Digilent Genesys 2 Reference Manual page 20

capital letter), then a shift character is sent in addition to the scan code, and the host must determine which ASCII () character
to use. Some keys, called extended keys, send an E0 ahead of the scan code (and they may send more than one scan code).
When an extended key is released, an E0 F0 key-up code is sent, followed by the scan code. Scan codes for most keys are
shown in Figure 12.
A host device can also send data to the keyboard. Table 8 shows a list of some common commands a host might send.
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 8. Keyboard commands.
The keyboard can send data to the host only when both the data and clock lines are high (or idle). Because the host is the bus
master, the keyboard must check to see whether the host is sending data before driving the bus. To facilitate this, the 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
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