Digilent Basys Board Rev.E - Manual (Page 6)

Digilent
Basys Reference Manual
www.digilentinc.com
Copyright Digilent, Inc. Page 6/12 Doc: 502-107
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).
PS2-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, a “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
Figure 10. PS/2 signal timing
Figure 9. PS/2 connector and Basys PS/2 circuit