Analog Input Using SPI

Introduction

A wide variety of sensors output analog voltage levels in response to environmental stimulus. Examples of analog sensors are TMP36 temperature sensor, and photo resistors. Raspberry Pi hardware does not have any built-in analog input ports. Too use analog sensors, you need an external ADC. In this example, you interface an ADC chip to Raspberry Pi hardware using the SPI peripheral.

Prerequisites

Required Hardware

To run this example you need the following hardware:

Overview of SPI

The Serial Peripheral Interface (SPI) is a synchronous serial data link that you use to interface one or more peripheral devices to a single controller SPI device. SPI uses three signal lines common to all peripherals:

Each peripheral is wired to an individual chip-select or peripheral-transmit enable line. During SPI communication, the controller selects only one peripheral device at a time. Any other peripherals on the bus that has not been activated using its chip-select line must disregard the input clock and SDO signals. It also must not drive the SDI line.

SPI communications are full duplex. When the controller sends a bit on the SDO line, the peripheral reads the bit and at the same time sends a bit on the SDI line that is in turn read by the controller. Therefore, to read from a peripheral device, you must write to it.

There are four different SPI modes you can use. Each mode defines a particular clock phase (CPHA) and polarity (CPOL) with respect to the data. Mode 0 (CPOL = 0, CPHA = 0) is the most common use case. The Raspberry Pi hardware has two SPI channels on the board: 0 and 1. It can support up to two SPI peripheral devices. Each SPI channel has a corresponding bus speed that can range from 500 kHz to 32 MHz. The number of data bits that are exchanged in each SPI transaction can vary between 8 to 16 bits with 8-bits being the most common.

View available SPI channels

Create a raspi object and inspect the AvailableSPIChannels property of the raspi object.

clear rpi
rpi = raspi();
rpi.AvailableSPIChannels

If the SPI peripheral is not enabled, you will see an empty cell array. To enable the SPI peripheral use the raspi-config utility that comes with the Raspbian Linux image. AvailableSPIChannels property of the rpi object should yield 'CE0' and 'CE1' SPI channels listed as available.

Overview of MCP3008

MCP3008 is a 10-bit, 8-channel analog to digital converter (ADC) with an SPI interface.

As seen in this diagram, pins 1 through 8 are analog input channels. Pin 16 is the digital supply voltage and pin 9 is the digital ground. and are the pins used for reference voltage level for analog measurements. The digital 10-bit voltage measurement value is scaled so that a value of 0 corresponds to and a value of 1023 (2^10 - 1) corresponding to . Pins 10 through 13 are connections for SPI interface.

Connect MCP3008

Connect MCP3008 to the Raspberry Pi hardware as seen in the following circuit diagram. To simulate a variable voltage applied to CH0, use a 10 kOhm potentiometer connected to CH0.

In this example, the potentiometer (POT) is a three-terminal device with terminals 1 and 3 comprising the end points of a resistor embedded in the POT. The second terminal is connected to a variable wiper. As the wiper moves, the resistance across terminals 1 and 2 and terminals 2 and 3 changes. In this circuit, POT acts as a variable voltage divider. As you move the knob of the potentiometer, the voltage seen at terminal 2 changes between 3.3 Volts and 0 volts.

Measure Voltage at CH0

MCP3008 uses the SPI interface to communicate with the SPI controller which in this case is Raspberry Pi hardware. An SPI transaction between MCP3008 and Raspberry Pi consist of 3 bytes. Raspberry Pi hardware sends a byte containing a value of '1' to MCP3008. At the same time, MCP3008 sends a do not care byte to Raspberry Pi hardware. Raspberry Pi hardware sends another byte to the MCP3008 with the most significant 4 bits containing a value of '1000'. This byte indicates to the MCP3008 that a single-ended voltage measurement at CH0 is requested. At the same time, MCP3008 sends the bits 9 and 10 of the ADC measurement. Finally, Raspberry Pi hardware sends a do not care byte and at the same time reads the least significant 8 bits of the voltage measurement. The 10-bit value read from MCP3008 is then converted to a voltage value.

Read Voltage

To read the voltage value from MCP3008, first create an spidev object connected to SPI channel 'CE0'. Perform the SPI transaction as previously described.

clear mcp3008
mcp3008 = spidev(rpi, 'CE0');
data = uint16(writeRead(mcp3008,[1, bin2dec('10000000'), 0]));
highbits = bitand(data(2), bin2dec('11'));
voltage = double(bitor(bitshift(highbits, 8), data(3)));
voltage = (3.3/1024) * voltage;

The variable voltage holds the voltage value read from the CH0 pin of the MCP3008. You can substitute various analog sensors, such as a TMP36 temperature sensor, in place of the POT in this circuit.

Advanced: Create a MATLAB class for MCP3008

You can create a MATLAB® class for MCP3008 that makes it easy to use this device from MATLAB command-line interface. You can condense the steps in this example using this MATLAB class.

clear mcp3008
mcp3008 = raspi.internal.mcp3008(rpi, 'CE0');
for i = 1:20
    voltage = readVoltage(mcp3008, 0);
    fprintf('Voltage = %0.2f\n', voltage);
    pause(0.2);
end

Experiment with the POT knob while executing the preceding loop. You see that the voltage value printed on the MATLAB prompt changes.

Summary

This example introduced the workflow for working with the SPI interface. You learned how to use MCP3008 ADC to read analog input voltages.