Direction register vs data register in Embedded C - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this embedded C code snippet?

Consider a microcontroller port with a direction register (DDR) and a data register (PORT). The DDR controls pin direction (1 for output, 0 for input). The PORT register controls output value if the pin is output, or enables pull-up resistor if input.

What will be the output on the port pins after running this code?

Embedded C

#include <stdint.h>
#include <stdio.h>

uint8_t DDR = 0x00;  // Direction register
uint8_t PORT = 0x00; // Data register

int main() {
    DDR = 0b00001111;  // Lower 4 pins output, upper 4 pins input
    PORT = 0b10101010; // Set alternating bits

    // Simulate reading port pins
    uint8_t pin_values = PORT & DDR; // Output pins show PORT value
    printf("Pin values: 0x%02X\n", pin_values);
    return 0;
}

A Pin values: 0x0F

B Pin values: 0x0A

C Pin values: 0xAA

D Pin values: 0x00

💡 Hint

Remember that only pins set as output in DDR show the PORT value on pins.

The DDR sets lower 4 pins as output (bits 0-3). PORT is 0b10101010. Masking PORT with DDR (0b00001111) keeps only lower 4 bits: 0b00001010 (0x0A). So output pins show 0x0A.

🧠 Conceptual

intermediate

Which statement correctly describes the difference between DDR and PORT registers?

Choose the correct description of the roles of the Direction Register (DDR) and Data Register (PORT) in microcontroller I/O ports.

A DDR and PORT both set output values but for different pins.

B DDR sets output value; PORT sets pin direction.

C DDR sets pin direction; PORT sets output value or pull-up resistor depending on DDR.

D DDR enables interrupts; PORT controls analog input levels.

💡 Hint

Think about what controls if a pin is input or output, and what controls the voltage level.

DDR configures pins as input or output. PORT sets output voltage for output pins or enables pull-up resistors for input pins.

🔧 Debug

advanced

Why does this code not set the output pins correctly?

The code below tries to set pins 0-3 as outputs and set them high, but the output pins remain low. What is the cause?

Embedded C

#include <stdint.h>
#include <stdio.h>

uint8_t DDR = 0x00;
uint8_t PORT = 0x00;

int main() {
    DDR = 0b00001111; // Set pins 0-3 as output
    PORT = 0b00001111; // Set pins 0-3 high

    // Simulate reading pins
    uint8_t pins = PORT & DDR;
    printf("Pins: 0x%02X\n", pins);
    return 0;
}

A The PORT register must be set before DDR to take effect.

B The pins variable should be DDR & ~PORT to read output pins.

C The code should use |= instead of = to set bits in DDR and PORT.

D The DDR and PORT variables are not linked to actual hardware registers, so setting them has no effect.

💡 Hint

Think about what DDR and PORT represent in real embedded systems versus this simulation.

DDR and PORT here are just variables, not actual hardware registers. Setting them does not affect real pins. In real embedded code, these would be memory-mapped registers.

📝 Syntax

advanced

What error does this code produce?

Analyze the following embedded C code snippet. What error will it produce when compiled?

Embedded C

#include <stdint.h>

uint8_t DDR = 0x00;
uint8_t PORT = 0x00;

int main() {
    DDR = 0b00001111;
    PORT = 0b10101010;
    return 0;
}

A SyntaxError: missing semicolon after DDR assignment

B TypeError: invalid binary literal

C No error, code compiles successfully

D Runtime error: uninitialized variable usage

💡 Hint

Check each line carefully for missing punctuation.

The line 'DDR = 0b00001111' is missing a semicolon at the end, causing a syntax error.

🚀 Application

expert

How many pins are configured as output and set high after this code?

Given the following code, how many pins are configured as output and set to high voltage?

Embedded C

#include <stdint.h>

uint8_t DDR = 0b11001100;
uint8_t PORT = 0b10101010;

int main() {
    uint8_t output_pins = DDR & PORT;
    int count = 0;
    for (int i = 0; i < 8; i++) {
        if ((output_pins >> i) & 1) {
            count++;
        }
    }
    return count;
}

A 2

B 3

C 4

D 5

💡 Hint

Count bits set to 1 in DDR and PORT at the same positions.

DDR = 0b11001100 (bits 7,6,3,2 set as output). PORT = 0b10101010 (bits 7,5,3,1 set). DDR & PORT = 0b10001000 (bits 7,3 set). The loop counts 2 pins.