Raspberry PI - Adding analogue inputs using MCP3008, MCP3004, MCP3208, MCP3204

These chips are analogue to digital converters with 8 inputs, they can read values from POT's, NTC sensors and other resistive passive sensors. The MCP3008 is 10 bit resolution and the MCP3208 is 12 bit resolution. There is also MCP3004 and MCP3204 these are the same but with 4 inputs. To connect to the Raspberry PI you need to enable the SPI (Serial Peripheral Interface) chip interface. The chip reads eight parallel inputs and converts them to a serial data stream. It is possible to chain more than one chip using this interface to increase the amount of inputs or use the another chip for outputs (74HC595) or digital inputs (74HC165). 

Install and enable the SPI interface

1. First make sure the system is up to date
   sudo apt-get update
   sudo apt-get upgrade
   sudo reboot
2. To install the development software that includes spi_dev type the following command. Note:Raspberry PI needs internet connection for this to work.
   sudo apt-get install python-dev python3-dev
   The command will indicate the package size before installing. Then install the spidev for Pyhon with the below commands.
   cd ~
   git clone https://github.com/doceme/py-spidev.git
   cd py-spidev
   make
   sudo make install
3. Enable the SPI interface 
4. sudo raspi-config select Advanced the enable SPI and set to load by default.

5. Reboot the Raspberry PI
6. sudo reboot
7. To check that the interface is enabled type lsmod and check that spi_bcm2835 is listed.

Chip connections

MCP3004/ MCP3204	MCP3008/ MCP3208	Signal	Description
1	1	CH0	Analogue Input 1
2	2	CH2	Analogue Input 2
3	3	CH2	Analogue Input 3
4	4	CH3	Analogue Input 4
-	5	CH4	Analogue Input 5
-	6	CH5	Analogue Input 6
-	7	CH6	Analogue Input 7
-	8	CH7	Analogue Input 8
5	-	NC	No Connection
6	-	NC	No Connection
7	9	DGND	Digital Ground
8	10	CS/SHDN	Chip Select/Shut Down
9	11	DIN	Digital Serial Input
10	12	DOUT	Digital Serial Output
11	13	CLK	Clock
12	14	AGND	Analogue Input Ground
13	15	VREF	Reference Voltage
14	16	VDD	Positive Supply Voltage

Chip Communication 

The input values are read and stored in chip addresses continuously. CLK is the clock for communication timing. The CS pin starts high then pulled low to enable to chip data to be read. The address of the input to be read is sent to DIN (MSB IN) and the requested data is sent to DOUT (MSB OUT) after a null data bit. The output of the MCP3008/4 will be 0 to 3FF hex and the MCP3208/4 output will be form 0 to FFF hex. For more details download the detailed data sheets MCP3004, MCP3008, MCP3204, MCP3208.

Wiring Connections

The wiring drawing shows the MCP3008 connections. Connections to all the chips are listed in table below:-

MCP3004/ MCP3204	MCP3008/ MCP3208	Signal	Description	GPIO	RPI Pin
1	1	CH0	10K NTC Sensor	-	-
2	2	CH1	10K POT	-	-
7	9	DGND	Digital Ground	GND	6
8	10	CS/SHDN	Chip Select/Shut Down	CS0	24
9	11	DIN	Digital Serial Input	MISI	19
10	12	DOUT	Digital Serial Output	MISO	21
11	13	CLK	Clock	SCLK	23
12	14	AGND	Analogue Input Ground	GND	6
13	15	VREF	Reference Voltage	3.3 Volts	1
14	16	VDD	Positive Supply Voltage	3.3 Volts	1

The drawing shows just two inputs connected. A 10KΩ POT is connected to GND and 3.3 volts and the wiper is connected to the analogue input 2. Wiring a sensor requires a resistor of the same value as the sensors reference value in this case 10KΩ. The sensor is connected to analogue input 1 and GND and the resistor is also connected to input 2 and 3.3 volts.

