HC-548 matrix keypad test code. Read the full blog post on www.teomaragakis.com/hardware/electronics/testing-the-hc-548-matrix-keypad/

hc-548.ino

/*
                                      +-----+
         +----[PWR]-------------------| USB |--+
         |                            +-----+  |
         |         GND/RST2  [ ][ ]            |
         |       MOSI2/SCK2  [ ][ ]  A5/SCL[ ] |   C5 
         |          5V/MISO2 [ ][ ]  A4/SDA[ ] |   C4 
         |                             AREF[ ] |
         |                              GND[ ] |
         | [ ]N/C                    SCK/13[ ] |   B5
         | [ ]v.ref                 MISO/12[ ] |   .
         | [ ]RST                   MOSI/11[ ]~|   .
         | [ ]3V3    +---+               10[X]~|   .
         | [X]5v     | A |                9[X]~|   .
         | [X]GND   -| R |-               8[X] |   B0
         | [ ]GND   -| D |-                    |
         | [ ]Vin   -| U |-               7[X] |   D7
         |          -| I |-               6[X]~|   .
         | [ ]A0    -| N |-               5[X]~|   .
         | [ ]A1    -| O |-               4[X] |   .
         | [ ]A2     +---+           INT1/3[X]~|   .
         | [ ]A3                     INT0/2[ ] |   .
         | [ ]A4/SDA  RST SCK MISO     TX>1[ ] |   .
         | [ ]A5/SCL  [ ] [ ] [ ]      RX<0[ ] |   D0
         |            [ ] [ ] [ ]              |
         |  UNO_R3    GND MOSI 5V  ____________/
          \_______________________/
      
      http://busyducks.com/ascii-art-arduinos
*/

#include <Keypad.h>
const byte ROWS = 4; //four rows
const byte COLS = 4; //four columns

//define the symbols on the buttons of the keypads
char hexaKeys[ROWS][COLS] = {
  {'1','2','3','A'},
  {'4','5','6','B'},
  {'7','8','9','C'},
  {'*','0','#','D'}
};

byte rowPins[ROWS] = {3, 4, 5, 6}; //connect to the row pinouts of the keypad
byte colPins[COLS] = {7, 8, 9, 10};  //connect to the column pinouts of the keypad

//initialize an instance of class NewKeypad
Keypad customKeypad = Keypad( makeKeymap(hexaKeys), rowPins, colPins, ROWS, COLS);

void setup(){
  Serial.begin(9600);
  Serial.println("Starting keypad...");
}
  
void loop(){
  char customKey = customKeypad.getKey();
  
  if (customKey){
    Serial.println(customKey);
  }
}