Digilent DIO5 - Manual (Page 10)

DIO5 Reference Manual Digilent, Inc.
www.digilentinc.com Page 10
Copyright Digilent, Inc. All rights reserved. Other product and company names mentioned may be trademarks of their respective owners.
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GND
VGA "DB15" Connector
R
270
G
B
270
270
HS
VS
The keyboard should 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 should check 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 must not
send any data until the clock is released (host-
to-keyboard data transmission will not be dealt
with further here).
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 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, 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).
VGA Port
The five standard VGA signals Red (R), Green
(G), Blue (B), Horizontal Sync (HS), and
Vertical Sync (VS) are routed directly to the
VGA connector, bypassing the CPLD. A 270-
ohm series resistor is used on each color
signal. This resistor forms a divider with the 75-
ohm VGA cable termination, resulting in a
signal that conforms to the VGA specification
(i.e., 0V for fully off and .7V for fully on).
VGA signal timings are specified, published,
copyrighted and sold by the VESA organization
(www.vesa.org). The following VGA system
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 P 1 0 1 0 0 1 1
Idle state
Start bit Stop bit
Start bit
Mouse status byte X direction byte Y direction byte
Stop bit
Start bit
Stop bit
Idle state