REV. B

–8–

ADN2850

OPERATIONAL OVERVIEW

The ADN2850 programmable resistor is designed to operate as

a true variable resistor. The resistor wiper position is determined

by the RDAC register contents. The RDAC register acts as a

scratch pad register which allows unlimited changes of resistance

settings. The scratch pad register can be programmed with any

position setting using the standard SPI serial interface by loading

the 24-bit data-word. The format of the data-word is that the first

4 bits are instructions, the following 4 bits are addresses, and the

last 16 bits are data. Once a specific value is set, this value can be

saved into a corresponding EEMEM register. During subsequent

power-ups, the wiper setting will automatically be loaded at that

value. Saving data to the EEMEM takes about 25 ms and con-

sumes approximately 20 mA. During this time the shift register

is locked, preventing any changes from taking place. The RDY pin

indicates the completion of this EEMEM saving process. There

are also 13 two-bytes addresses, of user defined data that can be

stored in EEMEM.

OPERATION DETAIL

There are 16 instructions that facilitate users’ programming

needs. Referring to Table II, the instructions are:

0. Do Nothing

1. Restore EEMEM setting to RDAC

2. Save RDAC setting to EEMEM

3. Save user data or RDAC setting to EEMEM

4. Decrement 6 dB

5. Decrement all 6 dB

6. Decrement one step

7. Decrement all one step

8. Reset all EEMEM settings to RDAC

9. Read EEMEM to SDO

10. Read Wiper Setting to SDO

11. Write data to RDAC

12. Increment 6 dB

13. Increment all 6 dB

14. Increment one step

15. Increment all one step

Tables VIII to XIV provide a few programming examples by using

some of these instructions.

Scratch Pad and EEMEM Programming

The basic mode of setting the programmable resistor wiper position

(programming the scratch pad register) is done by loading the

serial data input register with the instruction 11, the corresponding

address, and the data. Since the scratch pad register is a standard

logic register, there is no restriction on the number of changes

allowed. When the desired wiper position is determined, the user can

load the serial data input register with the instruction 2, which stores

the setting into the corresponding EEMEM register. The EEMEM

value can be changed at any time or permanently protected by

activating the

WP command. Table III provides a programming

example listing the sequence of serial data input (SDI) words and

the corresponding serial data output (SDO) in hexadecimal format.

Table III. Set and Save RDAC with Independent Data

to EEMEM Registers

SDI

SDO

Action

Wiper W1 moves to 1/4 full-scale

position.

Saves copy of RDAC1 register content

into corresponding EEMEM1 register.

Wiper W2 moves to 1/2 full-scale

position.

Saves copy of RDAC2 register contents

into corresponding EEMEM2 register.

At system power ON, the scratch pad register is automatically

refreshed with the value previously saved in the corresponding

EEMEM register. The factory preset EEMEM value is midscale.

During operations, the scratch pad register can also be refreshed

with the current contents of the EEMEM registers in three different

ways. First, executing instruction 1 retrieves the corresponding

EEMEM value. Second, executing instruction 8 resets the EEMEM

values of both channels. Finally, pulsing the

PR pin also refreshes

both EEMEM settings. Operating the hardware control

PR

function, however, requires a complete pulse signal. When

PR

goes low, the internal logic sets the wiper at midscale. The

EEMEM value will not be loaded until PR returns to high.

EEMEM Protection

The write-protect (

WP) disables any changes of the scratch pad

register contents regardless of the software commands, except

that the EEMEM setting can be refreshed and can overwrite the

WP by using commands 1, 8, and PR pulse. To disable WP, it is

recommended to execute a NOP command before returning

WP to logic high.

Linear Increment and Decrement Commands

The increment and decrement commands (14, 15, 6, 7) are useful

for linear step adjustment applications. These commands simplify

microcontroller software coding by allowing the controller to

just send an increment or decrement command to the device. The

adjustment can be individually or gang controlled. For incre-

ment command, executing instruction 14 will automatically move the

wiper to the next resistance segment position. The master increment

instruction 15 will move all resistor wipers up by one position.

Logarithmic Taper Mode Adjustment ( 6 dB/step)

There are four programming instructions which provide the

logarithmic taper increment and decrement wiper position con-

trol by either individual or gang control. 6 dB increment is

activated by instructions 12 and 13 and 6 dB decrement is acti-

vated by instructions 4 and 5. For example, starting at zero

scale, executing 11 times the increment instruction 12 will move

value of the RDAC register contents each time the command is

executed. When the wiper position is near the maximum setting,

the last 6 dB increment instruction will cause the wiper to go to

the full-scale 1023-code position. Further 6 dB per increment

instruction will no longer change the wiper position beyond its

full-scale, Table IV.

6 dB step increment and decrement are achieved by shifting the bit

internally to the left and right, respectively. The following infor-

mation explains the nonideal

±6dB step adjustment at certain