Silicon Laboratories Stepper Machine Bedienungsanleitung PDF herunterladen (Seite 11)

AN155

Rev. 1.1 11

5.2. User Interface

The primary user interface is an ASCII terminal with a

baud rate of 57,600 bps. The main loop of the code

parses the characters from the terminal and performs

the appropriate action. The flowchart for the main loop

is shown in Figure D1.

The main loop first checks to see if the motor is moving.

The LED bit is used to turn on the LED and is also used

as a state variable. The LED is turned on by the move

function and is turned off by the Timer ISR on

completion of the move. While the motor is moving the

main loop will periodically update the position to the

terminal display. This is accomplished by overwriting the

previous position. The position is written to the terminal

followed by a carriage return without the normal line

feed character. This overwrites the previous number.

The position is written to the terminal using the puts()

put string function. A string of four space characters are

used to blank trailing characters. This is necessary

when moving from a large 5-digit number to a small 1-

digit number. A short delay is used to prevent the

transmitter buffer from overflowing.

Next the main loop checks the doneFlag. The Timer

ISR sets the doneFlag when the move is complete. If

doneFlag is set, the main loop will call the doneMsg()

function. The doneMsg() function displays the word

“done” and readies the terminal for the next command.

The doneFlag is cleared by doneMsg().

Next the main loop enters the command parser. The

command parser is implemented using a large switch

statement. The parser gets one character from the

terminal and uses a case statement to do different

things depending on the character.

If the character is a carriage return, a newline and

prompt will be transmitted to the terminal. Repeatedly

entering a carriage return will display multiple prompt

characters, each on a new line.

If the character is a p, the parser will get a 16-bit

number from the terminal using the getuint()

function, and move to the new position.

If the character is an a, the parser will get an 8-bit

number from the terminal using the getuchar()

function, store the new value, and display the new value

to the screen for confirmation.

If the character is an s, the main loop will display the

current status information - position and acceleration.

The status command does not wait for any additional

characters.

If the parser does not recognize the character, an invalid

character message will be displayed.

5.3. Move Function

A flowchart for the move function is shown in Figure D2.

The move function first checks to see if the motor is

already at the target position. If the motor is already at

position zero and you enter the command p0, the motor

will not move and the done message will be displayed.

This is necessary to ensure that the algorithm works

properly. The smallest valid move is 1 step in either

direction. The current position is the value stored in the

global variable Position. The desired position,

passed as a parameter to the move() function, is called

targetPosition. The forward flag is set or cleared

depending on whether the target is greater than the

position. Both targetPosition and Position are

unsigned integers. The smaller is subtracted from the

larger to get the variable length.

The global variable TableMax defines how many steps

the motor will accelerate and decelerate. While

accelerating the TableIndex will be incremented until

it reaches TableMax. If length is less than 1024, the

value for TableMax is calculated by dividing length

by four. If length is greater than 1024, TableMax is

set to 255. This is the maximum allowable value for

TableIndex.

Taking length, subtracting two times TableMax for

the acceleration/deceleration steps, and subtracting one

more for the initial step gives the number of constant

velocity slewing steps. This ensures that the total

number of steps is exactly equal to the length.

The move function then schedules the timer to execute

the first step after a short delay. After the initial step, the

Timer Interrupt service routine Timer_ISR() will

control the stepper motor timing from then on.

5.4. Timer ISR

The timer interrupt service routine moves the motor

according to the prescribed profile. The interrupt service

routine will first commutate the motor. Then the

PatternIndex is incremented or decremented

according to the stepper motor direction. The

PatternIndex is used as a pointer to the step pattern.

It is not to be confused with the TableIndex that points

to the linear velocity table. The step pattern index for a

unipolar stepper motor is a modulus 8 counter. It counts

from zero to seven. The step pattern is written to P0. Bit

7 is set to a 1 to ensure that P0.7 remains configured as

an input.

The timer interrupt service routine determines what to

do next depending on the current state of the motor. The

four possible states are Accelerating, Decelerating,

Slewing, or Done. If the motor state is Done, the timer

ISR will disable further timer interrupts, turn off the LED,

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