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

Concept Flow - MCP3008 ADC over SPI

Start SPI Communication

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Send Start Bit + Channel Selection

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MCP3008 Converts Analog to Digital

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Receive 10-bit Digital Data

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Process and Use Digital Value

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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.