DS1868
Dual Digital Potentiometer Chip
www.dalsemi.com
FEATURES
§
Ultra-lowpower consumption, quiet, pumpless
design
§
Two digitally controlled, 256-position
potentiometers
§
Serial port provides means for setting and
reading both potentiometers
§
Resistors can be connected in series to
provide increased total resistance
§
20-pin TSSOP, 16-pin SOIC, and 14-pin DIP
packages are available.
§
Resistive elements are temperature
compensated to
±0.3
LSB relative linearity
§
Standard resistance values:
- DS1868-10
∼
10 kΩ
- DS1868-50
∼
50 kΩ
- DS1868-100
∼
100 kΩ
§
+5V or ±3V operation
§
Operating Temperature Range:
- Industrial: -40°C to 85°C
PIN ASSIGNMENT
V
B
DNC
H1
L1
W1
RST
CLK
DNC
DNC
GND
1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
15
14
13
12
11
V
CC
DNC
DNC
S
OUT
W0
H0
L0
C
OUT
DNC
DQ
V
B
H1
L1
V
CC
NC
W1
S
OUT
W0 RST
H0
L0 CLK
C
OUT
DQ GND
1
2
3
4
5
6
7
14
13
12
11
10
9
8
V
CC
S
OUT
W0
H0
L0
C
OUT
DQ
20-Pin TSSOP (173-mil)
V
B
NC
H1
L1
W1
RST
CLK
GND
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
DS1868S 16-Pin SOIC (300-mil)
14-Pin DIP (300-mil)
PIN DESCRIPTION
L0, L1
H0, H1
W0, W1
S
OUT
- Low End of Resistor
- High End of Resistor
- Wiper Terminal of Resistor
- Stacked Configuration Output
RST
- Serial Port Reset Input
DQ
- Serial Port Data Input
CLK
- Serial Port Clock Input
C
OUT
- Cascade Port Output
V
CC
- +5 Volt Supply
GND
- Ground Connections
NC
- No Internal Connection
V
B
- Substrate Bias Voltage
DNC
- Do Not Connect
*All GND pins must be connected to ground.
DESCRIPTION
The DS1868 Dual Digital Potentiometer Chip consists of two digitally controlled solid-state
potentiometers. Each potentiometer is composed of 256 resistive sections. Between each resistive section
and both ends of the potentiometer are tap points which are accessible to the wiper. The position of the
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DS1868
wiper on the resistor array is set by an 8-bit value that controls which tap point is connected to the wiper
output. Communication and control of the device is accomplished via a 3-wire serial port interface. This
interface allows the device wiper position to be read or written.
Both potentiometers can be connected in series (or stacked) for an increased total resistance with the same
resolution. For multiple-device, single-processor environments, the DS1868 can be cascaded or daisy
chained. This feature provides for control of multiple devices over a single 3-wire bus.
The DS1868 is offered in three standard resistance values which include 10, 50, and 100 kohm versions.
The part is available in 16-pin SOIC (300-mil), 14-pin DIP, and 20-pin (173-mil) TSSOP packages.
OPERATION
The DS1868 contains two 256-position potentiometers whose wiper positions are set by an 8-bit value.
These two 8-bit values are written to a 17-bit I/O shift register which is used to store the two wiper
positions and the stack select bit when the device is powered. A block diagram of the DS1868 is
presented in Figure 1.
Communication and control of the DS1868 is accomplished through a 3-wire serial port interface that
drives an internal control logic unit. The 3-wire serial interface consists of the three input signals:
RST
,
CLK, and DQ.
The
RST
control signal is used to enable the 3-wire serial port operation of the device. The
RST
signal is
an active high input and is required to begin any communication to the DS1868. The CLK signal input is
used to provide timing synchronization for data input and output. The DQ signal line is used to transmit
potentiometer wiper settings and the stack select bit configuration to the 17-bit I/O shift register of the
DS1868.
Figure 9(a) presents the 3-wire serial port protocol. As shown, the 3-wire port is inactive when the
RST
signal input is low. Communication with the DS1868 requires the transition of the
RST
input from a low
state to a high state. Once the 3-wire port has been activated, data is entered into the part on the low to
high transition of the CLK signal inputs. Three-wire serial timing requirements are provided in the timing
diagrams of Figure 9(b),(c).
