Digilent 410-094P User Manual

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Revision: August 3, 2006

Overview

The Digilent PmodPS/2 module board allows a
Digilent system board to send and receive
signals from a PS/2-style keyboard or mouse.

The PS/2 is designed for use with either a
Digilent programmable logic system board or a
Digilent embedded control system board.

Features include:

• a 6-pin header for connection to a
system board

• PS/2 connector for a keyboard or
mouse

• power routing jumpers

Functional Description

The PS/2 has a 6-pin header for easy
connection to a Digilent system board. Most of
Digilent’s programmable logic system boards
(like the Nexys™ or Basys™ boards) or
Digilent’s embedded control boards (like the
Cerebot™) have 6-pin connectors that allow
direct connection of the PS/2. To connect the
PS/2 to some older Digilent system boards, a
Digilent Modular Interface Board (MIB) and a
6-pin cable may be needed. The MIB plugs
into the system board, and the cable connects
the MIB to the PS/2.

Power to the keyboard or mouse can be
provided from the system board or an external
power supply. To power the keyboard or
mouse from the system board, set the shorting
block on jumper JP2 to the VB position. To
power the keyboard or mouse from an external
power supply, set the shorting bock on JP2 to
the VE position and attach the external power
supply to JP1. The external supply voltage is
attached to the VE pin of JP1 and the ground

JP1

for the external power supply is attached to the
GND pin of JP1.

NOTE: Some keyboards and mice can operate
with a 3.3V power supply but some require a 5V
power supply. Be careful to observe the correct
polarity when connecting an external power
supply to JP1 and do not use an external power
supply voltage greater than 5V.

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P2

P4

J1 Connection

GND

VCC

VCC
GND

External Power

PmodPS/2 Circuit Diagram

Data

GND

VCC

CLK

PS/2

Connector

JP2

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PS/2 Reference Manual Digilent, Inc.

Keyboard and Mouse Interface

Pin 1

Pin 2

1

2

6

PS/2

3

5

Pin 5Pin 6

Bottom-up

hole pattern

4

Connector

The keyboard and mouse both use identical
signal timings. 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 below. 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.

T

T

CK

Edge 0

CLK

DATA

T

SU

T

CK

HLD

'1' stop bit'0' start bit

Symbol Parameter Min Max

T

Clock time

CK

T

Data-to-clock setup time

SU

T

Clock-to-data hold time 5us 25us

HLD

Pin Definitions

Pin Function
1 Data
2 Reserved
3 GND
4 Vdd
5 Clock
6 Reserved

Edge 10

30us

5us

50us

25us

Keyboard

The keyboard uses open-collector drivers so that
either 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 simple input-only ports).

PS/2-style keyboards use scan codes to
communicate key-press data (nearly all
keyboards in use today are PS/2 style). Each
key has a single, unique scan code that is sent
whenever the corresponding key is pressed. If
the key is pressed and held, the scan code will
be sent repeatedly once every 100ms or so.
When a key is released, an “F0” 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 original scan
code, and the host device must determine which
character to use. Some keys, called extended
keys, send an “E0” 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 keyboard
diagram below.

A host device can also send data to the
keyboard. Below is a short list of some oft-used
commands.

ED Set Num Lock, Caps Lock, and Scroll Lock

LEDs. After receiving an “ED”, the keyboard
returns an “FA”, then the 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. Upon receiving an echo command, the

keyboard replies with “EE”.

F3 Set scan code repeat rate. The keyboard

acknowledges receipt of an “F3” by returning an
“FA”, after which the host sends a second byte
to set the repeat rate.

FE Resend. Upon receiving FE, the keyboard re-

sends the last scan code sent.

FF Reset. Resets the keyboard.

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