Digilent 410-178P-KIT - Manual (Page 17)

Atlys Reference Manual
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To access the USB host controller, EDK designs
can use the standard PS/2 core. Reference
designs posted on the Digilent website show an
example for reading characters from a USB
keyboard connected to the USB host interface.
Mice and keyboards that use the PS/2 protocol
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.
The keyboard can send data to the host only when both the data and clock lines are high (or idle).
Since 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
pulls the clock line low, the keyboard must not send any data until the clock is released. The keyboard
T
CK
T
SU
Clock time
Data-to-clock setup time 30us
5us
50us
25us
Symbol Parameter Min Max
T
HLD
Clock-to-data hold time 5us 25us
Edge 0
‘0’ start bit ‘1’ stop bit
Edge 10
Tsu
T hld
Tck Tck