MCP3008 ADC over SPI in Raspberry Pi - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the SPI library needed to communicate with MCP3008.

Raspberry Pi

import [1]

Drag options to blanks, or click blank then click option'

Agpiozero

Bspidev

CRPi.GPIO

Dtime

Attempts:

3 left

💡 Hint

Common Mistakes

Importing gpiozero or RPi.GPIO instead of spidev.

Forgetting to import the SPI library.

The spidev library is used to communicate over SPI with devices like the MCP3008 ADC.

2fill in blank

medium

Complete the code to open SPI bus 0, device 0 for communication.

Raspberry Pi

spi = spidev.SpiDev()
spi.[1](0, 0)

Drag options to blanks, or click blank then click option'

Aopen

Bstart

Cconnect

Dinit

Attempts:

3 left

💡 Hint

Common Mistakes

Using connect or init instead of open.

Not opening the SPI device before use.

The open method opens the SPI bus and device for communication.

3fill in blank

hard

Fix the error in the code to read channel 0 from MCP3008 correctly.

Raspberry Pi

def read_channel(channel):
    adc = spi.xfer2([1, (8 + [1]) << 4, 0])
    data = ((adc[1] & 3) << 8) + adc[2]
    return data

Drag options to blanks, or click blank then click option'

Aspi

B0

C1

Dchannel

Attempts:

3 left

💡 Hint

Common Mistakes

Using a fixed number like 0 or 1 instead of the channel variable.

Using an undefined variable.

The variable channel must be used to select the ADC channel dynamically.

4fill in blank

hard

Fill both blanks to set SPI speed and mode correctly for MCP3008.

Raspberry Pi

spi.max_speed_hz = [1]
spi.mode = [2]

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A1350000

B500000

C0

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using too high or too low SPI speed.

Setting wrong SPI mode causing communication errors.

The MCP3008 works well with SPI speed of 1.35 MHz and SPI mode 0.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that reads channels 0 to 3 and stores their values.

Raspberry Pi

adc_values = {ch: read_channel([1]) for ch in range([2], [3])}

Drag options to blanks, or click blank then click option'

Ach

B0

C4

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using a fixed number instead of the loop variable for reading channels.

Incorrect range boundaries causing missing or extra channels.

The comprehension uses ch as the argument to read_channel and loops from 0 to 4 (exclusive) to read channels 0,1,2,3.

Practice

1. What is the main purpose of using the MCP3008 chip with a Raspberry Pi?

easy

A. To read analog signals by converting them to digital values

B. To provide additional digital output pins

C. To increase the Raspberry Pi's processing speed

D. To connect multiple Raspberry Pis together

Solution

1. Step 1: Understand MCP3008 function

   The MCP3008 is an analog-to-digital converter (ADC) chip that converts analog signals into digital values readable by the Raspberry Pi.

2. Step 2: Identify Raspberry Pi limitation

   The Raspberry Pi does not have built-in analog input pins, so MCP3008 is used to read analog sensors.

3. Final Answer:

   To read analog signals by converting them to digital values -> Option A

4. Quick Check:

   MCP3008 = Analog to Digital Converter [OK]

Hint: MCP3008 converts analog sensor signals to digital for Pi [OK]

Common Mistakes:

• Thinking MCP3008 adds digital outputs
• Confusing it with a processor speed booster
• Assuming it connects multiple Pis

2. Which Python library is commonly used to communicate with the MCP3008 over SPI on Raspberry Pi?

easy

A. RPi.GPIO

B. smbus

C. serial

D. spidev

Solution

1. Step 1: Identify SPI communication library

   The spidev library is designed for SPI communication on Raspberry Pi.

2. Step 2: Match library to MCP3008 usage

   MCP3008 uses SPI protocol, so spidev is the correct library to communicate with it.