Data written to the DS1868 over the 3-wire serial interface is stored in the 17-bit I/O shift register (see
Figure 2). The 17-bit I/O shift register contains both 8-bit potentiometer wiper position values and the
stack select bit. The composition of the I/O shift register is presented in Figure 2. Bit 0 of the I/O shift
register contains the stack select bit. This bit will be discussed in the section entitled Stacked
Configuration. Bits 1 through 8 of the I/O shift register contain the potentiometer-1 wiper position value.
Bit 1 will contain the MSB of the wiper setting for potentiometer-1 and bit 8 the LSB for the wiper
setting. Bits 9 through 16 of the I/O shift register contain the value of the potentiometer-0 wiper position
with the MSB for the wiper position occupying bit 9 and the LSB bit 16.
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DS1868
DS1868 BLOCK DIAGRAM
Figure 1
I/O SHIFT REGISTER
Figure 2
Transmission of data always begins with the stack select bit followed by the potentiometer-1 wiper
position value and lastly the potentiometer-0 wiper position value.
When wiper position data is to be written to the DS1868, 17 bits (or some integer multiple) of data should
always be transmitted. Transactions which do not send a complete 17 bits (or multiple) will leave the
register incomplete and possibly an error in the desired wiper positions.
After a communication transaction has been completed the
RST
signal input should be taken to a low
state to prevent any inadvertent changes to the device shift register. Once
RST
has reached a low state,
the contents of the I/O shift register are loaded into the respective multiplexers for setting wiper position.
A new wiper position will only engage after a
RST
transition to the inactive state. On device power-up,
wiper position will be random.
STACKED CONFIGURATION
The potentiometers of the DS1868 can be connected in series as shown in Figure 3. This is referred to as
the stacked configuration and allows the user to double the total end-to-end resistance of the part. The
resolution of the combined potentiometers will remain the same as a single potentiometer but with a total
of 512 wiper positions available. Device resolution is defined as R
TOT
/256 (per potentiometer); where
R
TOT
equals the total potentiometer resistance.
The wiper output for the combined stacked potentiometer will be taken at the S
OUT
pin, which is the
multiplexed output of the wiper of potentiometer-0 (W0) or potentiometer-1 (W1). The potentiometer
wiper selected at the S
OUT
output is governed by the setting of the stack select bit (bit 0) of the 17-bit I/O
shift register. If the stack select bit has value 0, the multiplexed output, S
OUT
, will be that of the
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DS1868
potentiometer-0 wiper. If the stack select bit has value 1, the multiplexed output, S
OUT
, will be that of the
potentiometer-1 wiper.
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DS1868
STACKED CONFIGURATION
Figure 3
CASCADE OPERATION
A feature of the DS1868 is the ability to control multiple devices from a single processor. Multiple
DS1868s can be linked or daisy chained as shown in Figure 4. As a data bit is entered into the I/O shift
register of the DS1868 a bit will appear at the C
OUT
output after a minimum delay of 50 nanoseconds. The
stack select bit of the DS1868 will always be the first out the part at the beginning of a transaction. The
C
OUT
pin will always have the value of the stack select bit (b0) when
RST
is inactive.
CASCADING MULTIPLE DEVICES
Figure 4
The C
OUT
output of the DS1868 can be used to drive the DQ input of another DS1868. When connecting
multiple devices, the total number of bits transmitted is always 17 times the number of DS1868s in the
daisy chain.
An optional feedback resistor can be placed between the C
OUT
terminal of the last device and the first
DS1868 DQ, input thus allowing the controlling processor to read, as well as, write data, or circularly
clock data through the daisy chain. The value of the feedback or isolation resistor should be in the range
from 2 to 10 kohms.
When reading data via the C
OUT
pin and isolation resistor, the DQ line is left floating by the reading
device. When
RST
is driven high, bit 17 is present on the C
OUT
pin, which is fed back to the input DQ
pin through the isolation resistor. When the CLK input transitions low to high, bit 17 is loaded into the
first position of the I/O shift register and bit 16 becomes present on C
OUT
and DQ of the next device. After
17 bits (or 17 times the number of DS1868s in the daisy chain), the data has shifted completely around
and back to its original position. When
RST
transitions to the low state to end data transfer, the value (the
same as before the read occurred) is loaded into the wiper-0, wiper-1, and stack select bit I/O register.
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