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WMS7130100P

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P22
WMS7130 / 7131
Publication Release Date: July 2003
- 7 - Revision 1.0
7. FUNCTIONAL DESCRIPTION
7.1. RHEOSTAT AND DIVIDER OPERATIONS
The WMS7130 device can operate as either a two-terminal variable resistor or a three-terminal
voltage divider without an output buffer. However, the WMS7131 can only operate in a three-terminal
voltage divider with an output buffer.
7.1.1. Rheostat Configuration
In the rheostat mode, the WMS7130 can be configured as a two-terminal resistive element, where one
terminal is connected to one end of the resistor (R
A
or R
B
) and the other terminal is the wiper (R
W
).
The moving direction of the wiper depends upon the setting of U/
D control signal. When the U/ D is
set to Up, then the wiper moves towards R
A
. Conversely, when the U/ D is set to Down, then the
wiper moves towards R
B
. The wiper movement to either direction is controlled by toggling the INC
signal from HIGH to LOW.
This configuration controls the resistance between the wiper and either end. The wiper resistance can
be adjusted by either changing the wiper position or loading a stored wiper position value from
NVMEM0 upon power up.
7.1.2. Divider Configuration
Additionally, the WMS7130 can also be configured as a voltage divider. With an input voltage applied
to one end (usually V
A
), the ground is connected to the other end (usually V
B
). These input voltages
cannot exceed the V
DD
level or go below the V
SS
level. The voltage on the wiper, V
W
, is proportional
to the wiper position with respect to the voltage difference between V
A
and V
B
. The moving direction of
the wiper depends upon the setting of the U/
D control signal. When the U/D is set to Up, then the
wiper moves towards V
A
. Conversely, when the U/ D is set to Down, then the wiper moves towards
V
B
. The wiper movement to either direction is controlled by toggling the INC signal from HIGH to
LOW.
Nevertheless, the WMS7131 can only be configured as a voltage divider and operate similarly as the
WMS7130 device. The only difference is WMS7131 has an output buffer, but WMS7130 doesn’t have.
Besides, the resistance cannot be directly measured in this configuration.
7.2. NON-VOLATILE MEMORY (NVMEM0)
The WMS7130/7131 has one NVMEM0 location available for storing the current wiper position via the
Up/Down serial interface. This stored value is automatically recalled and loaded into the tap register
upon power up.
WMS7130 / 7131
- 8 -
7.3. SERIAL DATA INTERFACE
The WMS7130/7131 device has a 3-wire Up/Down Serial Interface consisting of CS , INC and U/D
control signals. The key features of this interface include:
• Enabling the device
• Determining the moving direction of the wiper
• Increment/Decrement operation on the wiper
• Non-volatile storage of the present wiper position into the NVMEM0 for automatic recall at
power up
• Entering into the standby mode
7.4. OPERATION OVERVIEW
The wiper position can be changed either up or down by operating the CS , U/ D and INC control
signals.
When
CS is LOW, the device is selected and the wiper can be moved by toggling the INC . As a
result, the wiper moves up when U/
D is HIGH and moves down when U/D is LOW. The status of the
U/
D can be changed even though the CS remains LOW. This allows the system to enable the
device and then move the wiper position either up or down until the desired position is reached.
When the wiper is already at the lowest position, further Down operation won’t change the wiper
position. Similarly, when the wiper is at the highest position, further Up operation won’t change the
wiper position too.
The current wiper position can be automatically stored into the NVMEM0 each time the
CS goes
from LOW to HIGH while the
INC remains HIGH. Adversely, if the INC is LOW when the CS goes
HIGH, the wiper position cannot be stored. Meanwhile, the NVMEM0 content is automatically loaded
into the wiper during power on.
When the
CS is held HIGH, the device enters into Standby mode and the wiper position cannot be
changed. Changing the CS to LOW exits the Standby mode and enables the device again.
The operating modes of Up/Down interface are summarized in the table below:
CS
U/D INC
Operation
LOW HIGH HIGH to LOW Move Wiper toward R
A
/V
A
LOW LOW HIGH to LOW Move Wiper toward R
B
/V
B
LOW to HIGH x HIGH Store Current Wiper Position
LOW to HIGH x LOW No Store, Return to Standby
HIGH x x Standby
Note: x means don’t care
WMS7130 / 7131
Publication Release Date: July 2003
- 9 - Revision 1.0
8. TIMING DIAGRAMS
Conditions: V
DD
= +2.7V to 5.5V, V
A
= V
DD
, V
B
= 0V, T = 25°C
FIGURE 3 –WMS7130/1 TIMING DIAGRAM
Note:
[1]
This only applies to the Power-Up sequence.
[2]
MI in the AC Timing diagram (Figure 3) refers to the minimum incremental change in the wiper output due to a change in the
wiper position.
U
/D
CS
INC
t
I
L
t
I
H
t
C
YC
V
W
MI
[
2
]
90%
90%
10%
(store)
t
CPH
t
I
W
t
PUD
[
1
]
t
CI
t
CI
t
DI
t
ID
t
F
t
R
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P22

WMS7130100P

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