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Digital-to-Analog Converter

Lab 3 Final Report

The Ion Cannons:

Shrinand Aggarwal

Cameron Francis

Nicholas Polito

Section 2

May 1, 2017

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Table of Contents

Introduction………………………………………………………………………………………..3

Rationale…………………………………………………………………………………………..3

Theory of Operation……………………………………………………………………….3

Theory of Interfaces……………………………………………………………………….3

Implementation……………………………………………………………………………………4

Explanation of Code and Circuitry………………………………………………………..4

Verification Testing……………………………………………………………………….4

Validation Testing………………………………………………………………………....4

Results from Integrated System…………………………………………………………...5

Discussion………………………………………………………………………………………....5

Value Statement…………………………………………………………………………………...6

Conclusion………………………………………………………………………………………...6

Appendices…………………………………………………………………………………...........7

Appendix A: Code, Block Diagrams, and Pinouts………………………………………..7

Appendix B: Bill of Materials……………………………………………………………13

Appendix C: Gantt Chart…………………………………………………………...........14

References………………………………………………………………………………..14

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Introduction

In Lab 3, we explored the process of working with embedded systems. It is common in

industry to take a large project and break it into multiple smaller projects in order to create

smaller and easier problems. For this lab, our project was broken into four subsystems that were

integrated together to determine what component was in the black box. The subsystems were the

keypad which controlled the actions done by the system, a Digital-to-Analog Converter, a Digital

Potentiometer, and an Analog-to-Digital Converter. The system diagram can be found in

Appendix A.

Our group, The Ion Cannons, was assigned to the Digital-to-Analog Converter (DAC).

The DAC accepted digital signals from the keypad and outputted an analog signal to the digital

potentiometer. The outputs from the DAC were either a sinusoid or a square wave with 1 or 10

Hz frequency. This was determined by what the input from the keypad was. These inputs

determined the type of wave, the frequency of the wave, and when to start and stop sampling.

The analog signal created by the DAC was significant because it allowed the DigiPot and black

box to manipulate the signal. After the signal was manipulated, the ADC outputted a digital

signal that we used to successfully determine the characteristics of the black box.

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Rationale

The DAC subsystem generates specific waveforms and receives messages from the

keypad.

Theory of Implementation

The main function of DAC is to convert a digital signal to a corresponding analog

sinusoidal or square wave. It allows the user to start, stop, and select the frequency and type of

the output waveform. The waves oscillate at a frequency of 1 Hz or 10 Hz with an amplitude

ranging from 0±0.05 𝑉 to 1±0.05 𝑉.

The DAC communicates with the Mbed LPC1768 microcontroller to output the

waveforms. The waveforms will then communicate with the DAC MCP4725 chip through I2C

serial communication. The converted analog signal from the DAC will then be output as the

input for the DigiPot subsystem.

Theory of Interfaces

The DAC uses the CAN bus interface to read the commands from the keypad. The DAC

and the CAN follow I2C communication protocol. I2C is synchronous and operates on master-

slave format. The CAN uses a 3.3V TTL logic signal to take inputs from the keypad and the

DAC output waveforms to the digital potentiometer.

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Implementation

Explanation of Code and Circuitry

In order to realize the Digital-Analog Converter (DAC) system we used a LCP1768

Mbed Microcontroller, a MCP4725 DAC, and a MCP2551 CAN Bus. The Mbed held the main

code and sent commands to the DAC and the CAN bus to convert and transmit the signals

needed by the keypad. To do this, we connected the DAC to pins 9 and 10 of the Mbed to utilize

the I2C communication protocols. This allowed us to use the Fast 400 kHz mode. We connected

the CAN bus to the CAN rd and td terminals to pins 30 and 29 respectively. This allowed our

circuit to read the commands sent from the keypad.

Using C code as shown in Figure 1, we instantiated the DAC output to achieve the 400

kHz mode of the I2C bus and utilized the functionality of pins 9 and 10. The device address was

determined to be 0x62. The serial instantiation and CAN instantiation allows for communication

between the computer and the Mbed, and between the Mbed and the CAN Bus respectively. We

reserved and defined some variables to keep the code aesthetic.

As shown in Figure 2, the DAC initiates as soon as the can bus sends 1 byte of data.

Next, we coded a 1 Hz sine wave that initiates as soon as the keypad sends a value of ‘2’. The

can bus reads the data from the keypad and starts a for loop nested within a while loop that

creates a 1 volt peak-to-peak wave. This signal is outputted to the DigiPot and modified by that

circuit. The output continues until the stop command is sent to the DAC. In Figure 3, we coded a

10 Hz sine wave that initiates as soon as the keypad sends a value of ‘4’ using the same process

as the 1 Hz wave. For the 1 Hz square wave, we coded a repeating step function from 0 to 1 Volt.

The wave initiates when the keypad sends a value of ‘1’. In Figure 4, we used the same process

to create a 10 Hz square wave when the keypad sends a value of ‘3’. Lastly, we coded an

interrupt to bring all outputs to 0 V, when a value of ‘12’ is received from the keypad.

