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Working with Hardware

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jmalins edited this page Dec 28, 2012 · 2 revisions

The VexOS hardware abstraction layer (HAL) is a set of classes that make it easier to work with VEX hardware. While the easyC libraries provide low-level access that is friendly to easyC block-diagrams, the overall API has a number of weaknesses:

  • Pin numbers must be repeated on each call, requiring the use of macros to minimize changes if ports are rearranged
  • The "Controller Configuration" dialog must be kept in sync with any code changes
  • Procedural calls without objects do not model the actual hardware
  • Most calls are write-only, there is no way to determine what state hardware is in, this makes generic/reactive routines more difficult to implement
  • Minimal error checking
  • Inconvenient data types and ranges (motor power is -127 to 127 instead of -1.0 to 1.0, analog output is 0 to 4096)
  • Some API calls are unwieldy to type

The VexOS HAL addresses all of these issues. The key features are:

  • Hardware objects encapsulate the pin assignments and easyC calls, providing a clean API that directly models the physical hardware on the robot
  • Cortex controller digital I/O, motor types and IME addresses are automatically configured based on the objects that are constructed
  • Rich exceptions alert you immediately of hardware errors or configuration issues
  • Better, more consistent method names

While the system is designed to be used Commnand-based Subsystems, the HAL works just as well for a simple event-based robot.

VexOS defines the following physical hardware objects:

Accelerometer - accelerometer
AnalogIn - Potentiometer, Light Sensor, Line Sensor
DigitalIn - Limit and Bump switches, Jumpers
DigitalOut - Pneumatic valve and LED
Encoder - Quadrature and legacy rotary encoder
Gyro - yaw rate gyro
LCD - LCD display
Motor - a motor, always used through a MotorGroup
PowerExpander - power expander
Servo - servo
Speaker - speaker
Sonar - ultrasonic range finder

It also defines the following logical objects:

MotorGroup - generic motor control system with automatic ganging, feedback and PID
SerialPort - serial port for debugging and custom hardware

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