MCP3008 ADC over SPI in Raspberry Pi - Step-by-Step Execution

Concept Flow - MCP3008 ADC over SPI

Start SPI Communication

↓

Send Start Bit + Channel Selection

↓

MCP3008 Converts Analog to Digital

↓

Receive 10-bit Digital Data

↓

Process and Use Digital Value

↓

End SPI Communication

This flow shows how the Raspberry Pi talks to the MCP3008 chip over SPI to get analog sensor data as digital values.

Execution Sample

Raspberry Pi

import spidev
spi = spidev.SpiDev()
spi.open(0,0)
channel = 0
resp = spi.xfer2([1,(8+channel)<<4,0])
data = ((resp[1]&3)<<8) + resp[2]

This code opens SPI, sends commands to MCP3008 to read a channel, and gets the 10-bit digital result.

Execution Table

Step	SPI Data Sent	SPI Data Received	Action	Result
1	[1, (8+0)<<4, 0]	[0, 2, 150]	Send start + channel 0, receive 3 bytes	Raw response bytes received
2	N/A	N/A	Extract 10-bit value	((2 & 3) << 8) + 150 = 662
3	N/A	N/A	Use digital value	Value 662 represents analog input voltage
4	N/A	N/A	End communication	SPI transaction complete

💡 All bytes sent and received; 10-bit ADC value extracted successfully

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
spi	Not opened	Opened SPI bus 0, device 0	Opened SPI bus 0, device 0	Opened SPI bus 0, device 0
resp	None	[0, 2, 150]	[0, 2, 150]	[0, 2, 150]
data	None	None	662	662

Key Moments - 3 Insights

Why do we send three bytes [1, (8+channel)<<4, 0] to MCP3008?

The first byte (1) is the start bit, the second byte encodes single-ended mode and channel number, and the third byte is a dummy to clock out the response. See execution_table row 1.

How is the 10-bit ADC value extracted from the received bytes?

The MCP3008 sends 3 bytes; the last 10 bits are split across resp[1] and resp[2]. We mask resp[1] with 3 (binary 11), shift left 8 bits, then add resp[2]. See execution_table row 2.

What does the digital value represent?

It represents the analog voltage level on the selected channel scaled to 0-1023 (10 bits). See execution_table row 3.

Visual Quiz - 3 Questions

Look at the execution_table row 1, what is the purpose of the first byte sent (value 1)?

A Start bit to begin communication

B Channel number

C Dummy byte for clocking

D End of transmission

💡 Hint

Refer to execution_table row 1 under 'SPI Data Sent' and 'Action'

At which step is the 10-bit ADC value calculated?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Check execution_table row 2 where 'Extract 10-bit value' happens

If channel is changed from 0 to 3, how does the SPI data sent change in step 1?

A [1, (3)<<4, 0]

B [1, (8+0)<<4, 0]

C [1, (8+3)<<4, 0]

D [1, (8)<<4, 3]

💡 Hint

Look at how channel is added in the second byte in execution_table row 1

Concept Snapshot

MCP3008 ADC over SPI:
- Open SPI bus on Raspberry Pi
- Send 3 bytes: start bit, channel + mode, dummy
- Receive 3 bytes response
- Extract 10-bit ADC value from response
- Use value to read analog sensors

Full Transcript

This visual trace shows how the Raspberry Pi communicates with the MCP3008 analog-to-digital converter chip over SPI. First, SPI communication starts by opening the bus. Then, three bytes are sent: a start bit (1), a byte encoding single-ended mode and the channel number shifted left 4 bits, and a dummy byte to clock out the response. The MCP3008 returns three bytes, from which the 10-bit digital value is extracted by masking and shifting bits. This value represents the analog voltage on the selected channel scaled from 0 to 1023. Finally, SPI communication ends after reading the value. Variables like spi, resp, and data change as the code runs, showing the process step-by-step. Common confusions include why three bytes are sent, how the 10-bit value is extracted, and what the digital value means. Quizzes test understanding of these steps.

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)