MCP23008-E/SO 8-Bit I/O Expander: Features, Application Circuit, and Interfacing Guide
In modern embedded system design, a common challenge is the scarcity of General-Purpose Input/Output (GPIO) pins on a microcontroller (MCU). The MCP23008-E/SO from Microchip Technology provides an elegant and efficient solution to this problem. This 8-bit I/O expander uses the I²C serial interface, enabling a single MCU to control up to eight additional digital I/O ports using only two I²C bus pins. Its small SOIC package makes it ideal for space-constrained applications.
Key Features of the MCP23008-E/SO
The MCP23008 is packed with features that make it incredibly versatile for a wide range of applications. Its core functionality revolves around expanding digital I/O capability via a simple two-wire serial interface.
I²C Serial Interface: Operates with a maximum serial clock frequency of 1.7 MHz, allowing for high-speed communication. It supports a wide range of voltage levels (1.8V to 5.5V), making it compatible with both 3.3V and 5V logic families.
Configurable I/O Ports: All eight GPIO pins are individually configurable as either inputs or outputs through the I/O Direction (IODIR) register. This flexibility allows designers to tailor the device to the specific needs of their circuit.
Internal Registers: A suite of internal registers provides powerful control, including:
Input Polarity Register (IPOL): Allows inversion of the input pin logic value, simplifying sensor interfacing.
Interrupt Control: Features like the GPIO Change Interrupt output pin and multiple internal interrupt control registers (GPINTEN, INTCON, DEFVAL) enable the expander to notify the MCU of input state changes without constant polling, significantly improving system efficiency.
High Sink/Source Current: Each pin can sink or source up to 25 mA, allowing it to drive LEDs directly without the need for external transistors in most cases.
Three Hardware Address Pins: These pins (A0, A1, A2) allow up to eight MCP23008 devices to share the same I²C bus, providing a potential expansion of 64 I/O pins on a single bus.
Typical Application Circuit
A basic application circuit for the MCP23008-E/SO is straightforward. The following components are essential for operation:
1. Power Decoupling: A 0.1µF ceramic decoupling capacitor must be placed between the VDD (pin 9) and VSS (pin 10) pins, as close to the IC as possible, to filter high-frequency noise.
2. I²C Pull-up Resistors: The serial data (SDA, pin 16) and serial clock (SCL, pin 15) lines require pull-up resistors (typically 4.7kΩ) to VDD. The I²C bus is open-drain, meaning these resistors are mandatory for proper communication.
3. Address Pins: The address pins (A0, A1, A2 on pins 13, 12, and 11) must be tied to either VDD or VSS to set the device's unique I²C address.
4. Reset Pin: The active-low RESET pin (pin 14) should be pulled high to VDD via a resistor (e.g., 10kΩ) for normal operation.
5. Interrupt Pin: The active-low INT pin (pin 6) can be connected to an interrupt-capable input pin on the host MCU to signal pin state changes. A pull-up resistor is typically used on this line.
The GPIO pins (GP0 to GP7, pins 1-5 and 7-8) can then be connected to switches, sensors, LEDs, or relays as required by the application.
Interfacing Guide with a Microcontroller
Interfacing the MCP23008 with an MCU like an Arduino or an ARM Cortex-based chip is a simple process involving software configuration.
1. Hardware Setup: Connect the SDA and SCL pins of the MCP23008 to the corresponding I²C pins on the MCU. Set the address pins to a unique value. Connect the INT pin if using interrupts.
2. Software Initialization: In the MCU's firmware, initialize the I²C peripheral. The first step is to configure the I/O direction by writing to the IODIR register. A value of '1' sets a pin as an input, and '0' sets it as an output.
3. Writing Outputs: To set output pins, write values to the GPIO register. A '1' sets a output pin high, a '0' sets it low.
4. Reading Inputs: To read the state of input pins, simply read the GPIO register. The value returned will reflect the logic level on each pin.
5. Using Interrupts (Advanced): To use the interrupt feature, configure the interrupt control registers:
Use GPINTEN to enable interrupt-on-change for specific pins.
Use INTCON to set whether to interrupt on pin change or compare against a default value (DEFVAL).
When an interrupt occurs on the INT pin, the MCU can read the GPIO register to determine which pin caused the interrupt. Reading the GPIO register automatically clears the interrupt condition.
ICGOOODFIND: The MCP23008-E/SO is an exceptionally versatile and cost-effective solution for GPIO expansion. Its simple I²C interface, robust interrupt functionality, and high drive capability make it a perennial favorite among engineers for applications ranging from industrial control and consumer electronics to hobbyist projects, effectively solving the problem of limited MCU pins.
Keywords: I²C Interface, GPIO Expander, Interrupt Function, Input/Output Expander, Microcontroller Interfacing.
