COMP541 Keyboard Interface

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COMP541Keyboard Interface

• Montek Singh
• April 9, 2007

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PS/2 Keyboard

• Uses a synchronous serial protocol
• What does that mean?

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Physical Interface

• Two lines
• Clock (10-20KHz)
• Data
• Pulled high by resistors
• Asserted low
• Takes away your pushbutton unless you do clever
  stuff

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Bidirectional

• Kybd-to-host and host-to-kybd on same wires
• CAPS LOCK light for example
• To send, host takes data line low
• Sometimes take clk low first
• Then kybd starts clocking
• Host sends data synced to kybd clock
• You shouldnt need to send to kybd

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Protocol

• 11 bits
• Start always 0
• 8 bits of data
• Odd parity bit
• Stop bit always 1
• Clocked by keyboard
• Value should be latched on negedge of keyboard
  clock

Illustration from http//www.beyondlogic.org/keybo
ard/keybrd.htm
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What is Sent

• ASCII is not sent
• Scan codes for keys
• Least significant bit first

Illustration from http//www.beyondlogic.org/keybo
ard/keybrd.htm
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Scan Codes

• Normally translated by software
• You remap your keys, for example
• Software takes care of
• Shift, caps lock, control

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Some Scan Codes Long

• Two code sequence common
• Have a look at Break key!

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Even More Complicated

• Scan code generated when you press
• And when you release
• Two bytes F0 followed by key scan code
• Example
• Space pressed, 29 sent
• Space released, F0 29 sent
• If you hold key, scan code repeated

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Resources

• Information
• http//www.beyondlogic.org/keyboard/keybrd.htm
• Scan codes
• http//www.barcodeman.com/altek/mule/scandoc.php
• There is also Verilog for keyboard interfaces
  available on web.

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My Verilog

• I will post my Verilog code on the web site
• You should memory-map the character code register
• Maybe add a single bit register to indicate that
  new character has arrived