Datasheet

Basys 3™ FPGA Board Reference Manual
Copyright Digilent, Inc. All rights reserved.
Other product and company names mentioned may be trademarks of their respective owners.
Page 9 of 19
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Ctrl
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Alt
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Alt
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F12
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Figure 9. Keyboard scan codes.
A host device can also send data to the keyboard. Table 3 shows a list of some common commands a host might
send.
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.
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.
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 3. Keyboard commands.