MCP3008 ADC over SPI in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Reading MCP3008 Channel Output

What is the output of this Python code snippet that reads from channel 0 of the MCP3008 ADC using SPI on a Raspberry Pi?

Raspberry Pi

import spidev
spi = spidev.SpiDev()
spi.open(0, 0)
spi.max_speed_hz = 1350000

# Function to read channel 0
adc = spi.xfer2([1, (8 + 0) << 4, 0])
data = ((adc[1] & 3) << 8) + adc[2]
print(data)

A A runtime error due to incorrect SPI command format

B A list of three integers representing raw SPI bytes

C A string showing the SPI device name

D An integer between 0 and 1023 representing the analog value on channel 0

💡 Hint

The MCP3008 returns a 10-bit value from 0 to 1023 for each channel read.

The code sends the correct SPI command to read channel 0 and processes the returned bytes to get a 10-bit integer value representing the analog input.

🧠 Conceptual

intermediate

Understanding MCP3008 SPI Command Structure

Which part of the SPI command byte sequence selects the MCP3008 channel to read?

A The second byte's bits 4 to 6 (shifted by 4) specify the channel number

B The channel is selected by the SPI clock speed

C The third byte contains the channel number

D The first byte always selects the channel

💡 Hint

Look at how the channel number is shifted and combined in the command bytes.

The MCP3008 expects the channel number in bits 4 to 6 of the second byte sent over SPI, shifted left by 4 bits.

🔧 Debug

advanced

Fixing Incorrect MCP3008 SPI Read Output

This code attempts to read channel 3 from MCP3008 but always returns 0. What is the cause?

Raspberry Pi

import spidev
spi = spidev.SpiDev()
spi.open(0, 0)
spi.max_speed_hz = 1350000

adc = spi.xfer2([1, (3 << 4), 0])
data = ((adc[1] & 3) << 8) + adc[2]
print(data)

A The channel number is not offset by 8 in the command byte, causing wrong channel selection

B The SPI device is not opened correctly

C The max_speed_hz is too high causing communication errors

D The bitwise AND operation on adc[1] is incorrect

💡 Hint

Check how the channel number is combined with the start bit in the command byte.

The MCP3008 command requires adding 8 to the channel number before shifting left by 4 to set the start bit and single-ended mode correctly.

📝 Syntax

advanced

Identify Syntax Error in MCP3008 SPI Read Code

Which option contains a syntax error that prevents the MCP3008 SPI read code from running?

A

adc = spi.xfer2([1, (8 + 2) << 4, 0])
data = ((adc[1] & 3) << 8) + adc[2]
print(data)

B

adc = spi.xfer2([1, (8 + 2) << 4 0])
data = ((adc[1] & 3) << 8) + adc[2]
print(data)

C

)atad(tnirp
]2[cda + )8 << )3 & ]1[cda(( = atad
)]0 ,4 << )2 + 8( ,1[(2refx.ips = cda

D

dc = spi.xfer2([1, (8 + 2) << 4, 0])
data = ((adc[1] & 3) << 8) + adc[2]
print(data)

💡 Hint

Look carefully at the commas separating list elements.

Option B is missing a comma between '(8 + 2) << 4' and '0' in the list, causing a syntax error.

🚀 Application

expert

Calculate Voltage from MCP3008 ADC Reading

Given a 3.3V reference and a 10-bit MCP3008 ADC reading stored in variable data, which formula correctly converts data to voltage?

A voltage = (data / 1023) * 5.0

B voltage = (data / 1024) * 3.3

C voltage = (data / 1023) * 3.3

D voltage = (data / 1024) * 5.0

💡 Hint

The MCP3008 outputs values from 0 to 1023 for 10 bits, and the reference voltage is 3.3V.

The maximum ADC value is 1023 (2^10 - 1), so dividing by 1023 and multiplying by 3.3V gives the correct voltage.