MCP3008 ADC over SPI in Raspberry Pi - Mini Project: Build & Apply

Reading Analog Sensor Values with MCP3008 ADC over SPI on Raspberry Pi

📖 Scenario: You have a Raspberry Pi connected to an MCP3008 analog-to-digital converter (ADC) via SPI. You want to read the analog value from a sensor connected to channel 0 of the MCP3008 and display the digital value.

🎯 Goal: Build a Python program that reads the analog input from channel 0 of the MCP3008 ADC using SPI communication and prints the digital value.

📋 What You'll Learn

Create a SPI connection using the spidev library

Set up the MCP3008 channel to read (channel 0)

Send the correct SPI command to read from channel 0

Parse the response to get the 10-bit ADC value

Print the ADC value

💡 Why This Matters

🌍 Real World

Reading analog sensors like temperature, light, or potentiometers with a Raspberry Pi requires an ADC like MCP3008 because the Pi has no built-in analog inputs.

💼 Career

Understanding SPI communication and ADC reading is important for embedded systems, IoT projects, and hardware interfacing roles.

1

Set up SPI connection

Import the spidev library and create a SPI object called spi. Open SPI bus 0, device 0 using spi.open(0, 0). Set the SPI max speed to 1 MHz with spi.max_speed_hz = 1000000.

Raspberry Pi

# Import spidev and create SPI object
# Your code here

Hint

Use import spidev to access SPI functions. Create SPI object with spidev.SpiDev(). Open bus 0, device 0 with spi.open(0, 0). Set speed with spi.max_speed_hz.

2

Prepare SPI command to read channel 0

Create a list called adc_command with three bytes to read from channel 0 of MCP3008. The first byte is 1 (start bit), the second byte is 0b10000000 (single-ended mode, channel 0), and the third byte is 0.

Raspberry Pi

import spidev
spi = spidev.SpiDev()
spi.open(0, 0)
spi.max_speed_hz = 1000000

# Create adc_command list to read channel 0
# Your code here

Hint

The MCP3008 expects 3 bytes: start bit (1), single-ended + channel bits (0b10000000 for channel 0), and a dummy byte (0).

3

Send SPI command and parse ADC value

Use spi.xfer2(adc_command) to send the command and receive a list called adc_response. Extract the 10-bit ADC value from adc_response by combining bits: mask the second byte with 3 (to get the last two bits), shift left 8 bits, then OR with the third byte. Store the result in a variable called adc_value.

Raspberry Pi

import spidev
spi = spidev.SpiDev()
spi.open(0, 0)
spi.max_speed_hz = 1000000

adc_command = [1, 0b10000000, 0]

# Send adc_command and get adc_response
# Extract 10-bit adc_value from adc_response
# Your code here

Hint

Use spi.xfer2() to send and receive SPI data. The 10-bit value is in the last two bytes: mask the second byte with 3, shift left 8 bits, then OR with the third byte.

4

Print the ADC value

Print the variable adc_value to display the digital reading from the MCP3008.

Raspberry Pi

import spidev
spi = spidev.SpiDev()
spi.open(0, 0)
spi.max_speed_hz = 1000000

adc_command = [1, 0b10000000, 0]

adc_response = spi.xfer2(adc_command)
adc_value = ((adc_response[1] & 3) << 8) | adc_response[2]

# Print adc_value
# Your code here

Hint

Use print(adc_value) to show the ADC reading. The actual number depends on your sensor input.

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)