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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PS/2 Reference Manual Digilent, Inc.
ESC
76
` ~ 0E
TAB
0D
Caps Lock
58
Shift
12
Ctrl
14
F105F206F304F4
0C
1 !162 @1E3 #264 $255 %
2E
Q
15W1DE24R2DT2C
A
1CS1BD23F2BG34
Z
1ZX22C21V2AB32
Alt 11
F503F60BF783F8
6 ^367 &3D8 *3E9 (460 )45- _4E= +55BackSpace
Y
35U3CI43O44P4D
Space
The keyboard sends 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 checks to see whether the host is sending data before driving the bus. To facilitate this, the clock line can be used as a “clear to send” signal. If the host pulls the clock line low, the keyboard will 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 8­bits 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 around 20 - 30KHz) when the data is sent, and data is valid on the falling edge of the clock.
Mouse
The mouse outputs a clock and data signal when it is moved, otherwise these signals remain at logic ‘1’. Each time the mouse is moved, three 11-bit words are sent from the mouse to the host device. Each of the 11-bit words contains a ‘0’ start bit, followed by 8 bits of data (LSB first), followed by an odd parity bit, and terminated with a ‘1’ stop bit. Thus,
0A
H
33J3BK42L4B
N
31M3A
29
, <41> .49/ ?
each data transmission contains 33 bits, where bits 0, 11, and 22 are ‘0’ start bits, and bits 11, 21, and 33 are ‘1’ stop bits. The three 8-bit data fields contain movement data as shown below. Data is valid at the falling edge of the clock, and the clock period is 20 to 30KHz.
The mouse assumes a relative coordinate system wherein moving the mouse to the right generates a positive number in the X field, and moving to the left generates a negative number. Likewise, moving the mouse up generates a positive number in the Y field, and moving down represents a negative number (the XS and YS bits in the status byte are the sign bits – a ‘1’ indicates a negative number). The magnitude of the X and Y numbers represent the rate of mouse movement – the larger the number, the faster the mouse is moving (the XV and YV bits in the status byte are movement overflow indicators – a ‘1’ means overflow has occurred). If the mouse moves continuously, the 33-bit transmissions are repeated every 50ms or so. The L and R fields in the status byte indicate Left and Right button presses (a ‘1’ indicates the button is being pressed).
F901F1009F1178F12
07
66
[ {
54
; :
4C
4A
Alt
E0 11
' "
52
] }
5B
\ |
5D
Enter
5A
Shift
59
Ctrl
E0 14
E0 75
E0 74
E0 6B
E0 72
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PS/2 Reference Manual Digilent, Inc.
Mouse status byte X direction byte Y direction byte
L R 0 1 XS YS XY YY P X0 X1 X2 X3 X4 X5 X6 X7 P Y0 Y1 Y2 Y3 Y4 Y5 Y6 Y7 P10 100 11
Idle state
Start bit Stop bit
Start bit
For more information see www.digilentinc.com
Stop bit
Start bit
Stop bit
Idle state
.
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