Digilent 410-274P-KIT - Manual (Page 12)

Nexys4™ FPGA Board Reference Manual
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Page 12 of 29
7.3 Mouse
Once entered in stream mode and data reporting, enabled, 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, as shown in Fig 10. 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 in the fig 9. 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).
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
Mouse status byte X direction byte Y direction byte
Stop bit Start bit Stop bit
Idle state
Stop bit Start bit
The microcontroller also supports Microsoft Intellimouse-type extensions for reporting back a third axis
representing the mouse wheel, as shown in Table 5.
Command Action
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.
Table 4. Keyboard commands
Figure 10. Mouse Data Format