DevGuide/DesignOverview

Airborne Functional Diagram

Design Goals

-Static approach
-Modularity
-Hardware abstraction
-Runtime efficiency

Static Approach

 -only "things that needs to be changed during flight are changeable
 -maximise compilation time resolutions

 advantages
   Error checking
   Efficiency
   Safety/Robustness

Modularity

 -Separation of concerns
 -Maintenability
 -Interface
 -C provide no dedicated mechanism for modularity
 -Main issues with modularity are configuration and dependancies

Hardware abstraction

 -Segregate hardware dependant modules

Runtime Efficiency

We have to take care that our code runs efficient on the autopilot board microprocessor. It is for example very unlikely that we will physically change the serial port on which we connected our telemetry modem during a flight.

bad coding practice

switch(UART) {
 case UART0 : UARTO_write(...); break;
 case UART1 : UART1_write(...); break;
}

good coding practice

#define UartWrite(x) UART ## _write(x)
UartWrite(...);

Design Solutions

* Heavy pre-processor usage
* C code generation
* Heavy Makefile usage
* Conventions to define our own module layer

pre-processor

header selection

conf/airframes/funjet1.xml

<makefile>
ap.CFLAGS += -DACTUATORS=\"servos_4017_hw.h\"
</makefile>

generated var/FJ1/Makefile.ac

ap.CFLAGS += -DACTUATORS=\"servos_4017_hw.h\"

sw/airborne/actuators.h

#include ACTUATORS

expanded by pre-processor to

#include "servos_4017_hw.h"

macros

conf/airframes/funjet1.xml

 ap.CFLAGS += -DXBEE_UART=Uart1

sw/airborne/xbee.h

 #define __XBeeLink(dev, _x) dev##_x
 #define _XBeeLink(dev, _x) __XBeeLink(dev, _x)
 #define XBeeLink(_x) _XBeeLink(XBEE_UART, _x)
 #define XBeeBuffer() XBeeLink(ChAvailable())

expanded by pre-processor to

Uart1ChAvailable()

C code generation

For problems too complex to solve with the pre processor, we use custom compilers to generate code from a description in xml

example 1 : command laws, aka mixers

conf/airframes/funjet1.xml

<command_laws>
 <let var="aileron" value="@ROLL * AILEVON_AILERON_RATE"/>
 <let var="elevator" value="@PITCH * AILEVON_ELEVATOR_RATE"/>
 <set servo="AILEVON_LEFT" value="$elevator + $aileron"/>
 <set servo="AILEVON_RIGHT" value="$elevator - $aileron"/>
</command_laws>

generated var/FJ1/airframe.h

#define SetActuatorsFromCommands(values) { \
  uint16_t servo_value;\
  float command_value;\
  int16_t _var_aileron = values[COMMAND_ROLL] * AILEVON_AILERON_RATE;\
  int16_t _var_elevator = values[COMMAND_PITCH] * AILEVON_ELEVATOR_RATE;\
[....]
  command_value = _var_elevator + _var_aileron;\
  command_value *= command_value>0 ? SERVO_AILEVON_LEFT_TRAVEL_UP :   SERVO_AILEVON_LEFT_TRAVEL_DOWN;\
  servo_value = SERVO_AILEVON_LEFT_NEUTRAL + (int16_t)(command_value);\
  actuators[SERVO_AILEVON_LEFT] = ChopServo(servo_value, SERVO_AILEVON_LEFT_MIN, SERVO_AILEVON_LEFT_MAX);\
  Actuator(SERVO_AILEVON_LEFT) = SERVOS_TICS_OF_USEC(actuators[SERVO_AILEVON_LEFT]);\

example 2 : flight plan

conf/flight plans/example.xml

  <block name="For loop (circles wp 1)">
     <for from="0" to="3" var="i">
       <circle radius="DEFAULT_CIRCLE_RADIUS+ $i*10" wp="1" until="NavCircleCount() > 1"/>
     </for>
     <deroute block="Standby"/>
   </block>

generated var/flight plan.h

 Block(29) // For loop (circles wp 1)
   switch(nav_stage) {
     static int8_t _var_i;
     static int8_t _var_i_to;
     Stage(0)
       _var_i = 0 - 1;
       _var_i_to = 3;
     Label(for_11)
     Stage(1)
       if (++_var_i > _var_i_to) Goto(endfor_12) else NextStageAndBreak();
     Stage(2)
       NavVerticalAutoThrottleMode(RadOfDeg(0.000000));
       NavVerticalAltitudeMode(WaypointAlt(3), 0.);
       NavCircleWaypoint(3, (DEFAULT_CIRCLE_RADIUS+(_var_i*10)));
       if ((NavCircleCount()>1)) NextStageAndBreak();
       break;
     Stage(3)
       Goto(for_11)
     Label(endfor_12)
     Stage(5)
       NextBlock();
       break;
   }

Makefile

Connecting and configuring modules

 ap.CFLAGS += -DDOWNLINK -DDOWNLINK_TRANSPORT=XBeeTransport -DXBEE_UART=Uart1
 ap.srcs += downlink.c xbee.c
 ap.CFLAGS += -DUSE_UART1 -DUART1_BAUD=B9600
 ap.srcs += \$(SRC_ARCH)/uart_hw.c

Modules configuration

3 sources of configuration:

 * airframe configuration file ( airframe.xml ) which "compiled" to C files and Makefile
 * specific per module configuration file ( eg radio.xml, flightplan.xml) compiled to C code to allow factorization between airframes.
 * specific per board configuration file ( eg tiny.h ) to describe modules configuration imposed by board routing

Generating Doxygen Code Documentation

see Doxygen