PS/2 Mouse

PS/2 Protocol

http://www.versalent.biz/ps2facts.htm

Also to avoid your confusion. A PS/2 bus (interface) by standard is limited to only one device. Although, you may hear it is limited to two devices. Do not become confused because your system hosting the interface is considered one of the two, and the keyboard or mouse for instance is actually considered the master (drives the clock line) and the computer (host) the slave.

You may wish to see the [PS2_Keyboard|PS2 Keyboard] page from here. As it stems from the PS/2 protocol.

Implementation Of The PS/2 Protocol

This greatly depends on your board. So for starters your going to need specific code to deal with this hardware that implements the PS/2 protocol or exposes it, then you are going to need to deal with the mouse hardware.

For example taking a look at:

drivers/input/serio/i8042.c

We can see an actual driver for the I8042 PS2 implementation/hardware. Now, on top of this Linux creates an serial abstraction type layer. This allows all sorts of different implementation of the PS/2 interface to exist but become exposes to say the mouse driver as no different than any other device. Otherwise, each mouse driver would have to specifically support each implementation of a PS2 bus interface. Which, is essentially a programming paradigm.

If your writing an operating system you always have time to go back and rewrite or expand a part of it. So at first you will be okay just writing a single driver to handle the hardware exposing the PS/2 and the mouse driver in one package. And, in other cases it may actually make more sense to create a single driver for instance for an embedded system.

Another example is the ARM Integrator/CP board which implements the PS/2 interface encapsulated in the PL050 (where the PL050 is analogous to the I8042) except you communicate with the PL050 differently than the I8042. But, after the PL050 is configured you then proceed with the PS/2 mouse protocol. For example:

	KMI_MMIO	volatile *mmio;
	uint32       tmp;
	mmio = (KMI_MMIO*)KMI_MS_BASE;
	mmio->cr = 0x4;
	/* talk to the PS2 controller and enable it */
	mmio->data = 0xF4;
	/* keyboard sends back ACK */
	while(!KMI_TXFULL(mmio->stat));
	tmp = mmio->data;

The above uses memory mapped input/output (MMIO), but other architectures may use I/O ports instead or a combination of I/O ports and MMIO for example the X84/64. So understanding your platform is very important to understand how to proceed in talking to the devices.

Above, the PL050 is also called the KMI (Keyboard And Mouse Interface). So first we have to configure the PL050 by enabling it. Now, it provides a interface to the PS2 interface which then interfaces with the mouse. So, the next thing I have to do is write the value 0xF4 which you can find in the table above to enable the mouse.

So, overall the hardware exposing the PS/2 interface may be light weight or heavy and it all depends on the system board (not the processor).

Mouse Extension

$linuxsrc/drivers/input/mouse/psmouse-base.c

http://www.computer-engineering.org/ps2mouse/

First, you have to enable the mouse on the PS/2 bus. This requires sending one byte which is clocked over the PS/2 interface. You will then get a response regarding the result.

Standard PS/2 Mouse Commands
Byte	Description
0xFF	Reset
0xFE	Resend
0xF6	Set Defaults
0xF5	Disable Data Reporting
0xF4	Enable Data Reporting
0xF3	Set Sample Rate
0xF2	Get Device ID
0xF0	Set Remote Mode
0xEE	Set Wrap Mode
0xEC	Reset Wrap Mode
0xEB	Read Data
0xEA	Set Stream Mode
0xE9	Status Request

The most command reply is 0xFA from the master (mouse), which means acknowledge. You may then get a variable number of bytes afterwards depending on the command. You may also receive other command replies which may state that the master (mouse) has encountered an error decoding your command. For a more detailed list check out some of the links above or look through the Linux source tree.

This is just to help get you started until someone comes and expands this page more.

Generic PS/2 Mouse Packet Bits
BYTE	7	6	5	4	3	2	1	0
0	yo	xo	ys	xs	ao	bm	br	bl
1	xm
2	ym

Code	Description
yo	Y-Axis Overflow
xo	X-Axis Overflow
ys	Y-Axis Sign Bit (9-Bit Y-Axis Relative Offset)
xs	X-Axis Sign Bit (9-Bit X-Axis Relative Offset)
ao	Always One
bm	Button Middle (Normally Off = 0)
br	Button Right (Normally Off = 0)
bl	Button Left (Normally Off = 0)
xm	X-Axis Movement Value
ym	Y-Axis Movement Value

Here, an example of mouse that supports extensions. To maintain backwards compatibility you should have to activate these features through the PS/2 bus. Various mouse devices use different ways. Linux mouse drivers for example sometimes handle multiple different devices which all share the same standard packet format above, or at least support the compatibility mode described above.

IntelliMouse Explorer
BYTE	7	6	5	4	3	2	1	0
3	vs	hs