Designing a USB Keyboard and PS/2Â® Mouse Combination Device ...

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Designing a USB Keyboard and PS/2 Mouse Combination Devicethe clock line LOW) to allow the high-level state machine,mouse_machine, to act.The high-level state machine (mouse_machine) is, in turn,driven by periodic calls from the application (the keyboard andPS/2 mouse firmware in this case). Whenever we initiate amouse command, mouse_machine must be called regularlyfrom the application in order to process its state sequences.Making these regular calls to mouse_machine is referred toas “operating” the mouse_machine. Every call tomouse_machine returns an idle/busy indication in the microcontroller’sC-bit (carry). It returns C-bit = 0 if it is idle (i.e., nomouse commands are currently in process), and C-bit = 1 ifa mouse command is currently being processed. No newmouse command should be initiated by the application unlessmouse_machine is idle. Further, mouse_machine can only bemade busy by a command initiated by the application or apower-up reset.A sample code in Figure 8 shows how to initiate a mousecommand and “operate” mouse_machine. Call to mouse_pollinitiates the Read Data command to the mouse, and sets themouse_machine to the corresponding state. The subsequentloop calls mouse_machine to sequence through the statesuntil the C-bit = 0, indicating the state machine is done andidle.MouseTask:call mouse_poll ;send poll cmdtw3:call mouse_machine ;wait until donejc tw3 ;jump back if C= 1Figure 8. Operating mouse_machinePS/2 Support SubroutinesA number of support subroutines are provided to facilitate theoperation of the PS/2 mouse. The main firmware code callsthese subroutines to initialize, set operation mode and readdata from the PS/2 mouse. Their descriptions and usages areas follows:• ps2_init: this subroutine initializes the PS/2 mouse port. Itmust be called from the main code within 500 ms of power-up,before the PS/2 mouse starts transmitting. Upon applicationof power and completion of its self-diagnostics,the mouse will automatically transmit a two-byte initializationpattern (0xAA and 0x00). Regular calls tomouse_machine must be made after the call to ps2_inituntil it returns idle, indicating the transmission of the initializationpattern is complete. The receipt of the pattern recognizesthat the mouse is attached to the device.• mouse_reset: this subroutine initiates a Reset commandto the mouse, causing it to execute its self-diagnostics andreturn the initialization pattern. After a call to mouse_reset,mouse_machine must be operated until it returns idle. Inthis instance, the returned data from a properly functioningmouse will be a three-byte string (0xFA, 0xAA, and 0x00).The first byte is the ACK byte to the Reset command, andthe next two bytes are the initialization pattern. Note thatthe initialization pattern will only be transmitted automaticallyby the mouse at power-up. Therefore, if the USB microcontrollergets reset by some reason, it is recommendedthat mouse_reset be called to duplicate the reset/initializationsequence, in order to make sure that the firmwarerecognizes the mouse is attached.• ps2_status(): this subroutine can be called oncemouse_machine goes idle after the reset/initialization sequenceto determine if a mouse is attached. It returns “0’in the accumulator (register A) if no mouse is attached;otherwise, “1” is returned.• mouse_init: Once the mouse has gone through the reset/initializationphase and is detected as a result,mouse_init must be invoked by the main code to preparethe mouse for data polling. This is done by sending the SetRemote Mode and Enable commands to the mouse.Mouse_init operates the mouse_machine through thecommand sending transactions, and only returns when themouse is properly configured. Mouse_init is also used toreinitialize the mouse upon leaving the suspend mode.Suspend/resume and remote wakeup issues will be discussedin the next section.• mouse_poll: Upon successful completion of mouse_init,data may be polled from the mouse by calling themouse_poll subroutine. Note that mouse_poll merely initiatesthe transaction, which is then driven to completionby operating mouse_machine until it returns idle. When thetransaction is completed, the 3-byte PS/2 mouse data reportwill be available in RAM area at addressmouse_packetV.• mouse_suspend: this subroutine is called before the microcontrollerenters suspend state. It initiates a Set StreamMode command and operates mouse_machine until thetransaction is complete.Keyboard and Mouse Firmware IntegrationMain LoopThe main loop of the firmware (Figure 9) runs two tasks continuously:ScanTask and MouseTask. The ScanTask is thesame keyboard task in the keyboard-only firmware. It scansthe keyboard scan matrix to determine any key statuschange, loads the keyboard report to endpoint 1 FIFO andMain LoopScanTaskMouseTask20 ms DelayFigure 9. Main Loop8
