File content as of revision 0:feb8c38ed947:

#include "mbed.h"

Serial pc(USBTX, USBRX);

DigitalOut rows[4]={dp2,dp1,dp28,dp6};
DigitalIn columns[4]={dp9,dp10,dp11,dp13};

Ticker ticker;
char giveChar()
{
    char value=0;
    for(int i=0;i<4;i++)
    {
        rows[i]=1;
        for(int j=0;j<4;j++)
        {
            if( i<3 && j<3 && columns[j] )
                value= '1'+(i*3+j);
                
            if(j==3 && columns[j] )
                value= 'a'+i;
            if(i==3 && columns[j] )
            {
                if(j==0)
                    value= '*';
                if(j==1)
                    value= '0';
                if(j==2)
                    value='#';
            }
        }
        rows[i]=0;
        if(value!=0)
            break;
    }            
    return value;   
}

void displayChar()
{
    char c=giveChar();
    if(c==0)
    return;
    if(c!='*')
        pc.putc(c);
    else if(c=='*')
        pc.putc('\n');
}
int main() {
    ticker.attach(displayChar,0.2);
    while(1) {
    
    }
}