Question

The 8051 program segment, which performs ‘software polling’ to check if the Timer-0 counting has completed, is:

• A. JNB TF0, 0FEh
• B. JB TF0, 0FEh
• C. JB TF1, 0FEh
• D. JNB TF1, 0FEh

Hint:

In 8051 software polling, the correct instruction is JB TFx, address, where the program loops until the Timer Flag (TF0 or TF1) is set.

Answer 1

The Correct Option is B

Step 1: In the 8051 microcontroller, Timer Flags (TF0, TF1) are set when the respective timers (Timer-0, Timer-1) complete their counting. 
Step 2: Software polling involves continuously checking a flag until it becomes set, indicating completion. 
Step 3: The correct instruction format for polling is: \[ \text{JB TFx, address} \] where:
- JB (Jump if Bit is Set) checks if TF0 = 1.
- If TF0 is set, it jumps to 0FEh (execution continues).
- If TF0 is not set, it keeps polling. 
Step 4: Evaluating options: 
- (A) JNB TF0, 0FEh (Incorrect): Jumps if TF0 is NOT set, which is opposite to polling behavior. 
- (B) JB TF0, 0FEh (Correct): Checks if TF0 = 1, making it the correct software polling approach. 
- (C) JB TF1, 0FEh (Incorrect): Monitors Timer-1 flag instead of Timer-0. 
- (D) JNB TF1, 0FEh (Incorrect): Jumps if TF1 is NOT set, which is not polling.