UAV V2 platform

- Core of the board is the SiRF III GPS system (EM406)
  - Baud rate is 4800, uses NMEA protocol (National Marine Electronics
    Association)
    - uses ASCII serial communications, allows for unidirectional
      communications, w/ listeners able to accurately parse information.
    - There is also some way to establish primary/secondary "talkers."
  - Antenna is built-in. it is best to orient the plane of the antenna
    parallel to the ground.
     

- CPU is a Microchip dsPIC30F4011
  - 16-bit architecture, 40-bit accumulators
  - C compiler
  - clock speed up to 120MHz
  - 2 USARTs (universal synchronous/asynchronous reciever/transmitter) on the
    dsPIC30F4011, one for GPS -> CPU, and one for CPU -> GPS
    - One connected to GPS, other can be used for debugging
  - Vectored, prioritized interrupts
    - dedicated ISRs for each resource, 7 levels of priority
  - Free running PWM generator, controlled by three 16-bit duty cycle
    registers
  - Free running buffered A/D convertor
    - samples 7 analog input channels    
  - 16-bit capture inputs measure pulse widths of each channel, generate
    separate interrupt for each
  - Mapping of program space into data space, easy to make lookup tables  

- Oscillator
  - can drive the CPU at 8, 16, 32, 64, or 120 MHz

- Gyros & 3-axis accelerometer
  - Three Analog Devices ADXRS401 gyroscopes, one per axis
    - sensitivity is approx 15 mV/deg/sec
    - max range is +/- 75 deg/sec
    - use a reference voltage so that the gyro output does not
      vary w/ supply voltage, i.e., discharging batteries
    
  - One 3-axis Freescale MMA7260 accelerometer
    - sensitivity is approx 800 mV/g
    - max range is +/- 1.5 g
    - accelerometer measures net acceleration minus g
    - output voltage is proportional to the supply voltage.

  - Both of these are high-gain devices, to improve noise/drift performance
  - Dynamic range is somewhat small
  - output of gyros and accelerometer are analog voltages. 
    - zero rotation & zero accel corresponds to approx. half voltage.  
    - motionless output voltages of gyros/accelerometers vary
      from unit to unit, so we should self-zero them by using the output
      voltages at power up to be baselines.

- Highest recommended update rate to servos is approx. 50Hz.
  - will want to use a decimation filter (makes signals discrete)
    between the sampling processing and control processing.
    - implement a first order digital low-pass filter w/ freq. between
      50Hz and 400Hz... filter updates its output w/ each 
      control processing read its output asynchronously
	
- Radio/Servo input/output pulses
  - interface b/w servos and RC receiver is 7 PWM I/O pins on the board.
  - 4 PWM inputs from RC receiver labeled "Radio," indicated as I1 - I4.
    - 4th input pin could be used to change from auto/manual control, for
      example.
    - PWM inputs are trhough interrupt-on-change pins. Each pin can be 
      assigned its own interrupt routine, and each pin can use a capture
      function to measure pulse width.
  - 3 PWM outputs, collectively labeled "Servos."
    - Standard RC 5V TTL servo pulses.
      - Pulse width is b/w 1 and 2ms., w/ approx. 20ms between pulses.
	- Between pulses, signal is a TTL low, and the pulse is a TTL
	  high. Can use this to measure the characteristics of our
	  own RC radio. 
	  - *** It is important to respect the delay between
	  pulses, because there is a pulse-stretcher within the servos,
	  so if we try to reduce this delay, the puse stretchers in
	  the servos will fuck up.	
	- Can use the PWN controllers to create properly-spaced
	  pulses. 
	  - Use the period to dictate spacing b/w pulses, and use
	    the duty cycle to control pulse widths. 		  
    - Servos can be connected directly to the board. 
    - No direct physical connection on board b/w PWN inputs and outputs.
      - Manual control of servos from radio can reliably be performed
	as a "pass-through" in software, by assigning a high priority    
	level to the input pins, and keeping manual control software
	simple. 
      - Can also connect servos directly ot the radio, if, for example,
	we want to use more than 3 servos, or we want to test one
	servo independently while manually controlling the other two.	   

- Board works well w/ 5V.
  - has not been tested w/ LiPo batteries.