AD5174

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THEORY OF OPERATION

The AD5174 is designed to operate as a true variable resistor for

position. The RDAC register acts as a scratchpad register, which

allows unlimited changes of resistance settings. The RDAC register

can be programmed with any position setting by using the SPI

interface. When a desirable wiper position is found, this value

can be stored in a 50-TP memory register. Thereafter, the wiper

position is always restored to that position for subsequent

power-ups. The storing of 50-TP data takes approximately 350 ms;

during this time, the AD5174 locks to prevent any changes from

taking place.

The AD5174 also feature a patented 1% end-to-end resistor

tolerance. This simplifies precision, rheostat mode, and open-

loop applications where knowledge of absolute resistance is

critical.

SERIAL DATA INTERFACE

The AD5174 contains a serial interface (SYNC, SCLK, DIN,

and SDO) that is compatible with SPI interface standards, as well

as most DSPs. This device allows writing of data via the serial

interface to every register.

SHIFT REGISTER

The shift register is 16 bits wide, as shown in Figure 2. The

16-bit word consists of two unused bits, which should be set to

0, followed by four control bits and 10 RDAC data bits. Data is

loaded MSB first (Bit D9). The four control bits determine the

function of the software command as listed in Table 6. Figure 3

shows a timing diagram of a typical AD5174 write sequence.

The write sequence begins by bringing the SYNC line low. The

SYNC pin must be held low until the complete data-word is

loaded from the DIN pin. When SYNC returns high, the serial

data-word is decoded according to the instructions in

.

The command bits (Cx) control the operation of the digital

potentiometer. The data bits (Dx) are the values that are loaded

into the decoded register. The AD5174 has an internal counter

that counts a multiple of 16 bits (a frame) for proper operation.

For example, AD5174 works with a 32-bit word but does not

work properly with a 31-bit or 33-bit word. The AD5174

does not require a continuous SCLK when

Table 6

SYNC is high.

To minimize power consumption in the digital input buffers,

RDAC REGISTER

The RDAC register directly controls the position of the digital

rheostat wiper. For example, when the RDAC register is loaded

with all 0s, the wiper is connected to Terminal A of the variable

resistor. The RDAC register is a standard logic register, and there

is no restriction on the number of changes allowed. The basic

mode of setting the variable resistor wiper position (programming

the RDAC register) is accomplished by loading the serial data

input register with Command 1 (see Table 6) and with the desired

wiper position data.

50-TP MEMORY BLOCK

The AD5174 contains an array of 50-TP programmable memory

registers, which allow the wiper position to be programmed up

to 50 times. Table 10 shows the memory map. When the desired

wiper position is determined, the user can load the serial data

input register with Command 3 (see Table 6), which stores the

wiper position data in a 50-TP memory register. The first address

to be programmed is Location 0x01 (see Table 10); the AD5174

increments the 50-TP memory address for each subsequent

program until the memory is full. Programming data to 50-TP

consumes approximately 4 mA for 55 ms, and takes approx-

imately 350 ms to complete, during which time the shift register

locks to prevent any changes from occurring. Bit C2 of the

control register can be polled to verify that the fuse program

command was completed properly. No change in supply voltage

is required to program the 50-TP memory; however, a 1 μF

capacitor on the EXT_CAP pin is required (see Figure 28).

Prior to 50-TP activation, the AD5174 presets to midscale

on power-up.

WRITE PROTECTION

At power-up, the serial data input register write commands for

both the RDAC register and the 50-TP memory registers are

disabled. The RDAC write protect bit, C1, of the control register

(see Table 8 and Table 9) is set to 0 by default. This disables any

change of the RDAC register content regardless of the software

commands, except that the RDAC register can be refreshed

from the 50-TP memory using the software reset, Command 4

(see Table 6). To enable programming of the RDAC register,

the write protect bit (Bit C1), of the control register must first

be programmed by loading the serial data input register with

Command 7. To enable programming of the 50-TP memory,

the program enable bit (Bit C0) of the control register, which

is set to 0 by default, must first be set to 1.