Digilent DIO1 - Manual (Page 6)

Digilent DIO1 Reference Manual Digilent, Inc.
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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 10,
21, and 32 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’ 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 Stop bit
Start bit
Stop bit
Start bit
Stop bit
Idle state
Mouse status byte X direction byte Y direction byte