Designing a USB Keyboard and PS/2 Mouse Combination Devicesends it to the host system if necessary. For more details onits operation please see the USB keyboard application note.Similarly, the MouseTask calls mouse_poll subroutine to readdata from the PS/2 mouse, copies the mouse report frommemory buffer to endpoint 1 FIFO and transmits it to the hostsystem. An arbitrary delay of 20 ms was added before thecode loops back. This is to make sure that the PS/2 mousegets ready to respond to the next polling command.USB Suspend/Resume and Remote WakeupAccording to the USB Specification 1.0, a device has to gointo suspend mode after 3 ms of no bus activity. The 1 ms ISRdetermines when this condition occurs (by reading the BusActivity bit, bit 3, of the USB Status and Control Register, 1Fh)and suspends the microcontroller. This is accomplished bysetting bit 0 (Run bit) and bit 3 (Suspend bit) of the ProcessorStatus and Control Register (FFh). The microcontroller willresume operation upon one of the following three events:USB bus activity, keyboard activity, or PS/2 mouse activity.USB bus activity resume is carried out automatically by themicrocontroller. On the other hand, keyboard and PS/2mouse activity during suspend are detected through GPIOinterrupts. For this reason, before entering suspend, GPIOinterrupts for the keyboard and mouse must be properly enabled.For the keyboard, prior to suspending the microcontroller, the1 ms ISR pulls down all the column port lines and enable afalling edge interrupt on all Port 2 lines (Row port). If any keyis pressed while in suspended state, one of the row lines willbe pulled LOW and trigger a GPIO interrupt. The GPIO ISRwill then send a resume signal (force a K state where D+ isHIGH and D– is LOW for 10 ms) to wake up the host system(remote wake up).The PS/2 mouse normally operates in Remote (polling)mode, where the mouse only transmits a data report in responseto a Read Data command. When the microcontrollerenters suspend state, however, it no longer polls the mousefor data. Thus, it is desirable to allow the mouse to be capableof autonomously waking up the microcontroller if it has datato transmit (mouse movement or button press). This is accomplishedby placing the mouse in Stream mode before enteringsuspend. In the Stream mode, the PS/2 mouse willtransmit data at regular intervals any time it has new dataavailable, generating GPIO interrupts and waking up the microcontroller.Before entering suspend the main code must call functionmouse_suspend, which initiates a Set Stream Mode commandand operates mouse_machine until the transaction iscomplete. Then the main code will enable suspend, leavingthe Port 3 GPIO interrupt enabled. During suspend state, anyactivity on the PS/2 port will cause an entry to the GPIO ISR,which will immediately inhibit the mouse transmission. Duringthe wakeup sequence, the firmware must also call mouse_initto place the mouse back to Remote mode for normal datapolling operations.GPIO Interrupt Service RoutineIn the keyboard-only firmware, the GPIO ISR was only usedto handle the keyboard resume and the sending of the remotewakeup signal after leaving the suspend state. For the keyboardand mouse combination device, however, the GPIO interruptis also used to drive the PS/2 low-level state machine(mouse_int) at each falling edge of the PS/2 clock signal.NoBus StillIdle?YesSend RemoteWakeup SignalGPIO ISRIn SuspendMode?ReturnYesCall MouseBit/byte LevelState Machinemouse_intFigure 10. GPIO Interrupt Service RoutineUpon entering the GPIO ISR (See Figure 10), a test is doneto determine whether the firmware is running in normal operationmode or has just left suspend mode. If it is in normaloperation, the mouse_int state machine is called; otherwise,the remote wakeup sequence begins.1-ms Interrupt Service RoutineFor the keyboard and mouse combination firmware, a fewchanges were added to the keyboard-only 1 ms ISR (Figure11):• The keyboard scan task is no longer scheduled every 4 msas in the keyboard-only firmware.• Before entering suspend state, the PS/2 mouse is placedin Stream mode. Also, after coming out of suspend, themouse is set back to Remote mode.• The PS/2 mouse time-out counter increment and check isincluded. If the PS/2 is in the active state for more than 25ms and no transfer happens, a time-out condition will occur,resetting the PS/2 bus and setting the state machines toidle.PS/2 Signal Timing IssueFor reliable PS/2 bus operation, the GPIO interrupt serviceprovided by the main code must ensure that the latency betweena falling edge on the PS/2 clock line and resultant entryinto the bit/byte level state machine (mouse_int) is no morethan 17 µs. This requirement precludes the disabling of interruptsfor long periods of time by other code modules, whilemouse_machine is busy. Special attention should be given tothe 1 ms ISR or other ISRs that may disable interrupt for longperiods of time.No9
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