You need to send to cols or rows, an 'F' because when you press a key, the '0' will be reflected on cols or rows...

I mean, if you are controlling cols, the first thing you send is a E (for example) that is '1110' the '0' will be reflected on rows if a key is pressed.

Then you read rows and knows which key was pressed because it has a 0. If you don't send an F to the rows at the end, How will you know which key was pressed? You can have two 0's if there are two keys pressed consecutivly.

Hope that helped.