PCA9539BS

The PCA9539BS from NXP Semiconductors is a 16-bit I2C-bus GPIO expander that provides 16 bidirectional I/O pins controlled via an I2C interface. It is designed to expand the I/O capability of MCUs in applications where pin count is limited. The device features an interrupt output that activates when any input pin changes state, eliminating the need for I2C polling. Per-pin polarity inversion allows direct interfacing with active-low signals without software inversion. Three address pins allow up to 8 PCA9539 devices on the same I2C bus, providing up to 128 additional I/O pins. Each output can sink 25mA, sufficient for direct LED driving. The BS suffix denotes the TSSOP-24 package.

The PCA9539BS is controlled entirely through I2C register access. The 8 internal registers include: Input Port (read pin states), Output Port (set output levels), Polarity Inversion (invert input logic), Configuration (set pin direction: 0=output, 1=input), plus registers for bus transactions. When a pin is configured as output (Configuration bit=0), the Output Port register bit drives the pin high or low. When configured as input (Configuration bit=1), the pin is high-impedance and its state can be read from the Input Port register. The Polarity Inversion register XORs the input data, so setting a bit to 1 inverts the corresponding input reading — useful for active-low buttons. The interrupt output (INT) is asserted low whenever any input pin changes state from the last read. Reading the Input Port register clears the interrupt. The active-low RESET pin restores all registers to their default states (all inputs, no inversion). The I2C address is set by the three hardware address pins (A0, A1, A2), allowing 8 unique addresses from 0x74 to 0x77 on the I2C bus.

PCA9539BS uses the HVQFN24 (4x4x0.85mm, SOT616-1) surface-mount package from NXP: Pin 1 = VDD, Pin 2 = A0 (address), Pin 3 = A1 (address), Pin 4 = SCL, Pin 5 = SDA, Pin 6 = RESET (active-low), Pin 7 = INT (interrupt, active-low), Pins 8-15 = P00-P07 (Port 0 I/O), Pin 16 = VSS (GND), Pins 17-24 = P10-P17 (Port 1 I/O), Exposed Pad = GND. The RESET pin replaces the A2 address pin found on PCA9555, limiting the bus to 4 devices instead of 8. Pin 1 is marked by a dot in the corner. The exposed pad must be soldered to the PCB ground plane for thermal and electrical performance. All 16 I/O pins default to inputs with weak internal pull-ups on power-up. For TSSOP-24 version, PCA9539PW provides the same pinout.

• Keyboard Scanning: 16 I/O pins form 8×8 matrix for 64 keys; INT pin alerts MCU on any keypress change
• LED Control: 25mA sink per pin directly drives 16 indicator LEDs via I2C without additional drivers
• Industrial I/O: Polarity inversion simplifies interface to active-low PLC inputs without software overhead
• Server Management: 8 addresses × 16 pins = 128 I/O for fan, power, and status monitoring across a single I2C bus
• Display Segments: 16 outputs multiplexed with 8 port pins drive 7-segment displays with minimal MCU pins

PCA9539BS is a 16-bit I2C GPIO expander from NXP in HVQFN24. The PCA9539PW provides the same in TSSOP-24 (still in production). The PCA9555 offers 16-bit I/O with internal pull-up resistors in DIP-24/TSSOP-24. The MCP23017 from Microchip provides 16-bit I2C GPIO with configurable interrupt in DIP-28/SOIC-28. The MAX7316 from ADI/Maxim offers 16-bit I2C GPIO with hot-insertion support. The TCA9555 from TI provides 16-bit I2C GPIO in TSSOP-24. For lower pin count, PCA9536 offers 4-bit in TSSOP-8.