Datasheet

clock line is used as a “clear to send” signal. If the host drives the clock line low, the keyboard must not send any data
until the clock is released. 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 20 to 30 kHz) when the data is sent, and data is valid on the falling edge of the clock.
11.3. 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 Figure 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 figure above. 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 the button is
being pressed).
The microcontroller also supports Microsoft Intellimouse-type extensions for reporting back a third axis representing the
mouse wheel, as shown in Table 9.
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.
Table 9. Microsoft Intellimouse type extensions, commands, and actions.