Terminal Emulator

In order to combine the new video module with the keyboard module, a top level structural VHDL file was created.  This file is responsible for sending the signals back and forth between the two modules as well as performing some functions for both the keyboard and video module to make the project work. (Download Terminal Emulator Files).

Structural Top DLX
The main function of the top level file is to perform the communications between the keyboard and video modules, as well as the Altera UP1 Board.  This is done by routing the signals from keyboard to video and vise versa (See Figure 1).  It maintains the system clock and receives the video and keyboard inputs from the board.

Figure 1.  Schematic of keyboard emulator  

The other function of the Top DLX module is to perform some functions for the video and keyboard that are not implemented in the respective modules.  Some of these functions include:

• Providing the functionality for the backspace and enter keys
• Incrementing the video row and column after the video writes a character to the screen

The reason these functions are done in the top module is to provide more flexibility and reuse of the video and keyboard modules.

The following events happen to print a character from the keyboard to the screen:
 

• The keyboard module is constantly watching the keyboard clock and data lines coming in from the keyboard.  When a key is pressed, the keyboard module reads in the scan code (see keyboard documentation for more details), and sends the ascii code and key-stroke signal to Top DLX.
  
  
•  After receiving the key-stoke signal, the top module checks to see if the ascii code is either a return character or a backspace character and adjusts the video accordingly.  
  
  
• If not, it sends the ascii code to the video module and sets the video_write signal high.
  
  
• The video module, when it receives a video_write signal, takes the inputs from the Top module (video_row, video_col, and key_data) and writes the value to the screen (see video documentation for more details)

The End Result

After putting all three modules together, compiling, and programming the Flex 10K20 University board, you can type away as shown here.