Digital Potentiometer Control and Configuration with the Microchip MCP42100-I/ST
The evolution from mechanical potentiometers to digital counterparts represents a significant leap forward in design flexibility and precision control. The Microchip MCP42100-I/ST is a prime example of this technology, integrating two independently controllable 8-bit (256-tap) potentiometers into a single, compact TSSOP-14 package. This device is engineered for applications requiring programmable voltage division, resistance adjustment, or digital control of analog signals, making it indispensable in modern electronic systems.
Device Overview and Key Features
The MCP42100 is part of Microchip's extensive digital POT family, communicating via a standard SPI (Serial Peripheral Interface). This serial protocol allows for straightforward connection to a wide array of microcontrollers (MCUs) and microprocessors, minimizing the number of required I/O pins. Each potentiometer has a nominal end-to-end resistance of 10 kΩ (with 50 kΩ and 100 kΩ variants also available) and features a programmable wiper that can be set to any of the 256 positions.
A critical feature of this device is its non-volatile memory. The wiper settings can be stored in EEPROM, ensuring that the last configured resistance value is retained even after a complete power loss. This eliminates the need for re-calibration upon power-up, which is crucial for applications requiring consistent start-up conditions.
SPI Communication Protocol
Controlling the MCP42100 is achieved through a simple SPI command structure. The MCU initiates communication by pulling the Chip Select (CS) pin low. This is followed by a 16-bit data packet sent over the Serial Data Input (SDI) line, synchronized by the Serial Clock (SCK).
The command byte is the first 8 bits of the packet. It specifies the target potentiometer (P0 or P1) and the command type, most commonly a write command to update the wiper register. The subsequent data byte contains the actual 8-bit value (0x00 to 0xFF) that defines the wiper's position. Upon receiving the command, the device updates the wiper register and the analog output immediately reflects the change.
Basic Configuration and Circuit Design
A typical application circuit is remarkably simple. The device requires standard power supply decoupling capacitors (e.g., a 0.1 µF capacitor between VDD and VSS) for stable operation. The three SPI lines (SDI, SCK, CS) are connected directly to the MCU's SPI peripheral. The potentiometer terminals (PAx, PWx, PBx) are connected just like a traditional three-terminal potentiometer.
The device can be configured in two primary modes:
Variable Resistor Mode: The unused terminal (either PA or PB) is left unconnected or tied to the wiper (PW). This creates a digitally controlled rheostat.
Voltage Divider Mode: A voltage potential is applied across terminals PA and PB, and the wiper (PW) provides a divided output voltage proportional to the wiper setting.
Application Areas
The MCP42100's versatility makes it suitable for a broad spectrum of applications. It is extensively used for:
Programmable Gain/Offset Adjustment in operational amplifier circuits.
Contrast and Brightness Control in LCD displays.
Sensor Calibration and Trimming, replacing manual trimpots for factory calibration.
Volume Control in audio equipment.
System Parameter Tuning in power supplies and motor control systems.
The Microchip MCP42100-I/ST stands out as a robust and highly reliable solution for integrating digital control into analog circuits. Its combination of SPI interface, non-volatile memory, and dual-channel architecture provides designers with an unparalleled level of precision and automation, effectively bridging the digital and analog domains.
Keywords: Digital Potentiometer, SPI Interface, MCP42100-I/ST, Non-Volatile Memory, Programmable Resistance
