Datasheet

Data Sheet AD5291/AD5292
THEORY OF OPERATION
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RDAC REGISTER
The RDAC register directly controls the position of the digital poten-
tiometer wiper. For example, when the RDAC register is loaded
with all zeros, the wiper is connected to Terminal B of the variable
resistor. The RDAC register is a standard logic register; there is no
restriction on the number of changes allowed.
20-TP MEMORY
Once a desirable wiper position is found, the contents of the RDAC
register can be saved into a 20-TP memory register (see Table
14). Thereafter, the wiper position is always set at that position for
any future on-off-on power supply sequence. The AD5291/AD5292
have an array of 20 one-time programmable (OTP) memory regis-
ters. When the desired word is programmed to 20-TP memory,
the device automatically verifies that the program command was
successful. The verification process includes margin testing. Bit
C3 of the control register can be polled to verify that the fuse
program command was successful. Programming data to 20-TP
memory consumes approximately 25 mA for 550 µs and takes
approximately 8 ms to complete. During this time, the shift register
is locked, preventing any changes from taking place. The RDY
pin can be used to monitor the completion of the 20-TP memory
program and verification. No change in supply voltage is required
to program the 20-TP memory. However, a 1 µF capacitor on the
EXT_CAP pin is required (see Figure 68). Prior to 20-TP activation,
the AD5291/AD5292 preset to midscale on power-up.
Table 14. Write and Read to RDAC and 20-TP Memory
DIN SDO Action
0x1803 0xXXXX Enable update of wiper position and 20-TP memory contents through digital interface.
0x0500 0x1803 Write 0x100 to the RDAC register; wiper moves to ¼ full-scale position.
0x0800 0x0500 Prepare data read from the RDAC register.
0x0C00 0x0100 Stores RDAC register content into 20-TP memory. The 16-bit word appears out of SDO, where the last 10 bits contain the contents of the
RDAC register (0x100).
0x1C00 0x0C00 Prepare data read from the control register.
0x0000 0x000X NOP Instruction 0 sends 16-bit word out of SDO, where the last four bits contain the contents of the control register. If Bit C3 = 1, the fuse
program command is successful.
Table 15. Memory Map of Command 5
Data Bits[DB9:DB0]
1
Register ContentsD9 D8 D7 D6 D5 D4 D3 D2 D1 D0
X X X X X 0 0 0 0 0 1
st
programmed wiper location (0x00)
X X X X X 0 0 0 0 1 2
nd
programmed wiper location (0x01)
X X X X X 0 0 0 1 0 3
rd
programmed wiper location (0x02)
X X X X X 0 0 0 1 1 4
th
programmed wiper location (0x03)
X X X X X 0 0 1 0 0 5
th
programmed wiper location (0x04)
… … … … … … … … … … …
X X X X X 0 1 0 0 1 10
th
programmed wiper location (0x09)
X X X X X 0 1 1 1 0 15
th
programmed wiper location (0x0E)
X X X X X 1 0 0 1 1 20
th
programmed wiper location (0x13)
X X X X X 1 0 1 0 0 Programmed memory status (thermometer encoded)
2
(0x14)
X X X X X 1 0 1 0 1 Programmed memory status (thermometer encoded)
2
(0x15)
1
X = don’t care.
2
Allows the user to calculate the remaining spare memory locations.