NXP PCAL9535AHF: A Comprehensive Overview of the Low-Voltage 16-Bit I²C-Bus I/O Expander

In the realm of modern electronic design, the constant challenge is to maximize functionality while minimizing space and power consumption. The NXP PCAL9535AHF addresses this challenge head-on, serving as a pivotal component for system expansion in a vast array of applications, from servers and networking equipment to industrial controllers and IoT devices. This device is a low-voltage 16-bit I²C-bus I/O expander that provides a simple and efficient method for a microcontroller to control a multitude of digital I/O signals via a two-wire serial interface.

The core functionality of the PCAL9535AHF is to add sixteen general-purpose input/output (GPIO) ports to a design using only two pins (Serial Data Line (SDA) and Serial Clock Line (SCL)) on the controlling microcontroller. This efficient use of the I²C-bus is crucial for systems where the main processor has a limited number of available GPIO pins. The device operates at voltages from 1.65 V to 3.6 V, making it ideally suited for low-power and low-voltage systems, including those powered by a single lithium-ion cell or multiple AA batteries.

A key advancement of the PCAL9535A family over standard I/O expanders is its inclusion of advanced features that significantly enhance system performance and reliability. These include:

Programmable Output Drive Strength: This allows designers to tailor the rise and fall times of the output signals. By reducing drive strength, electromagnetic interference (EMI) can be minimized. Conversely, increasing strength ensures signal integrity for longer traces or higher capacitive loads.

Internal Input Pull-Up and Pull-Down Resistors: Each port can be configured with an internal ~100 kΩ resistor, either pull-up or pull-down. This eliminates the need for external discrete resistors, saving board space, reducing the bill of materials (BOM) cost, and simplifying PCB layout.

Input Latch Functionality: The inputs can be configured to latch, meaning they will hold their state until read by the master controller. This is critical for capturing and retaining very short pulse glitches or transient events that might otherwise be missed.

Maskable Interrupt Output: The INT (interrupt) pin provides a signal to the host controller whenever an input change occurs. This allows the microcontroller to operate in a low-power sleep mode and only wake up when necessary, rather than constantly polling the expander, leading to significantly reduced system power consumption.

Bus Recovery: This feature helps the device recover from a situation where the I²C bus has hung, enhancing the overall robustness of the system.

The device is offered in a space-saving TSSOP-24 package and is fully specified over the industrial temperature range of -40 °C to +85 °C. Communication is fast, supporting I²C-bus frequencies up to 1 MHz, ensuring rapid data transfer between the host and the expander. With three hardware address pins, up to eight of these devices can be connected on the same I²C-bus, providing the capability to add up to 128 GPIO ports with minimal wiring.

ICGOODFIND: The NXP PCAL9535AHF is far more than a simple port expander. It is a feature-rich, high-performance solution that empowers designers to overcome I/O limitations efficiently. Its programmability, interrupt-driven operation, and integrated resistors make it an indispensable component for creating compact, low-power, and robust embedded systems.

Keywords: I²C-bus GPIO Expander, Low-Voltage Operation, Programmable Output Drive, Interrupt Output, Bus Recovery.