of VREG and NVREG