Protocol Documentation

This module implements a command line interface for the A4988 stepper motor driver.

Implementing a protocol for a command line interface is not easy. One has to know what a protocol and a state machine is and how they work. Please see the following modules for more information:

• Module 5.3.1 - Protocols
• Module 5.3.2 - Analysis and Selection
• Module 5.4.1 - Finate-State Machines

Refer to Module 5.3.3 for an implementation example with a Visual Studio and a Code::Blocks solution.

Protocol Definition

• Start Condition: A, M, S, T
• Stop Condition: space
• Message Format: 'COMMAND''ARGUMENT'
  • A num1 num2 - Advance the motor num steps
  • M O/F - Switch motor on (O) or off (F)
  • S - Advance the motor one step
  • T R/L - Turn right (R) or left (L)

Protocol Implementation

The protocol is implemented by the following finite state machine:

The description of the states is:

State	Description
C	Parse command.
A1	Parse first digit of advance argument.
A2	Parse second digit of advance argument.
D	Parse argument of direction command.
M	Parse argument of motor state command.
T	Parse terminal character.
E	Execute command.

The transitions are:

State	Condition	Transition	Description
C	token = A	A1	Command is Advance. → Parse first digit of argument.
	token = T	D	Command is Turn. → Parse argument.
	token = M	M	Command is Motor State. → Parse argument.
	token = S	T	Command is Step. → No argument required, parse terminal.
A1	token = digit	A2	First digit parsed. → Parse second digit.
A2	token = digit	T	Second digit parsed. → Parse terminal.
D	token = R	T	Argument parsed. → Parse terminal.
	token = L	T	Argument parsed. → Parse terminal.
M	token = O	T	Argument parsed. → Parse terminal.
	token = F	T	Argument parsed. → Parse terminal.
T	token = space	E	Terminal parsed. → Execute.
E	-	C	Command executed. → Parse next command.

Every parsing function gets the new token and returns the subsequent state. Some functions return extra information (command, argument) via a reference parameter. The basic workflow is as follows:

while (1)
{
  switch (state) {
    case CLI_STATE_GET_COMMAND:
      token = new_token();
      state = cli_parse_command(token, &command);
      break;
    case CLI_STATE_GET_ARGUMENT:
      token = new_token();
      state = cli_parse_argument(token, &argument);
      break;
    case CLI_STATE_GET_TERMINAL:
      token = new_token();
      state = cli_parse_terminal(token);
      break;
    case CLI_STATE_EXECUTE:
      state = cli_execute(command, argument);
      break;
  }
}