Keyboard And Mouse Interface - Digilent PmodPS/2 Reference Manual

PmodPS/2™ Reference Manual
3

Keyboard and Mouse Interface

The keyboard and mouse both use identical signal timings. 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 below. 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.
Pin 2
2 1
4 3
6
5
Pin 6
Bottom-up
PS/2
hole pattern
Connector
Symbol
Tck
Tsu
Thld
3.1 Keyboard
The keyboard uses open-collector drivers so that either 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 simple input-only ports).
PS/2-style keyboards use scan codes to communicate key-press data (nearly all keyboards in use today are PS/2
style). Each key has a single, unique scan code that is sent whenever the corresponding key is pressed. If the key is
pressed and held, the scan code will be sent repeatedly once every 100ms or so. 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 original scan code, and the host
device must determine which character to use. Some keys, called extended keys, send an "E0" ahead of the scan
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Pin Number
1
2
3
4
5
6
Table 1. Pin description table.
Pin Definitions
Pin 1
Pin Function
1 Data
2 Reserved
3 GND
Pin 5
4 Vdd
5 Clock
6 Reserved
Parameter
Clock time
Data-to-clock setup time
Clock-to-data hold time
Table 2. Bus timings.
Description
Data
Not Used
Clock
Not Used
Ground
VCC
T T
CK
Edge 0
CLK
T
HLD
T
SU
DATA
'0' start bit
Min Max
30us 50us
5us 25us
5us 25us
CK
Edge 10
'1' stop bit
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