CPE/EE 422/522, Laboratory Assignment 1
PS 2, PC Keyboard Interface
(CPE/EE 422 Students 5% of Final Grade -- CPE/EE 522 Students 2.5% of Final Grade)
The purpose of this laboratory project is to give each student the opportunity to develop a practi-
cal logic design that interfaces a 104 key PS-2 style keyboard to the Altera UP 2 Educational
Trainer. The design should be created in a manner that will allow it to display in hexadecimal for-
mat on two seven-segment LEDs, the ANSII encoding that correspond to each key that is pressed.
Background
PS-2 type keyboards contain the embedded logic necessary to scan all 104 keys, produce a unique
keyboard scan code for each key that is pressed, and to send this scan code value to another device
in a serial manner. (A hardware realization of this embedded logic will be the focus of the scan-
ning keypad design that is to be developed in Laboratory Assignment 3. The focus of this labora-
tory, though, is not to implement this low-level logic but rather be able to decode the scan code
after it is sent out in serial form and then convert this code to an ANSII representation and display
it on the seven segment LEDs.) The PS-2 type interface includes four main signals that include
VCC, Ground, a keyboard clock, and a keyboard data line. The VCC and ground line are used to
power the keyboard and data is transferred in and out of the keyboard using the bidirectional key-
board clock (KEY_CLK) and data (DATA) lines. In this lab, we will use the keyboard as an out-
put only device meaning the from our digital design’s point of view the keyboard clock and
keyboard data lines are always inputs to the design. (It is possible, and common, for an external
design to drive these lines in the other direction to perform such functions as resetting the key-
board, switching keyboards scan code character sets, etc. The keyboard is very flexible. We will
only be using its basic default features in this laboratory assignment.)
In its default mode, every time a key is pressed a sequence of bytes is sent serially across the data
line from the keyboard to the external device. There is a separate sequence of bytes that is sent
when each key is pressed and another sequence that is sent when each key is released. The
sequence that is sent when the key is pressed is often called the Make Code and the sequence that
is sent when the key is released is called the Break Code. Except for a few special keys, the last
byte that is sent in the make code and break code sequence for a given key are identical to one
another with its value uniquely represents the key that was pressed. Table 1 lists the final byte of
the make/break sequences for each of the keys on the PS/2 keyboards that will be used in the lab-
oratory assignment. This table assumes the key labeling that is shown in Figure 1.
