4x4 keypad with Raspberry Pi Pico 2 W — MicroPython PIN lock that avoids erratum E9

A 4×4 membrane keypad is eight wires — four rows, four columns — and a scan loop. On the Pico 2 W the direction of that scan matters: drive the rows LOW and read the columns with pull-ups, so the RP2350’s pull-down erratum never comes into play.

Rows go to GP6–GP9, columns to GP10–GP13. The script prints each key and checks a four-digit PIN (# to enter, * to clear), lighting the on-board LED when it is right.

Wiring

Every connection
Raspberry Pi Pico 2 W	4×4 matrix membrane keypad	Wire	Notes
GP6	R1	red	Physical pin 9.
GP7	R2	orange	Physical pin 10.
GP8	R3	yellow	Physical pin 11.
GP9	R4	green	Physical pin 12.
GP10	C1	blue	Physical pin 14.
GP11	C2	purple	Physical pin 15.
GP12	C3	white	Physical pin 16.
GP13	C4	brown	Physical pin 17.

No supply — passive switch matrix part, 3.3 V board. The keypad has no electronics, so it works at 3.3 V or 5 V. It does need eight GPIOs; on ESP32 avoid the strapping pins (0, 2, 5, 12, 15) and the input-only pins (34–39) for the row outputs.

Board quirks

The scan drives one row LOW at a time and reads the columns with Pin.PULL_UP. Many online examples do the opposite (rows HIGH, columns PULL_DOWN) — on RP2350 A2 chips that hits erratum E9 and keys appear stuck.
The keypad is just switches, so it needs no power wire. Eight GPIOs in a row (GP6–GP13) keep the ribbon cable straight; GND pin 13 sits between them and is not used.
Pressing two keys in the same column at once can ghost a third. For PIN entry that never matters; for a game controller, add a diode per key.
This is a demo lock. A real door lock also needs a delay after wrong PINs and a relay or solenoid driver (with its own supply).

Code

main.pyMicroPython

# 4x4 membrane keypad on a Raspberry Pi Pico 2 W (MicroPython): scan keys and check a PIN code.
# Wiring: R1-R4 -> GP6-GP9 (pins 9, 10, 11, 12), C1-C4 -> GP10-GP13 (pins 14, 15, 16, 17).
# Rows are driven LOW one at a time; columns use the internal PULL-UPs and read LOW when pressed.
# (Many tutorials drive rows HIGH and use PULL_DOWN on columns - that fails on RP2350 A2 chips,
# erratum E9, where an internal pull-down can hold the pin at about 2 V.)
from machine import Pin
import time

SECRET = "1234"
KEYS = ("123A", "456B", "789C", "*0#D")

rows = [Pin(n, Pin.OUT, value=1) for n in (6, 7, 8, 9)]
cols = [Pin(n, Pin.IN, Pin.PULL_UP) for n in (10, 11, 12, 13)]
led = Pin("LED", Pin.OUT)


def scan():
    """Return the key held down, or None."""
    for r, row in enumerate(rows):
        row.value(0)
        time.sleep_us(10)  # let the line settle
        for c, col in enumerate(cols):
            if col.value() == 0:
                row.value(1)
                return KEYS[r][c]
        row.value(1)
    return None


print("Keypad ready. Type the PIN then #, * to clear")
entered = ""
last = None
while True:
    key = scan()
    if key and key != last:  # act once per press
        if key == "*":
            entered = ""
            print("Cleared")
        elif key == "#":
            if entered == SECRET:
                print("PIN OK - unlocked")
                led.on()
                time.sleep(3)
                led.off()
            else:
                print("Wrong PIN")
            entered = ""
        else:
            entered += key
            print("Key {}   entered: {}".format(key, "*" * len(entered)))
    last = key
    time.sleep_ms(30)  # also debounces

# 4x4 membrane keypad on a Raspberry Pi Pico 2 W (MicroPython): scan keys and check a PIN code.
# Wiring: R1-R4 -> GP6-GP9 (pins 9, 10, 11, 12), C1-C4 -> GP10-GP13 (pins 14, 15, 16, 17).
# Rows are driven LOW one at a time; columns use the internal PULL-UPs and read LOW when pressed.
# (Many tutorials drive rows HIGH and use PULL_DOWN on columns - that fails on RP2350 A2 chips,
# erratum E9, where an internal pull-down can hold the pin at about 2 V.)
from machine import Pin
import time

SECRET = "1234"
KEYS = ("123A", "456B", "789C", "*0#D")

rows = [Pin(n, Pin.OUT, value=1) for n in (6, 7, 8, 9)]
cols = [Pin(n, Pin.IN, Pin.PULL_UP) for n in (10, 11, 12, 13)]
led = Pin("LED", Pin.OUT)


def scan():
    """Return the key held down, or None."""
    for r, row in enumerate(rows):
        row.value(0)
        time.sleep_us(10)  # let the line settle
        for c, col in enumerate(cols):
            if col.value() == 0:
                row.value(1)
                return KEYS[r][c]
        row.value(1)
    return None


print("Keypad ready. Type the PIN then #, * to clear")
entered = ""
last = None
while True:
    key = scan()
    if key and key != last:  # act once per press
        if key == "*":
            entered = ""
            print("Cleared")
        elif key == "#":
            if entered == SECRET:
                print("PIN OK - unlocked")
                led.on()
                time.sleep(3)
                led.off()
            else:
                print("Wrong PIN")
            entered = ""
        else:
            entered += key
            print("Key {}   entered: {}".format(key, "*" * len(entered)))
    last = key
    time.sleep_ms(30)  # also debounces

Expected serial output

REPL / Thonny shellOutput

Keypad ready. Type the PIN then #, * to clear
Key 1   entered: *
Key 2   entered: **
Key 3   entered: ***
Key 4   entered: ****
PIN OK - unlocked

Keypad ready. Type the PIN then #, * to clear
Key 1   entered: *
Key 2   entered: **
Key 3   entered: ***
Key 4   entered: ****
PIN OK - unlocked

Common errors

Keys come out as the wrong characters

Your keypad’s connector order differs. Check which pin is R1 with a multimeter (continuity while pressing “1”) and reorder the GP lists in code.

Whole row or column never responds

A loose jumper in the 8-pin connector. Press it home, or use a male header strip.