AN238| Application Note
Maxim > App Notes > DIGITAL POTENTIOMETERS
GENERAL ENGINEERING TOPICS
Keywords: NV, system configuration, digital potentiometer, digipot, digital pots, NV variable resistors, dip switch
Mar 05, 2003
APPLICATION NOTE 238
2-Wire Controlled Digital DIP Switch Using the DS3904
Abstract: The DS3904 contains three digitally controlled nonvolatile (NV) variable resistors. It has the capability of placing each of the variable resistors into a high-impedance state. Using this feature, it is possible to create a 2-wire controlled NV digital DIP switch. This application note will explain the resistor settings necessary to create a digital DIP switch using the DS3904, and how to create NV outputs that can be used to configure digital systems.
Resistor Register Settings
To place one of the DS3904 in high-impedance mode, the MSB of the selected resistor register must be written to a '1'. For example, writing 80h to one of the variable resistor registers enables the high-impedance mode for that resistor. This value can be used to generate a HIGH when combined with a pullup resistor. To attain the smallest resistance available from the variable resistors (typically 400!&), the resistor register must be set to 00h (position 0). This will generate a LOW.
Hardware Requirements
When using a mechanical DIP switch to control a digital input, pullup resistors are used to create a HIGH state (see Figure 1). A LOW is generated by closing the DIP switch.
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