Start and stop conditions in Embedded C - Step-by-Step Execution

Concept Flow - Start and stop conditions

Initialize variables

↓

Check start condition

Yes↓

Start main operation

↓

Check stop condition

No↓

Continue operation

↓

Check stop condition

Yes↓

Stop operation and exit

The program waits for a start condition to begin operation, then repeatedly checks a stop condition to know when to end.

Execution Sample

Embedded C

int start = 0;
int stop = 0;

// Wait for start
while(!start) {
  start = check_start_signal();
}

// Run until stop
while(!stop) {
  perform_task();
  stop = check_stop_signal();
}

This code waits until a start signal is true, then runs a task repeatedly until a stop signal is true.

Execution Table

Step	start	stop	Condition Checked	Action Taken	Output
1	0	0	start == 0	Check start signal	start remains 0, loop continues
2	0	0	start == 0	Check start signal	start becomes 1, loop exits
3	1	0	stop == 0	perform_task() called	stop remains 0
4	1	0	stop == 0	perform_task() called	stop remains 0
5	1	0	stop == 0	perform_task() called	stop becomes 1, loop exits
6	1	1	Loop ends	Stop operation	Program ends
exit	1	1	stop == 1	Stop condition met	Exit main loop

💡 stop becomes 1, so stop == 0 condition is false and loop ends

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	Final
start	0	0	1	1	1	1	1
stop	0	0	0	0	0	1	1

Key Moments - 2 Insights

Why does the program wait in the first loop before starting the main operation?

Because the start variable is 0 initially, the condition 'while(!start)' keeps the program waiting until start becomes 1, as shown in execution_table rows 1 and 2.

Why does the main loop stop running when stop becomes 1?

The main loop condition is 'while(!stop)', so when stop is 1, '!stop' is false, causing the loop to exit as shown in execution_table row 5.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'start' after step 2?

A 1

B 0

C Undefined

D 2

💡 Hint

Check the 'start' column in execution_table row 2.

At which step does the stop condition become true, causing the main loop to exit?

A Step 3

B Step 1

C Step 5

D Step 6

💡 Hint

Look at the 'stop' column and 'Condition Checked' in execution_table rows.

If the start signal never becomes true, what happens to the program?

A It runs the main loop anyway

B It waits forever in the first loop

C It exits immediately

D It crashes

💡 Hint

Refer to execution_table rows 1 and 2 where start is checked.

Concept Snapshot

Start and stop conditions control program flow.
Use a loop to wait for start condition (e.g., while(!start)).
After start, run main loop until stop condition is true (e.g., while(!stop)).
Check conditions each iteration to decide when to start or stop.
This pattern ensures controlled operation start and safe stop.

Full Transcript

This example shows how a program waits for a start signal before running its main task. Initially, the variable 'start' is zero, so the program stays in the first loop checking for the start signal. Once 'start' becomes one, the program exits the waiting loop and enters the main loop. In the main loop, it repeatedly checks the 'stop' variable. As long as 'stop' is zero, the program performs its task. When 'stop' becomes one, the main loop ends and the program stops. This pattern helps control when a program begins and ends its operation safely.