Datasheet

AD5122/AD5142 Data Sheet
Rev. 0 | Page 24 of 32
±6 dB Increment and Decrement Instructions
Two programming instructions produce logarithmic taper
increment or decrement of the wiper position control by
an individual potentiometer or by a ganged potentiometer
arrangement where all RDAC register positions are changed
simultaneously. The +6 dB increment is activated by Command 6,
and the −6 dB decrement is activated by Command 7 (see Table 16).
For example, starting with the zero-scale position and executing
Command 6 ten times moves the wiper in 6 dB steps to the full-
scale position. When the wiper position is near the maximum setting,
the last 6 dB increment instruction causes the wiper to go to the
full-scale position (see Table 14).
Incrementing the wiper position by +6 dB essentially doubles the
RDAC register value, whereas decrementing the wiper position by
−6 dB halves the register value. Internally, the AD5122/AD5142 use
shift registers to shift the bits left and right to achieve a ±6 dB
increment or decrement. These functions are useful for various
audio/video level adjustments, especially for white LED brightness
settings in which human visual responses are more sensitive to
large adjustments than to small adjustments.
Table 14. Detailed Left Shift and Right Shift Functions for
the ±6 dB Step Increment and Decrement
Left Shift (+6 dB/Step) Right Shift (−6 dB/Step)
0000 0000 1111 1111
0000 0001 0111 1111
0000 0010 0011 1111
0000 0100
0001 1111
0000 1000 0000 1111
0001 0000 0000 0111
0010 0000 0000 0011
0100 0000 0000 0001
1000 0000 0000 0000
1111 1111 0000 0000
Reset
The AD5122/AD5142 can be reset through software by executing
Command 14 (see Table 16) or through hardware on the low pulse
of the
RESET
pin. The reset command loads the RDAC registers
with the contents of the EEPROM and takes approximately 30 µs.
The EEPROM is preloaded to midscale at the factory, and initial
power-up is, accordingly, at midscale. Tie
RESET
to V
LOGIC
if
the
RESET
pin is not used.
Shutdown Mode
The AD5122/AD5142 can be placed in shutdown mode by
executing the software shutdown command, Command 15 (see
Table 16); and by setting the LSB (D0) to 1. This feature places
the RDAC in a special state. The contents of the RDAC register are
unchanged by entering shutdown mode. However, all commands
listed in Table 16 are supported while in shutdown mode. Execute
Command 15 (see Table 16) and set the LSB (D0) to 0 to exit
shutdown mode.
Table 15. Truth Table for Shutdown Mode
Linear Gain Setting Mode
Potentiometer Mode
A2 AW WB AW WB
0 N/A
1
Open Open R
BS
1 Open N/A
1
N/A
1
N/A
1
1
N/A = not applicable.
EEPROM OR RDAC REGISTER PROTECTION
The EEPROM and RDAC registers can be protected by disabling
any update to these registers. This can be done by using software or
by using hardware. If these registers are protected by software,
set Bit D0 and/or Bit D1 (see Table 18 ), which protects the
EEPROM and RDAC registers independently.
When RDAC is protected, the only operation allowed is to copy
the EEPROM into the RDAC register.
INDEP PIN
If the INDEP pin is pulled high at power-up, the part operates
in linear gain setting mode, loading each string resistor, R
AWx
and
R
WBx
, with the value stored into the EEPROM (see Table 17). If
the pin is pulled low, the part powers up in potentiometer mode.
The INDEP pin and the D2 bit are connected internally to a logic
OR gate, if any or both are 1, the part cannot operate in
potentiometer mode (see Table 18).