3. Final Answer:

   spidev -> Option D

4. Quick Check:

   SPI communication = spidev library [OK]

Hint: Use spidev for SPI devices like MCP3008 on Pi [OK]

Common Mistakes:

• Using RPi.GPIO which is for GPIO pins, not SPI
• Choosing smbus which is for I2C devices
• Selecting serial which is for UART communication

3. Given the following Python code snippet reading channel 0 from MCP3008, what is the output if the analog input corresponds to a digital value of 512?

import spidev
spi = spidev.SpiDev()
spi.open(0, 0)
spi.max_speed_hz = 1350000
adc = spi.xfer2([1, (8 + 0) << 4, 0])
data = ((adc[1] & 3) << 8) + adc[2]
print(data)

medium

A. 512

B. 0

C. 1023

D. 256

Solution

1. Step 1: Understand SPI transfer and data extraction

   The spi.xfer2 sends 3 bytes and receives 3 bytes. The returned adc list contains the 10-bit ADC value split across adc[1] and adc[2].

2. Step 2: Calculate the 10-bit value from response

   The code extracts the 10-bit value by combining bits: ((adc[1] & 3) << 8) + adc[2]. If the analog input corresponds to 512, the printed value will be 512.

3. Final Answer:

   512 -> Option A

4. Quick Check:

   Extracted ADC value = 512 [OK]

Hint: Combine bits from adc[1] and adc[2] to get 10-bit value [OK]

Common Mistakes:

• Not masking adc[1] with & 3
• Shifting bits incorrectly
• Confusing channel bits with data bits

4. Identify the error in this MCP3008 SPI read code snippet:

import spidev
spi = spidev.SpiDev()
spi.open(0, 0)
spi.max_speed_hz = 1350000
channel = 1
adc = spi.xfer2([1, (8 + channel) << 4, 0])
data = (adc[1] << 8) + adc[2]
print(data)

medium

A. Wrong shift amount for channel bits

B. Incorrect SPI bus number in spi.open()

C. Missing mask to extract only 2 bits from adc[1]

D. Using xfer2 instead of xfer

Solution

1. Step 1: Review data extraction from SPI response

   The MCP3008 returns a 10-bit value where only the last 2 bits of adc[1] are part of the data. The code incorrectly uses adc[1] << 8 without masking.

2. Step 2: Identify correct masking

   Correct extraction requires masking with adc[1] & 3 before shifting to keep only the 2 relevant bits.

3. Final Answer:

   Missing mask to extract only 2 bits from adc[1] -> Option C

4. Quick Check:

   Mask adc[1] with & 3 before shifting [OK]

Hint: Always mask adc[1] with & 3 before shifting bits [OK]

Common Mistakes:

• Skipping bit masking on adc[1]
• Incorrect SPI bus or device numbers
• Confusing xfer and xfer2 usage

5. You want to read all 8 channels of the MCP3008 and store their values in a dictionary with keys as channel numbers. Which code snippet correctly implements this using spidev?

hard

A. values = {ch: ((spi.xfer2([1, (8 + ch) << 4, 0])[1] & 3) << 8) + spi.xfer2([1, (8 + ch) << 4, 0])[2] for ch in range(8)}

B. values = {}
for ch in range(8):
adc = spi.xfer2([1, (8 + ch) << 4, 0])
values[ch] = ((adc[1] & 3) << 8) + adc[2]

C. values = [spi.xfer2([1, (8 + ch) << 4, 0]) for ch in range(8)]

D. values = {ch: spi.xfer2([1, (8 + ch) << 4, 0])[1] << 8 + spi.xfer2([1, (8 + ch) << 4, 0])[2] for ch in range(8)}

Solution

1. Step 1: Understand correct SPI read and bit extraction

   Each channel read requires one SPI transfer. The 10-bit value is extracted by masking adc[1] with 3 and shifting before adding adc[2].

2. Step 2: Check code correctness

   values = {} for ch in range(8): adc = spi.xfer2([1, (8 + ch) << 4, 0]) values[ch] = ((adc[1] & 3) << 8) + adc[2] uses a for loop to read each channel once, extracts the value correctly, and stores it in a dictionary with channel keys.

3. Final Answer:

   Option B code snippet -> Option B

4. Quick Check:

   Loop + mask adc[1] & 3 + shift + store in dict [OK]

Hint: Use a loop with masking and shifting to read all channels [OK]

Common Mistakes:

• Calling spi.xfer2 twice per channel (Option A)
• Not masking adc[1] bits (Option D)
• Storing raw SPI response without processing (Option C)