Digilent 410-183P-KIT - Manual (Page 10)

Basys3™ FPGA Board Reference Manual
6.3 Mouse
Once entered in stream mode and data reporting has been 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. 10. Data is valid at the falling edge of
the clock, and the clock period is 20 to 30 KHz.
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 that 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
Figure 10. Mouse data format.
The microcontroller also supports Microsoft® IntelliMouse®-type extensions for reporting back a third axis
representing the mouse wheel, as shown in Table 4.
Table 4. Microsoft Intellimouse-type extensions, commands, and actions.
Command Action
EA
Set stream mode. The mouse responds with "acknowledge" (0xFA) then resets its movement
counters and enters stream mode.
F4
Enable data reporting. The mouse responds with "acknowledge" (0xFA) then enables data
reporting and resets its movement counters. This command only affects behavior in stream
mode. Once issued, mouse movement will automatically generate a data packet.
F5
Disable data reporting. The mouse responds with "acknowledge" (0xFA) then disables data
reporting and resets its movement counters.
F3
Set mouse sample rate. The mouse responds with "acknowledge" (0xFA) then reads one more
byte from the host. This byte is then saved as the new sample rate, and a new “acknowledge”
packet is issued.
FE Resend. FE directs mouse to re-send last packet.
FF Reset. The mouse responds with "acknowledge" (0xFA) then enters reset mode.
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