The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Keyboard Interface Anselmo Lastra.

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

Keyboard Interface

Anselmo Lastra

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

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PS/2 Keyboard

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

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

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Physical Interface

• Two lines♦ Clock (10-20KHz)♦ Data

• Asserted low• There’s documentation in

NEXYS board manual, starting on P. 7

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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 shouldn’t 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♦ Slow enough that you could easily build

simple FSM and clock on your 50 MHz clock

Illustration from http://www.beyondlogic.org/keyboard/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/keyboard/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

• Informationhttp://www.beyondlogic.org/keyboard/keybrd.htm

• Scan codeshttp://www.barcodeman.com/altek/mule/scandoc.php

• There is also Verilog for keyboard interfaces available on web.

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My Verilog

• Have Verilog for keyboard• Have not ported to new FPGA• To use♦ You should memory-map the character

code register♦ Maybe add a single bit register to

indicate that new character has arrived

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Mouse

• Very similar interface (clk & data)

• 3 words sent w/ mouse movement or button press

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Movement

• Movement is relative• XS, YS are sign (+ is up/right)• XV, YV are overflow (too fast)• L, R are buttons (the 0 is

middle?)

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Scroll Wheel, etc.

• Extensions to original 2 button PS/2 mouse

• Seehttp://www.computer-engineering.org/ps2mouse/

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