Owner's manual

Digilent

Basys Reference Manual

www.digilentinc.com

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

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

Mouse status byte X direction byte Y direction byte

Stop bit

Start bit

Stop bit

Idle state

The five standard VGA signals Red, Green, Blue,

Horizontal Sync (HS), and Vertical Sync (VS) are

routed directly from the FPGA to the VGA connector,

resulting in a simple video system that can produce

eight colors. The color signals from the FPGA include

270-ohm series resistors that form a divider with the

75-ohm termination resistance of the VGA display.

This simple circuit ensures that the video signals

cannot exceed the VGA-specified maximum voltage,

and results in color signals that are either fully on

(.7V) or fully off (0V).

Spartan 3E

FPGA

RED

GRN

BLU

270

200

Basys VGA Circuit Diagram

VGA signal timings are specified, published, copyrighted and sold by the VESA organization

(www.vesa.org). The following VGA system timing information is provided as an example of how a