PS/2 Keyboard Controller

Nabil ChoubaNabil.chouba@gmail.com

http://nabil.chouba.googlepages.com

Keyboard PS/2

• IBM introduced a new keyboard with desktop computer.– IBM PC/XT Keyboard (1981)– IBM AT Keyboard (1984) – IBM PS/2 Keyboard (1987)

• Modern PS/2 (AT) compatible keyboards: – Any number of keys (usually 101 or 104) – 5-pin or 6-pin connector; adaptor usually included – Bi-directional serial protocol – Only scan code set 2 guaranteed. – Acknowledges all commands; may not act on all of them.

Pinout 6-pin Mini-DIN (PS/2)

Male

Female

6-pin Mini-DIN (PS/2): 1 - Data 2 - Not Implemented 3 - Ground 4 - Vcc (5V DC at 275 mA) 5 - Clock 6 - Not Implemented

Open-collector interface

• Vcc/Ground provide power to the keyboard/mouse

• Vcc = +4.5V to +5.5V • Max Current = 275 mA. • The Data and Clock lines are

both open-collector with pullup resistors to Vcc

Matrix of keys

• keyboard's processor scan the matrix of keys• Send a packet of information the host when :

• Key pressed• released• held down

• information known as a "scan code“• key is associated to code :

• "scan code set" table • Default scan code set for all modern keyboards

Make & Break Codes

Key Make Code Break Code

"A" 1C F0,1C

"5" 2E F0,2E

"F10" 09 F0,09

Right Arrow E0,74 E0,F0,74

Right Ctrl E0,14 E0,F0,14

When a key is pressed, make code is sent to the computer. When a key is released a break code is sent When you press and hold down a key, that key becomes typematic, keep sending that key's make code (0.25sec to 1sec ) until the key is released or another key is pressed. Exp : hold the key “A" down : 1C 1C 1 C . . . 1C FO 1C

Keyboard make code

* no defined relationship between a make code and an ASCII code

Device-to-host communication protocol

• The clock frequency must be in the range 10 - 16.7 kHz. • 1 start bit.  This is always 0.

• 8 data bits, least significant bit first.

• 1 parity bit (odd parity).

• 1 stop bit.  This is always 1.

• 1 acknowledge bit (host-to-device communication only)

Waveform "Q" key

Scan code for the "Q" key (15h) being sent from a keyboard to the host.  Channel A is the Clock signal; channel B is the Data signal.