PC/CP320 Physical Computing

Raspberry Pi SPI Digital-to-Analog Converter

Overview

Digital transmission of analog data, (such as sound and video), requires analog input signals to be converted to digital signals for processing. Once received, the signals then have to be converted to back analog. These conversions are done by analog to digital (A/D) and digital to analog (D/A) converters, respectively. Although you could construct these converters from discrete components, integrated circuits specifically designed for these purposes are usually used.

Objectives

1. To introduce the SPI interface on the Raspberry Pi
2. To become familiar with digital-to-analog conversion
3. To write a test program to show the SPI D/A in operation

Background

The SPI (Serial Peripheral Interface bus is a synchronous interface allowing a single master device to communicate with multiple slave devices. The master device controls a clock signal for the slaves. The bus consists of two data lines (one for each direction), the clock signal, and select lines for each slave. Although it may not always be mentioned, it's also important that the GROUND lines for the master and slaves be connected. It is now very commonly used to connect sensors to microprocessors or microcontrollers, and so will remain relevant for the forseable future. It's also one of the simplest serial protocols, and so it's easy to study. The Raspberry Pi has an SPI bus built-in to handle up to two slave devices, so it can be used for SPI communications.

Preparation

Equipment

• Raspberry Pi
• TTL-serial cable
• MAX522 SPI digital-to-analog converter
• 0.1 μ F capacitor
• 0.01 μ F capacitor
• oscilloscope

Procedure

As always, do all of the wiring before turning on power to the Raspberry Pi.

The Rapsberry Pi GPIO pins can easily be damaged with improper handling. The DAC in this lab can use a 3.3V supply, so it can be powered from the 3.3V GPIO pin. Now that you are going to be using the 3.3V supply, use blue wires exclusively for 3.3V so that they don't get confused with 5V supply wires.

To wire the DAC, there are 4 different sub-circuits to wire. They are:

• Supply voltages and inputs from Raspberry Pi
• Voltage reference
• SPI connections to Raspberry Pi
• Outputs

You will wire each of these sections in turn.

1. SPI GPIO pins

   Note: The Spidev library has laready been installed.
   
   From the GPIO diagram, identify all of the SPI pins, namely:
   • MOSI
   • MISO
   • SCLK
   • CE0
   • CE1
   
   
   
2. Connect V_DD to the appropriate GPIO pin.
3. Connect GND to the appropriate GPIO pin.
4. Conect the REF input of the DAC to the 3.3V pin of the Raspberry Pi. The output voltage will be proportional to the voltage on the REF input.
5. Conect the Raspberry Pi SPI pins as follows:
   Since communication is only going from the Raspberry Pi to the DAC, which signal line do you need?
   Remember to connect the SPI clock and GROUND lines as well.
   How many slave select lines does the Raspberry Pi have? Be sure your program is correct for the one you choose.
6. Note: The two DACs have different current limits, so the capacitor required depends on which one you're using. See the datasheet for details.
   Connect either the 0.1 μ F capacitor or the 0.01 μ F capacitor from the output of the DAC to GROUND, depending on which DAC you are using.
7. Conect the output to the oscilloscope.
   
   
8. Turn on the Pi and connect to it with the serial monitor.
   
   
9. Write a program to send values to the DAC. Note that every transfer to the DAC should be a two byte transfer.
   Download the test program from the website.
   Replace the question marks with appropriate values as needed.
10. If you have done everything correctly, you should have a circuit which can take in an 8-bit digital value and produce an appropriate analog output between 0 and 3.3 volts.
    What is the significance of the value "49" in the code example? (Hint: Look at the DAC datasheet.)
    Demonstrate your circuit to the lab supervisor.
    Keep this circuit for future labs.
    Don't take it apart.
    
    As always, remember to shut down properly when you are finished.

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