Nexys2 Reference Manual
Digilent
www.digilentinc.com
Copyright Digilent, Inc.
Page 8/17
Doc: 502-134
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).
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
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
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 around 20 - 30KHz) when the data is sent, and data is valid on the falling edge of the
clock.
Scan codes for most PS/2 keys are shown in the figure below.
Figure 13: PS/2 signal timings
