ADN2850(RevB) Просмотр технического описания (PDF) - Analog Devices
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ADN2850 Datasheet PDF : 20 Pages
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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
B00100H
20xxxxH
B10200H
21xxxxH
SDO
Action
XXXXXXH
B00100H
20xxxxH
B10200H
Loads data 100H into RDAC1 register,
Wiper W1 moves to 1/4 full-scale
position.
Saves copy of RDAC1 register content
into corresponding EEMEM1 register.
Loads 200H data into RDAC2 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
the wiper in 6 dB per step from the 0% of the full-scale RWB to
the full-scale RWB. The 6 dB increment instruction doubles the
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 ± 6 dB step adjustment at certain
–8–
REV. B
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