Analog Input Using SPI

Introduction

The Serial Peripheral Interface (SPI) is a very common device communication interface. BeagleBone Black hardware has SPI interface available on the expansion headers. In this example, you interface an ADC chip to BeagleBone Black 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 slave peripheral devices to a single master SPI device. SPI uses three signal lines common to all slave peripherals:

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

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

Enable SPI Peripheral

There are 2 SPI buses available on BeagleBone Black hardware: SPI0 and SPI1. Use showAllPins method to determine the pins on the P8 / P9 expansion header the SPI peripheral is mapped to. Use enableSPI method to enable SPI0. After enabling SPI0, view AvailableSPIChannels property of the beaglebone object to verify that you see new SPI channels:

clear bbb
bbb = beaglebone;
enableSPI(bbb, 0)
bbb.AvailableSPIChannels

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 BeagleBone Black 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 master which in this case is BeagleBone Black hardware. An SPI transaction between MCP3008 and BeagleBone Black consist of 3 bytes. BeagleBone Black hardware sends a byte containing a value of '1' to MCP3008. At the same time, MCP3008 sends a do not care byte to BeagleBone Black hardware. BeagleBone Black 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, BeagleBone Black 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 'spidev1.0'. Perform the SPI transaction as previously described.

clear mcp3008
mcp3008 = spidev(bbb, 'spidev1.0');
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 = beaglebone.mcp3008(bbb, 'spidev1.0');
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.