Figure 1. PS/2 Controller

The PS/2 controller interface is illustrated in Figure 1, “PS/2 Controller”, with inputs shown on the left, outputs on the right, and PS2_CLK and PS2_DAT lines being bidirectional. The blue lines are to be connected to the corresponding pins on the DE2 board, the green lines are used for receiving commands, and the red lines are used for sending commands. The red lines might be left unconnected if the user does not need to send the data to the PS/2 device. Ports are used as follows:

The controller interface consists of two parts: command-in and command-out. If using the optional command lines to send commands to the PS2/device, the command output will always take a precedence over the command input.

The commands are received automatically, and the most recent command is stored in the register connected to the received_data port. The signal received_data_en is driven high for one clock cycle whenever a new command is received. The protocol timing is illustrated in Figure 2, “Timing diagram for receiving commands”.

Figure 2. Timing diagram for receiving commands

PS/2 commands may be sent using the four command-out ports: the_command, send_command, command_was_sent, and error_communication_timed_out. Once the_command lines have been set to the desired value, the send_command line is driven high, and the command transmission commences. The send_command line has to be kept high for at least one clock cycle. After the command transmission is complete. either the command_was_sent or the error_communication_timed_out signal is activated, which indicate a transmission success or failure, respectively. These status signals are held until either the send_command line is de-asserted, or for one clock cycle, whichever is less. The protocol timing is illustrated in Figure 3, “Timing diagram for sending commands”.

Figure 3. Timing diagram for sending commands

The keyboard will send a code consisting of one or more 8 bit words whenever the key is pressed, held down, or released. The codes for the conventional keyboard are listed in Appendix A, List of key codes. "Make" code is sent when the key is pressed down, and repeated periodically if the key is held down. The "break" code is sent when the key is released. The codes will appear on the received_data port of the PS/2 controller as they are received. It is the user's responsibility to check the code before it is overwritten.

In addition to the key codes, the keyboard might also send commands to the host. The most common commands are listed in Table 1, “Keyboard-to-host commands”.

The host may send the commands to the keyboard to control its behavior. The most common commands are described in Table 2, “Host-to-keyboard commands”, and you may refer to [ce] for an exhaustive list.

Command transmission takes precedence over the data transmission from the keyboard, and will inhibit the transmission of any key codes. Instead, the keyboard will transmit an acknowledge code upon receiving the command. The PS/2 controller provides a simple interface to send the commands, as described in the section called “Interface description”.

When in the streaming mode of operation, mouse will periodically send the data packets containing the following information:

Data packet format is illustrated in Table 4, “Mouse data packet”.

The mouse has two internal counters to keep track of the relative position. The counters are incremented/decremented every time the mouse is moved. The counter step with respect to the physical distance depends on the resolution (which can be changed as described in [ce]), with the default being 4 counts/mm. The counters are reset every time the position data is transmitted to the host.

In addition to the data packets, the mouse may also send the commands described in Table 5, “Mouse-to-host commands”.

The host may send the commands to the mouse to control its operating modes and other parameters. The most useful commands are described in Table 6, “Host-to-mouse commands”. If the mouse is in streaming mode, the host should disable the data reporting prior to sending any other commands. The mouse will acknowledge every command.