Atlys Reference Manual
Doc: 502-178
page 15 of 19
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 20 to 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.
Mouse
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.
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
PS/2 Keyboard Scan Codes