Python Code

import spidev

import time
import math

spi = spidev.SpiDev()

spi.open(0,0)

#Read SPI Sensor input 0 to 7 for MCP3008
def ReadInput(Sensor):
  adc = spi.xfer2([1,(8+Sensor)<<4,0])
  data = ((adc[1]&3) << 8) + adc[2]
return data

#Convert Voltage to Resistance
def VoltToOhm(V):
  Rd = 10000
    Rd_effective =(Rd * 16800.0) / (Rd + 16800.0)
  Ohm = (3.3 - V) / V * Rd_effective
return Ohm

#Convert resistance to temperature
def OhmToCelsius(ohms):
    #NTC Sensor Characteristics
    A = 0.00335401643468053
    B = 0.0002744032
    C = 0.000003666944
    D = 0.0000001375492
    r = math.log(ohms/10000.0)
    temp = 1.0 / (A + B*r + C*r**2 + D*r**3)
    temp = temp - 273.15
    return temp

while True:

    for i in range(0,2):

    RawValue = ReadInput(i)    #Read the raw input from the chip

    percent = (RawValue/10.24) #Convert to a percentage

  Voltage = (percent /100.0)*3.3 #Convert % to a 0 to 3.3 Volts

  if (Voltage > 0): #Skip If 0 to avoid divide by zero error

  Ohms = VoltToOhm(Voltage) #Convert Voltage to resistance

DGC = OhmToCelsius(Ohms) #Convert to temp in Celsius

  else:

  Ohm = 0.0

  DGC =-50.0

  #Format values to 2 decimal places

PCTStr = ",PCT={0:.2f}".format(percent)+"%"

VoltsStr = ",Volts={0:.2f}".format(Voltage)+"V"

OhmsStr = ",Ohms={0:.2f}".format(Ohms)+"ohms"

DGCStr = ",Temperature={0:.2f}".format(DGC)+"DGC"

print "Sensor No "+str(i),"Raw="+str(RawValue), PCTStr, VoltsStr, OhmsStr, DGCStr

time.sleep(2) #Delay to slow down display for use humans

The SPIDEV function is imported to communicate with the MCP3008. I have also imported time for delays and math for the conversion to temperature. The SPI interface is defined and opened on channel 0 at start-up. The ReadInput function reads an input from the chip and returns the raw value. The VoltagetoOhm function converts the calculated voltage to a resistance value. OhmToCelsius converts the resistance to a temperature. In the main program the for loop reads the first two sensors (could be changed depending on the inputs used and chip used). Percentage, Voltage etc.. are calculated and displayed for the two inputs. 

The program output

NTC Sensor Characteristics

The sensor characteristics can sometimes be obtained from the supplier but not always. Here is a table of the sensors I have found and there characteristics. There is no guarantee but this helped me to get sensors reading reliably. When testing I would recommend testing values around the target temperatures against a know sensor. The value may need an offset or +/- 10 on the final value. The below are for 10K NTC sensors if you don't know the NTC sensor number just try each one until you get the best fit.

NTC Sensor	A	B	C	D
3490	0.003354016	0.000292425	3.45753E-06	2.33526E-07
3590	0.003354016	0.000255056	1.62764E-06	2.21525E-07
3740	0.003354016	0.000274403	3.66694E-06	1.37549E-07
3977	0.003354016	0.000256985	2.62013E-06	6.38309E-08
3435	0.003354016	0.000307404	1.01915E-05	9.09371E-07
3960	0.003354016	0.000255056	1.62764E-06	2.21525E-07
3610	0.003354016	0.000282875	2.98965E-06	4.84086E-08
3570	0.003354016	0.000286452	3.25226E-06	4.5945E-08
3940	0.003354016	0.000257077	1.89389E-06	1.89729E-07
3430	0.003354016	0.000303944	6.31344E-06	-6.85454E-08
3984	0.003354016	0.000256524	2.60597E-06	6.32926E-08
4090	0.003354016	0.000249339	2.2881E-06	7.98311E-08
3435	0.003354016	0.000300131	5.08516E-06	2.18765E-07