Question

The PORTD is connected to 2 LEDs: A green LED is connected to PD1 and a red LED to PD4. All LEDs are ON when a logic 0 is output to PORTD pins. What is the output of the following AVR assembly program?

LDI R20, $12
OUT DDRD, R20
LDI R20, $ED
OUT PORTD, R20

a. Both Green and Red LEDs are OFF
b. Green LED is ON and Red LED is OFF
c. Both Green and Red LEDs are ON
d. Green LED is OFF and Red LED is ON
e. None of the mentioned

Answer 1

The program sets PD4 (red LED) to logic 0 which turns it ON, and PD1 (green LED) to logic 1 which turns it OFF. Hence, the green LED is OFF and the red LED is ON.

Step-by-step explanation:

The AVR assembly program in question is designed to control the state of LEDs connected to the PORTD register of an AVR microcontroller. The program consists of a sequence of instructions that configure the data direction register (DDRD) and then set the output values of PORTD.

Let's break down the program step by step:

1. LDI R20, $12: This instruction loads the hexadecimal value 12 into register R20. The binary representation of $12 is 0001 0010. This sets PD4 and PD1 as outputs and the rest of the pins as inputs.
2. OUT DDRD, R20: This instruction writes the value from R20 to DDRD, configuring the data direction of PORTD's pins according to the value in R20.
3. LDI R20, $ED: This instruction loads the hexadecimal value ED into register R20. The binary representation of $ED is 1110 1101. This sets PD4 to logic 0, causing the red LED to turn ON and PD1 to logic 1, causing the green LED to turn OFF, as specified that logic 0 turns an LED ON.
4. OUT PORTD, R20: This instruction writes the value from R20 to PORTD, actually setting the outputs to the specified states.

Therefore, the correct answer is d. Green LED is OFF and Red LED is ON.