Digilent 410-138P-KIT - Manual (Page 23)

Genesys Reference Manual
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Both the mouse and keyboard use a two-wire serial bus (clock and data) to communicate with a host
device. Both use 11-bit words that include a start, stop, and odd parity bit, but the data packets are
organized differently, and the keyboard interface
allows bi-directional data transfers (so the host
device can illuminate state LEDs on the keyboard).
Bus timings are shown in the figure. The clock and
data signals are only driven when data transfers
occur, and otherwise they are held in the “idle”
state at logic ‘1’. The timings define signal
requirements for mouse-to-host communications
and bi-directional keyboard communications. A
PS/2 interface circuit can be implemented in the
FPGA to create a keyboard or mouse interface.
Keyboard
The keyboard uses open-collector drivers so the keyboard or an attached host device can drive the
two-wire bus (if the host device will not send data to the keyboard, then the host can use input-only
ports).
PS/2-style keyboards use scan codes to communicate key press data. Each key is assigned a code
that is sent whenever the key is pressed; if the key is held down, the scan code will be sent repeatedly
about once every 100ms. When a key is released, an “F0” key-up code is sent, followed by the scan
code of the released key. If a key can be “shifted” to produce a new character (like a 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 the figure. A host device can
also send data to the keyboard. Below is a short list of some common commands a host might send.
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.