IC and Sockets

The Raspberry Pi is a great computer but there's just one thing that most hardware hackers will find missing: a few inputs for connecting analog sensors. This is where the MCP3008 chip comes in handy, with eight 10-bit channels accessible over SPI. Thanks to the Raspberry Pi's SPI interfaces and its two 'Chip Select' pins available off of the main GPIO connector, the problem is solved.

Features

• 10-bit resolution
• ± 1 LSB max DNL
• ± 1 LSB max INL
• 8 input channels
• Analog inputs programmable as single-ended or pseudo-differential pairs
• On-chip sample and hold
• SPI serial interface (modes 0,0 and 1,1)
• Single supply operation: 2.7V - 5.5V
• 200 ksps max. sampling rate at VDD=5V
• 75 ksps max. sampling rate at VDD=2.7V
• Low power CMOS technology
• 5 nA typical standby current, 2 µA max.
• 500 µA max. active current at 5V
• Industrial temp range: -40°C to +85°C

Applications

• Sensor Interface
• Process Control
• Data Acquisition
• Battery Operated Systems

How-To

The following sample assumes that you're running the latest official Raspbian distribution, that you're logged in as root and that you haven't yet customized Raspbian to use SPI from a Python script. By default, the kernel module managing SPI communications is not loaded on Raspbian, so let's ensure that the 'spidev' device is available.

[email protected]:/# lsmod
Module                  Size  Used by  
snd_bcm2835            12808  0  
snd_pcm                74834  1 snd_bcm2835  
snd_seq                52536  0  
snd_timer              19698  2 snd_seq,snd_pcm  
snd_seq_device          6300  1 snd_seq  
snd                    52489  5 snd_seq_device,snd_timer,snd_seq,snd_pcm,snd_bcm2835  
snd_page_alloc          4951  1 snd_pcm  

If 'spidev' device is not listed, we need to remove it from the kernel module 'blacklist' so that it's automatically loaded on the next boot:

[email protected]:/# nano /etc/modprobe.d/raspi-blacklist.conf  

Put a hash character in front of the 'blacklist spi-bcm2708' statement and save the file:

#blacklist spi-bcm2708  

then, manually load the SPI device:

[email protected]:/# modprobe spidev  

The command should just complete without returning an error. Validate the list of modules:

[email protected]:/# lsmod  
Module                  Size  Used by  
spi_bcm2708             4401  0  
spidev                  5136  0  
snd_bcm2835            12808  0  
snd_pcm                74834  1 snd_bcm2835  
snd_seq                52536  0  
snd_timer              19698  2 snd_seq,snd_pcm  
snd_seq_device          6300  1 snd_seq  
snd                    52489  5 snd_seq_device,snd_timer,snd_seq,snd_pcm,snd_bcm2835  
snd_page_alloc          4951  1 snd_pcm  

The 'spidev' device should now be available.

Next, install the 'git' package. It will be needed to install other packages from Github.

apt-get install git-core  

Install the 'python-dev' package. It will be needed to compile the 'py-spidev' package.

apt-get install python-dev  

Download and compile the 'py-spidev' package.

[email protected]:/# cd /home  
[email protected]:/home# git clone git://github.com/doceme/py-spidev  
[email protected]:/home# cd py-spidev/  
[email protected]:/home# python setup.py install  

At this point, SPI is available from Python 2.7. Assuming that you have wired the MCP3008 to the Raspberry Pi's SPI bus using CE0 as the 'chip select' pin, you can read all eight analog inputs using the following Python script.

[email protected]:/home# nano adc.py  

Paste the script below in the editor and save it.

import spidev  
import time    
spi = spidev.SpiDev()  
spi.open(0,0)    

# read SPI data from MCP3008 chip, 8 possible adc's (0 thru 7)  
def readadc(adcnum):
          if ((adcnum > 7) or (adcnum < 0)):
                  return -1
          r = spi.xfer2([1,(8+adcnum)<<4,0])
          adcout = ((r[1]&3) << 8) + r[2]
          return adcout

while True:
          for adcInput in range(0,8):
                  print "ADC(", adcInput,")=", readadc(adcInput)
          time.sleep(1)  

To execute the script

[email protected]:/home# python adc.py  

Sample output using a 10K Ohm trimpot connected to the ADC's pin 3 and all other pins pulled down to ground using a 10k resistor.

ADC( 0 )= 0  
ADC( 1 )= 0  
ADC( 2 )= 0  
ADC( 3 )= 507  
ADC( 4 )= 0  
ADC( 5 )= 0  
ADC( 6 )= 0  
ADC( 7 )= 0  

Credits go to Jeremy for his work on the 'py-spidev' Python module which makes working with the MCP3008 straight forward. Alternatively, checkout wiringPi's SPI functions if you prefer using a 'C' API.

Related Tutorials & Samples

• Loading I2C, SPI and 1-Wire drivers on the Raspberry Pi
• Jeremy's Blog: Raspberry Pi hardware SPI analog inputs using the MCP3008
• wiringPi 'C' API for interfacing with SPI, I2C, GPIOs and more
• Adafruit Learning System
  • Save and Graph Analog Temperature Data
  • Converting analog inputs to digital and control audio volume