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Digilent 410-258P-KIT User Manual

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Anvyl Reference Manual

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Copyright Digilent, Inc. All rights reserved. Other product and company names mentioned may be trademarks of their respective owners.

Mice and keyboards that use the PS/2 protocol

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use a two-wire serial bus (clock and data) to

communicate with a host device. Both use 11-bit words that include a start, stop, and odd parity bit,
but the data packets are organized differently, and the keyboard interface allows bi-directional data
transfers (so the host device can illuminate state LEDs on the keyboard). Bus timings are shown in
the figure. The clock and data signals are only driven when data transfers occur, and otherwise they
are held in the idle state at logic ‘1’. The timings define signal requirements for mouse-to-host
communications and bi-directional keyboard communications. A PS/2 interface circuit can be
implemented in the FPGA to create a keyboard or mouse interface.

Keyboard

The keyboard uses open-collector drivers so the keyboard, or an attached host device, can drive the
two-wire bus (if the host device will not send data to the keyboard, then the host can use input-only
ports).

PS/2-style keyboards use scan codes to communicate key press data. Each key is assigned a code
that is sent whenever the key is pressed. If the key is held down, the scan code will be sent
repeatedly about once every 100ms. When a key is released, an F0 (binary “11110000”) key-up code
is sent, followed by the scan code of the released key. If a key can be shifted to produce a new
character (like a capital letter), then a shift character is sent in addition to the scan code, and the host
must determine which ASCII character to use. Some keys, called extended keys, send an E0 (binary
“11100000”) ahead of the scan code (and they may send more than one scan code). When an
extended key is released, an E0 F0 key-up code is sent, followed by the scan code. Scan codes for
most keys are shown in the figure. A host device can also send data to the keyboard. Below is a short
list of some common commands a host might send.

ED

Set Num Lock, Caps Lock, and Scroll Lock LEDs. Keyboard returns FA after receiving ED,
then host sends a byte to set LED status: bit 0 sets Scroll Lock, bit 1 sets Num Lock, and bit 2
sets Caps lock. Bits 3 to 7 are ignored.

EE

Echo (test). Keyboard returns EE after receiving EE.

F3

Set scan code repeat rate. Keyboard returns F3 on receiving FA, then host sends second byte
to set the repeat rate.

FE

Resend. FE directs keyboard to re-send most recent scan code.

FF

Reset. Resets the keyboard.


The keyboard can send data to the host only when both the data and clock lines are high (or idle).
Since the host is the bus master, the keyboard must check to see whether the host is sending data
before driving the bus. To facilitate this, the clock line is used as a “clear to send” signal. If the host
pulls the clock line low, the keyboard must not send any data until the clock is released. The keyboard
sends data to the host in 11-bit words that contain a ‘0’ start bit, followed by 8-bits of scan code (LSB
first), followed by an odd parity bit and terminated with a ‘1’ stop bit. The keyboard generates 11 clock
transitions (at 20 to 30KHz) when the data is sent, and data is valid on the falling edge of the clock.


Not all keyboard manufacturers strictly adhere to the PS/2 specifications; some keyboards may not
produce the proper signaling voltages or use the standard communication protocols. Compatibility
with the USB host may vary between different keyboards.

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