PS2 Keyboard Interface Using Spartan-3 Starter Kit Board Structure of Computer Systems Brateanu Vlad Group 30442

Introduction PS2 port was introduced in IBM’s Personal System 2 personnel computers. It is a widely supported interface for a keyboard and mouse to communicate with the host.

The PS2 Connector The PS2 port contains two wires for communication purposes. One wire is for data, which is transmitted in a serial stream. The other wire is for the clock information, which specifies when the data is valid and can be retrieved.

The PS2 Connector Pins

Timing Diagram Information is sent via 11-bit Words. Each word contains a Start bit, 8 Data bits, a Parity bit and a Stop bit. The keyboard interface allows for bidirectional data transfer.

PS2 Keyboard Scan Codes

Block Diagram

Block Diagram (2)

Design

Design – The 7 Segment DIsplay The Four Digit 7 Segment will be used for displaying the result, which will be the SCAN code coresponding to the pressed key Since the scan code of each button is formed of 8 bits, the value will be displayed in hexadecimal using 2 seven segment displays

Design – The Interpreter It has the job of Interpreting the 8 bit character received by the PS2 Controller. It checks to see if received data is either a character, or a Caps Lock or Num Lock command. In case of the latter, it also sends the required signal back to the PS2 Controller so that it may inform the Keyboard that the coresponding LED should be switched on.

Flow Chart – The Interpreter

Design – The PS2 Controller Is made up of two Finite State Machines. One for the reading operation and the other for the Writing operation. While a writing operation occurs, the read is blocked.

PS2 Controller Read Operation The falling edge of the PS2_CLOCK signal is used as refference for data retrieval. At each falling edge, another bit is loaded into a shift register. After all bits are loaded, the Char_Ready signal is asserted and the Interpreter may read the whole character

Read Operation Flow Chart

PS2 Controller Write Operation The keyboard sends data only when both the data and clock lines are High (the idle state) The host is the buss master, meaning that before sending data, the keyboard must first check if the host isn’t already sending data.

PS2 Controller Write Operation (2) Same as for the Read operation, this time, the keyboard is the receiver. The host drives the clock signal low, then sends an “ED” (in hexadecimal) command to which the keyboard replies with “FA” as acknowledgement. After this, the host sends another byte to set led status.

PS2 Controller Write Operation (3)

Bibliography [1] Spartan 3 StarterKit Board User Guide -for basic information [2] FPGA Prototyping by VHDL Examples – Xilinx Spartan 3 Version by Pong P. Chu Cleaveland State University - Chapters 8 and 9 describe in detail the operations and also gives examples