MCP3008 ADC (SPI)

The MCP3008 is an 8-channel, 10-bit analog-to-digital converter in a DIP-16 package. It costs $4 and is the classic way to give a Raspberry Pi analog inputs: the Pi has no ADC of its own, so a potentiometer, an LDR or a soil probe is read through this chip over four SPI wires, turning a voltage into a number from 0 to 1023. The analog inputs primer explains why that is needed in the first place.

Pins

CH0–CH7 are the analog inputs; the wiper of a pot or the middle of a divider goes to one of them. VDD and VREF both go to 3V3 — VREF sets the conversion range and VDD powers the logic — and AGND and DGND join at the Pi's ground. CLK, DIN, DOUT and CS are the SPI side: the Pi's SCLK, MOSI, MISO and CE0.

Wiring it in circuit.py

A potentiometer on channel 0, using the Pi's hardware SPI pins:

from wiring import *

pi = Pi()

pot = Potentiometer(10000)
adc = MCP3008()

pi.V3_3 >> pot.pin3
pot.pin1 >> pi.GND
pot.wiper >> adc.ch0

pi.V3_3 >> adc.vdd
adc.vdd >> adc.vref
adc.agnd >> pi.GND
adc.dgnd >> pi.GND
pi.SCLK >> adc.clk
pi.MISO >> adc.dout
pi.MOSI >> adc.din
pi.CE0 >> adc.cs

The knob + ADC kit is this circuit pre-wired: it exposes vdd, gnd, clk, dout, din and cs, and the pot sits on channel 0.

Driving it in main.py

from gpiozero import MCP3008
from time import sleep

knob = MCP3008(channel=0)

while True:
    print(f"{knob.value:.3f}  {knob.voltage:.2f} V  raw {knob.raw_value}")
    sleep(0.5)

value is the reading divided by VREF (0.0–1.0), voltage is the input in volts and raw_value is the code from 0 to 1023. Pass differential=True to read a channel against its neighbour instead of ground.

Electrical notes

• Supply is 2.7–5.5 V, but power it from 3V3. At 5 V the chip's digital output swings to 5 V, which the SPI pins must not see; VREF may still be 3.3 V, since it only sets the conversion range.
• 10 bits over VREF: code = 1024 × Vin ÷ VREF. With VREF on 3V3 that is one step of about 3.2 mV.
• The model adds the datasheet's errors: a smooth bow of up to 1 LSB across the range (integral non-linearity) plus Gaussian noise of 0.5 LSB σ by default. Both are datafeed knobs; offset and gain errors are not modelled.
• It converts up to about 200 kSPS at 5 V (75 kSPS at 2.7 V), draws 425 µA, and answers in SPI mode 0. The datasheet's other mode, 3, is not modelled: a driver that asks for it gets a warning and a reply shifted by one bit.
• Leave unused channels grounded: a floating input reads whatever its neighbour last held.
• VREF at or below 0.25 V reads 0, which is a handy test — an unwired reference gives all-zero codes.

Simulator vs real hardware

• SPI is decoded at bit level: after CS goes low the chip waits for a start bit, the single-ended/differential bit and three channel bits, samples, then shifts out a null bit, the 10-bit result MSB first and the same bits LSB first — exactly as the datasheet's timing diagram, so gpiozero, spidev and hand-rolled drivers all work.
• The eight channels are also differential pairs (CH0–CH1, CH2–CH3, …), so MCP3008(channel=0, differential=True) reads CH0 minus CH1 like the real chip.
• Conversions are instant: no sample rate, no minimum clock, no droop. On hardware, reading faster than the sample rate starts overlapping conversions.
• gpiozero's MCP3008 uses spidev (port 0, device 0 → CE0) in the simulator. For a different chip select, wire it and drive the transfer yourself.

Going deeper

• Primer: analog inputs and ADCs compares this chip with the ADS1115.
• Analog sources: potentiometer, light sensor, soil moisture.
• Pre-wired modules: knob + ADC, light sensor + ADC, soil sensor + ADC.
• Libraries: gpiozero's MCP3008 and spidev.
• Recipe: the night light reads an LDR through channel 0.

Further reading

• MCP3004/3008 datasheet (Microchip DS21295) — conversions, SPI framing and the channel map.
• gpiozero: SPI devices — MCP3008(channel=0), differential and raw_value.
• Adafruit: MCP3008 SPI ADC — the chip and the pot-to-ADC circuit on a bench.