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.
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