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                             characteristics. Command 105 releases the lock and
                             discards the stored event (command 10 would 
                             return zero immediately after command 105 is 
                             issued).

Read Byte Data, Command =11: Report the "IR Triggered Event Mode" setting (one
                             data byte). 1 = First Event (stop storing events
                             after the first one is triggered), 2 = Last Event
                             (overwrite the stored event each time one is
                             detected, so that the most recent event is always
                             the one that's stored), and 3 = Maximum Event
                             (only overwrite the stored event if an event is
                             detected with a bigger detection spike than the
                             stored event). 0 = IR sensing completely disabled.

Read Byte Data, Command =12: Report the IR triggered event ADC bitcount (one
                             data bye).  Host should use command 10 first to
                             check if a detection event occurred, and to
                             "lock" it (i.e. prevent it from being
                             overwitten by another event).

Read Byte Data, Command =13: Report the 8-bit timer value at the time the
                             stored IR event was triggered (one data byte).
                             Host should use command 10 first to
                             check if a detection event occurred, and to
                             "lock" it (i.e. prevent it from being
                             overwitten by another event).

Read Byte Data, Command =14: Report the current 8-bit timer value (one data
                             byte). This is a free running counter that 
                             increments by one every 20ms (the servo drive 
                             period), so it wraps around every 5.12 seconds.

Read Byte Date, Command =15: Report the status of the PWM outputs.
                             0 = outputs disabled (servos limp). This is the
                             initial default, low power mode. In this mode,
                             the free running timer counter and the infrared
                             sensing is also disabled.
                             1 = PWM output active (timer counter and infrared
                             sensing also enabled).

Write Byte Data, Command = 100 : Set the Pan servo position setpoint (one data
                                 byte).

Write Byte Data, Command = 101 : Set the Tilt servo position setpoint (one data
                                 byte).

Write Byte Data, Command = 102 : Set the "Maximum Change Per Period" setting
                                 (one data byte). This is the maximum change 
                                 per update period (which is 20ms) of a
                                 servo's driven position. The smaller this 
                                 value is, the slower (and smoother) the 
                                 servos will appear to respond to changes in 
                                 setpoint. For example, a value of 1
                             means that a servo's actual driven position can
                             only change by a maximum of 1 count per period.
                             Since the maximum range of driven position is 
                             expressed as a 1 byte value (0 to 255), the time
                             it would take to pan from one stop to the other
                             is [20ms x 256], which is just over 5 seconds. A
                             setting value of 2 would halve that time. If you
                             want to get meaningful Pan position data from the
                             IR sensor triggered detections, this setting
                             should be low enough to ensure that the loaded
                             servos can keep up with driven positions. 
                             Another advantage of driving the servos in this
                             way is that the smoother motion will put less
                             stress on the servos' gears and bearings,
                             prolonging their life. If you're not interested in
                             the IR sensor data and you just want the servos to
                             move to a new setpoint position as fast as 
                             possible, use 255 as the value for this setting.
                             This configuration setting is stored in 
                             nonvolatile EEPROM.
                     
Write Byte Data, Command = 103 : Set the "IR Sensor Neutral Value" setting
                                 (one data byte). This configuration setting
                                 is stored in nonvolatile EEPROM.

Write Byte Data, Command = 104 : Set the "IR Sensor Trigger Threshold" setting
                                 (one data byte). This configuration setting
                                 is stored in nonvolatile EEPROM.

Send Byte, Command = 105       : Reset the IR triggered event (no data bytes).

Write Byte Data, Command = 106 : Set the "IR Triggered Event Mode" setting
                                 (one data byte). 1 = First Event (stop storing
                                 events after the first one is triggered), 
                                 2 = Last Event (overwrite the stored event
                                 each time one is detected, so that the most
                                 recent event is always the one that's stored),
                                 and 3 = Maximum Event (only overwrite the 
                                 stored event if an event is detected with a 
                                 bigger detection spike than the stored event).
                                 0 = IR sensing completely disabled. This
                                 configuration setting is stored in nonvolatile
                                 EEPROM.

Send Byte, Command = 107       : Reset the 8-bit timer counter (no data bytes).

Send Byte, Command = 108       : Activate PWM Output. Also enables the free
                                 running timer counter and infrared sensing.

Send Byte, Command = 109       : Disable PWM Output. In this low power mode, 
                                 the free running timer counter and the 
                                 infrared sensing are also disabled. This is 
                                 the initial default mode of the module.

PEC (Packet Error Checking) is disabled for all SMBus commands.


FIRMWARE NOTES
--------------

The software for the PICMicro is written in assembly language, and compiles 
using the GNU PIC Assembler, gpasm. To compile the source code into a hex file 
suitable for programming a PIC16F819 device, type

    gpasm -n head_control_module.asm

The resulting hex file will be called head_control_module.hex  The -n option 
puts DOS-style CR-LF newline sequences in the .hex file, required by most 
device programmers. 

It also compiles with the MPASM compiler for Microsoft Windows that is a part
of the MPLAB suite from Microchip. However, you must make sure the assembly
(.asm) file is in Windows/DOS format (i.e. with CR-LF sequences at the end of
each line) before it will successfully compile in MPASM for Windows.

Some device programmers require the configuration bits to be set manually. It
is very important that these are set correctly. The Head Control Module
requires the following configuration bit settings:

 - Flash Code Protection off
 - CCP1 on RB3
 - In-circuit Debugger disabled
 - Flash write enable off
 - EE Code Protection off
 - Low Voltage Programming Enable off, RB3 is I/O
 - Brown-out Reset Enable on
 - RA5/MCLR pin function is digital I/O
 - Power-Up Timer on
 - Watchdog Timer off
 - Oscillator: INTRC, port I/O function of RA6 and RA7

The above configuration settings are represented by the configuration word
2F50 in hexadecimal.