Getting Started with the DS1840K . . .
3 of 13
Evaluating the DS1845
The DS184X family of digital potentiometers use the Dallas 2-Wire serial interface. Since it is a multi-
drop interface, address pins are used to give each device on the bus its own unique address. To provide the
most flexibility, the DS1840K Demo Board allows you to set the address (A0, A1, and A2) using DIP
switches (see Figure 3). A DIP switch (A0, A1, A2, and WP) in the
“ON” position grounds the signal. A DIP switch in the “OFF” position,
allows a pull-up resistor to pull the signal to Vcc. All of the address
signals have an external pull-up resistor, while the Write Protect (WP)
signal has an internal pull-up resistor. If the device is “write protected,”
then the wiper can not be moved and the memory can not be modified.
The default for these first 4 switches is the “ON” position. This
corresponds to a device address of 000h and NO write protection.
Figure 3. DIP switches
The DS1845 contains two non-volatile potentiometers. Pot 0 is a 100 position pot and Pot 1 is a 256
position pot. Valid wiper settings for Pot 0 (located at F9h in the memory map) are from 0 to 63 hex. Pot 1
(located at F8h) is valid from 0 to FF hex. The physical device pins associated with Pot 0 are L0, W0, and
H0. The pins for Pot 1 are L1, W1, and H1. Each of the pot terminals can be accessed directly at the “test
points” near the front of the board (see Figure 4). Using a multi-meter, the resistance can be measured
from these test points (make sure no jumpers are installed on J4 or J5). Measuring from Hx (x being 0 for
pot 0, or 1 for pot 1) to Lx you will see the end-to-end resistance of the pot. For a DS1845-010, you would
measure 10KOhms. Measuring from Wx to Lx will result in a resistance depending on where the wiper
was last set. If you require a ground terminal, you may use either of the black test points labeled GND,
even though they lie within the DS1846 area.
Depending on your application, you may
wish to use the potentiometer as a voltage
divider. This is quite simple using the
DS1840K Demo Board. J5 is the jumper
block for Pot 0. It contains the three pot
terminals as well as Vcc and GND. This
makes it very easy to add a jumper
between H0 and Vcc, as well as L0 and
GND. The result will be a varying
voltage between Vcc and GND at the W0
terminal. This same principal can be
applied to Pot 1 by using jumper block J4. Figure 4. Potentiometer terminals and jumper blocks.
Using either the jumper blocks or the test points, the pots can also be configured as a variable resistor (two
terminal device). By shorting Wx to Lx, a variable resistor is made. The resistance can then be measured
between Hx and Lx (which is shorted to Wx). Another way to convert the three terminal pot into a 2
terminal variable resistor is by adding jumpers to Lx to GND, and Wx to GND. The resistance can then be
measured between Hx and GND. If you use this approach with Pot 0, please be aware that R9 connects H0
to Vcc. This is an error. The ability to jumper the resistor in and out of the circuit will be included in a
later revision. The reason why R9 is even necessary is because this board needs to also work with the
DS1847/48 variable resistors.