Verification Testing

Our circuit and code was fully functioning at the end of verification testing. We went

through the code and tested each wave to see if it was reaching the right voltage peak-to-peak

using an oscilloscope. We made sure that the frequency was correct and that there were no lapses

in the output waves. Next, we tested to see how well it communicated with the keypad. The

signal was not being read by our CAN bus, due to incorrectly wiring the received and transmitted

pins. Once resolved, the code responded to all inputs upon command.

Validation Testing

During integration testing, there were no major problems from our DAC module. All

waves were output continuously and stopped upon demand. The Digipot module had a few

problems due to the components frequently malfunctioning. It worked a few times and it was

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able to manipulate all the waves sent out by our DAC. Figure 5 shows all components of the

system during integration testing.

Results

Our DAC group successfully completed our module. We were able to determine that the

black box inverted the signal and modulated the amplitude to be ¼ of the original signal as

shown in Figure 6. The Keypad was able to send all five signals essential to operate the DAC.

We output the signals to the DigiPot to be modified and those signals were input into the black

box. The ADC read the output of the black box and output the voltage values coming from the

black box. The ADC output the data onto a graph that showed the modified wave.

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Discussion

Approach to Module, Network Interface, and Debugging

Our team started working on the project and familiarized ourselves with the operations

and requirements of the DAC. After familiarizing ourselves with the data sheets of the DAC,

microcontroller, and the CAN bus, we observed that I2C communication protocols had to be

used to communicate within the Digital to Analog converter subsystem and CAN bus to

communicate with the keypad.

The debugging process mainly consisted of CAN bus related circuitry. We struggled to

get the right connection for successful communication with other subsystems. After changing

some lines in the code and integrating our DAC with other subsystems, the systems

communicated successfully.

The I2C bus master slave configuration is convenient for intra-circuit communication

whereas the CAN bus is efficient and inexpensive. SPI can also be used to communicate with

other subsystems. In our project, SPI would have not been the best choice since, the keypad

needs to communicate with only one subsystem at a time.

Modification to Design during Integration

The only modification our team made after integration and verification testing was to

change a few lines in the code. Since the DAC’s input depends upon the keypad, it was crucial to

get the right command values. After changing the input values in the code, our DAC successfully

received commands from the keypad and could output the waveforms to the DigiPot.

Changes to be made if Lab was Done Again

There were no major flaws that our team experienced as a whole. But minor changes on

the organization of the project teams might help. Our group was ahead of the schedule and were

waiting on the keypad team to give us the command values required to integrate the two

subsystems. Since the keypad was not ready to communicate with other subsystems, we could

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not test or validate the DAC for a week past the schedule. If there was a set of common ground

rules and protocols, the efficiency of all teams would have improved.

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Value Statement

The DAC subsystem played an integral role in the functionality of the whole system and

the determining of the black box. The outputs were created using the Mbed microcontroller,

Mbed compiler based on C code, CAN bus communication, DAC interface, and a simple

physical circuit. The Mbed online compiler was user-friendly and led to easy coding, testing, and

debugging. The DAC interface and Mbed microcontroller had easy pinouts which helped

simplify our subsystem. Overall, the experience of collaborating with other small projects for the

full system was engaging, interesting, and fun. Our team feels that other people who complete

this lab will find the same benefits.

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Conclusion

The efforts of all four teams working together led us to the successful completion of the

project. Individually, we reached our goals in a timely fashion and verified the functionality of

our modules. The factors that kept us successful were good communication and a strong

commitment to the work at hand. Integration was the most stressful part of the project, but we

believe we all came out as better engineers because of the complications that we faced. We will

take these experiences and learn from them to improve our understanding of technology well into

our professional careers.

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Appendices

Appendix A: Code, Block Diagrams, and Pinouts

Figure 1: First Section of Code

Figure 2: Second Section of Code

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Figure 3: Third Section of Code

Figure 4: Fourth Section of Code

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Figure 5: Integration Testing

Figure 6: Oscilloscope Capture of System Output with Black Box

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Figure 7: Diagnostics System Block Diagram

Figure 8: Communication Requirements

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Figure 9: MBED Pinout

Figure 10: DAC Pinout

Figure 11: CAN Bus Transceiver

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Figure 12: CAN Bus Pinout

Figure 13: Black Box Pinout

Figure 14: Black Box Pin Specifications

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Appendix B: Bill of Materials

Item Price

Breadboard $20.00

120Ω Resistor $0.01

LCP1768 MBED Microcontroller $64.00

MCP4725 DAC $4.95

MCP2551 CAN Bus $1.22

TOTAL PRICE $90.18

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Appendix C: Gantt Chart

Section Task Week

1

Week

2

Week

3

Week

4

Week

5

Week

6

Week

7

Introduction Understanding

the MBED

Team

Inventory

Individual

Subsystem

Learning about

DAC

Programming

DAC

Building DAC

Circuit

Testing DAC

Programming

CAN Bus

Testing CAN

Bus

Entire

Subsystem

Integration of

all subsystems

Testing entire

system

Wrap-Up Demonstration

CDR writing

References

EE 300W Canvas Page and Resources.

Online Mbed compiler. <https://developer.mbed.org/compiler/>

CAN Bus Datasheet. <http://ww1.microchip.com/downloads/en/DeviceDoc/21667f.pdf>