Digital I/O Expander Board

Revision Status

Revision	Status	Changes
`Rev A`	Active	(Initial release)

The Digital I/O Expander Board is a deterministic, shift-register-based expansion module providing 16 digital inputs and 16 digital outputs. It allows you to significantly scale the number of digital inputs and outputs available to a microcontroller using only a small, dedicated set of control lines.

You can follow its development on www.jktdev.co.uk.

Features

32-Channel Expansion: Adds 16 inputs and 16 outputs to your design.
Deterministic Interface: Uses simple serial shift registers instead of bus-based GPIO expander ICs, preventing bus lockup risks and offering predictable timing.
Robust Connections: The host microcontroller connects via a centralized 12-pin header, while all 32 external I/O channels, along with 3.3 V and GND rails, are broken out to screw terminal blocks.

Circuitry & Component Details

Input Side: Employs 2 × 74HC165 parallel-in, serial-out shift registers.
Output Side: Employs 2 × 74HC595 serial-in, parallel-out shift registers.

[!IMPORTANT] Rev A Silkscreen Labeling Issue: On Rev A boards, the input channel labels are reversed across each 8-input bank: * Physical input I0 is labeled as I7 (and vice-versa). * Physical input I8 is labeled as I15 (and vice-versa).

The electrical connections are fully functional, but you must map the inputs in software or account for this reversal in your wiring. This silkscreen mapping issue has been resolved in the Rev B PCB design.

Pinout & Interface

12-Pin Control Header

Pin	Label	Direction	Description
1	3.3V	Input	Logic Supply (Typically 3.3 V)
2	INDATA	Output	Serial Data Line (74HC165)
3	/INDATA	Output	Inverted Serial Data Line (74HC165, optional spare)
4	/INLOAD	Input	Input Parallel Load line (Active Low, 74HC165)
5	INCLK	Input	Input Clock line for input shift registers (74HC165)
6	GND	Input	Logic Ground Reference
7	OUTCLK	Input	Output Clock line for output shift registers (74HC595)
8	LATCH	Input	Output Latch line to update output pins (74HC595)
9	/OUTEN	Input	Output Enable (Active Low, 74HC595)
10	OUTDATA	Input	Output Serial Data Line (74HC595)
11	/OUTCLR	Input	Master Reset / Clear (Active Low, 74HC595)
12	GND	Input	Logic Ground Reference

Bill of Materials (BOM)

Reference	Qty	Value	Footprint	LCSC Part No	MFR Part No	Description
U1,U2	2	74HC165	Package_SO:SOIC-16_4.55x10.3mm_P1.27mm	C5613	74HC165D	Shift Register, 8-bit, Parallel Load
U3,U4	2	74HC595	Package_SO:SOIC-16_4.55x10.3mm_P1.27mm	C5947	74HC595D	-40°C~+125°C 100MHz [email protected], 50pF 2V~6V 74HC Series Serial-to-Serial or Parallel Tri-State SOIC-16 Shift Registers ROHS
R41,R42,R43,R44,R45,R46,R47,R48,R49,R50,R51,R52,R53,R54,R55,R56	16	1K	Resistor_SMD:R_0402_1005Metric	C11702	0402WGF1001TCE	Resistor
C1	1	10uF	Capacitor_SMD:C_1206_3216Metric	C13585	CL31A106KBHNNNE	Unpolarized capacitor, small symbol
C2,C3,C4,C5	4	100nF	Capacitor_SMD:C_0603_1608Metric_Pad1.08x0.95mm_HandSolder	C14663	CC0603KRX7R9BB104	Unpolarized capacitor, small symbol
R89	1	0R	Resistor_SMD:R_1206_3216Metric_Pad1.30x1.75mm_HandSolder	C17888	1206W4F0000T5E	Resistor
R57,R58,R59,R60,R61,R62,R63,R64,R65,R66,R67,R68,R69,R70,R71,R72	16	330R	Resistor_SMD:R_0603_1608Metric_Pad0.98x0.95mm_HandSolder	C23138	0603WAF3300T5E	Resistor
R24	1	470R	Resistor_SMD:R_0402_1005Metric	C25117	0402WGF4700TCE	Resistor
R17,R18,R19,R20,R21,R22,R23,R25,R26,R27,R28,R29,R30,R31,R32,R33,R34,R35,R36,R37,R38,R39,R40	23	10K	Resistor_SMD:R_0402_1005Metric	C25744	0402WGF1002TCE	Resistor
D9	1	LED	LED_SMD:LED_0603_1608Metric_Pad1.05x0.95mm_HandSolder	C510025	SML-D12M8WT86	Light emitting diode
J2,J3	2	Inputs	TerminalBlock_Phoenix:TerminalBlock_Phoenix_MPT-0,5-10-2.54_1x10_P2.54mm_Horizontal	C918128	DB125-2.54-10P-GN-S	Generic screw terminal, single row, 01x10, script generated (kicad-library-utils/schlib/autogen/connector/)
J4,J5	2	Logic Level Outputs	TerminalBlock_Phoenix:TerminalBlock_Phoenix_MPT-0,5-10-2.54_1x10_P2.54mm_Horizontal	C918128	DB125-2.54-10P-GN-S	Generic screw terminal, single row, 01x10, script generated (kicad-library-utils/schlib/autogen/connector/)

Test Verification & Code

This board has been verified using a Raspberry Pi Pico 2 W running MicroPython. The following code reads the 16 inputs and mirrors their states directly onto the 16 outputs (accounting for the physical channel layout).

import machine
import time

# Initialize status LED
led = machine.Pin("LED", machine.Pin.OUT)

# Output Shift Register (74HC595) Pins
outclk = machine.Pin(1, machine.Pin.OUT)
outlatch = machine.Pin(2, machine.Pin.OUT)
_outen = machine.Pin(3, machine.Pin.OUT)
outdata = machine.Pin(4, machine.Pin.OUT)
_outclr = machine.Pin(5, machine.Pin.OUT)

# Input Shift Register (74HC165) Pins
indata = machine.Pin(6, machine.Pin.IN)
_inload = machine.Pin(8, machine.Pin.OUT)
inclk = machine.Pin(9, machine.Pin.OUT)

# Enable outputs and clear any existing register data
_outen.value(0)
_outclr.value(0)
time.sleep(0.1)
_outclr.value(1)

while True:
    led.toggle()

    # Latch the current states of physical inputs into the 74HC165 registers
    _inload.value(0)
    time.sleep(0.0001)
    _inload.value(1)

    bitstream = 0

    # Clock in the 16 bits of input data
    for i in range(16):
        readbit = indata.value()

        # Build bitstream
        if readbit:
            bitstream = bitstream | (1 << i)
        else:
            bitstream = bitstream & ~(1 << i)

        inclk.value(0)
        time.sleep(0.0001)
        inclk.value(1)

    # Latch line low to prepare for serial output transmission
    outlatch.value(0)
    outclk.value(0)
    time.sleep(0.0001)

    # Clock out the 16 bits to the 74HC595 outputs
    for i in range(16):
        outclk.value(0)
        time.sleep(0.0001)

        # Extract bit to send
        bittoset = (bitstream >> (15 - i)) & 1

        outdata.value(bittoset)
        led.value(bittoset)

        outclk.value(1)
        time.sleep(0.0001)

    # Latch the data to the physical outputs
    outlatch.value(1)

Gerber Files

Project Files

Digital I/O Expander Board KiCad Project (ZIP)