Digital-to-Analog

Converter subsystem

T e a m m e m b e r s :

D e n g f e n g L i

E r i c H i l l

G o n g h a o S u n

2 0 1 4 / 5 / 3

Team Johnny Bravo

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Table of Contents

ABSTRACT: ............................................................................................................................................... 2

INTRODUCTION: ...................................................................................................................................... 2

RATIONALE: ............................................................................................................................................. 2

IMPLEMENTATION ................................................................................................................................... 3

CODE AND CIRCUITRY ........................................................................................................................................... 3

1. Main Function ...................................................................................................................................... 3

2. DAC Function ........................................................................................................................................ 4

3. Log function .......................................................................................................................................... 5

BLOCK DIAGRAM ................................................................................................................................................. 6

FLOW CHART ....................................................................................................................................................... 6

VERIFICATION TESTING .......................................................................................................................................... 7

1. Description (and sketch) of sensor and relation to overall system ...................................................... 7

2. Module and interface schematics (complete with mbed) .................................................................... 8

3. Method of testing and verification of requirements ............................................................................ 8

4. Testing results (plots, tables, etc.) ........................................................................................................ 9

5. Percentage Error Analysis .................................................................................................................. 11

VALIDATION TESTING .......................................................................................................................................... 11

RESULTS FROM INTEGRATED SYSTEM ..................................................................................................................... 11

DISCUSSION ........................................................................................................................................... 12

VALUE STATEMENT ................................................................................................................................ 14

CONCLUSION ......................................................................................................................................... 14

APPENDIX .............................................................................................................................................. 14

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Abstract:

A diagnostic suite was designed using 4 mbed controllers and related circuitry to analyze the

input-output response of a “black box” circuit. This suite consists of a DAC subsystem, digital

potentiometer subsystem, ADC subsystem, and keypad subsystem. The keypad is the control

subsystem that communicates to the others through a common CAN bus. Team “Johnny Bravo” was

responsible for the DAC subsystem, which outputs sine and square waves with an amplitude of 3.3 v,

with frequency of either 1 or 10 Hz, all controlled by the keypad subsystem through the CAN bus.

Introduction:

In this lab, a system of systems was created using 4 mbed controllers and accessory circuits that

control the input and characterize the output of a “black box” circuit. The subsystems for the

diagnostic suite consist of a DAC, digital potentiometer, ADC, and keypad control. The DAC is to

put out a sine wave or square wave with a frequency of either 1 Hz or 10 Hz, at 3.3 V, the digipot

then shifts the amplitude of this wave to levels of 1, 2, 3, 4, or 5 v. This signal is then input into the

black box circuit, and the resulting waveform outputted is written to a CRV file on a computer for

characterizing. The DAC subsystem’s importance in this system is to create a waveform of

appropriate shape and frequency which is then used to excite the “black box” circuit, so that it

generates an output that can be characterized.

In order to accomplish these criteria, our group created code for the mbed controller that outputs the

appropriate waveform depending commands addressed to it through the CAN bus. The output waves

themselves are selected using a switch-case structure, the case that is selected depending on the input

commands.

Rationale:

The DAC that team “Johnny Bravo” was charged with building is essentially a digital signal

generator with settings that can be controlled by the user through a central keypad attached to a CAN

bus. Per the design requirements for the lab, this DAC signal generator was required to output a sine

wave and a square wave, each with selectable frequencies of 1 Hz or 10 Hz. These analog

waveforms would then be scaled by a digipot module, and then fed into the circuit of interest to see

how the circuit would respond.

In order for our module to receive commands for the central keypad module, it needed to be hooked

up to some kind of common data bus. The data bus protocol specified for this lab was CAN

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(Controller Area Network). CAN allows multiple independent microcontrollers, such as mbeds, to

communicate to each other asynchronously, which makes integrating subsystems into a larger system

easier. In order to prevent messages from colliding with each other on the CAN network, CAN

implements a sort of bit dominance, in which the logic 0 is the dominant bit and logic 1 is the

recessive bit. When two nodes start transmitting CAN messages at the same time, the node with the

more dominant (i.e. numerically smaller) ID will override the other nodes message.

Implementation

Code and Circuitry For the code of DAC subsystem, there are 3 main parts including the main function, DAC

generate function, and log function.

1. Main Function

Figure 1: Main Function Code

The main function code is showed in Figure 1 above. We can receive a 3-byte code sent by the

keypad to control the switch, wave form, and frequency of our module via the CAN bus. The

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keyboard subsystem first sends an ID number to the CAN bus. This ID number is then compared

with our group ID of 2, and if it matches only then is the CAN message data read and acted upon.

In the case of our module, the first three characters after the message ID controlled the setting.

The protocol for this is listed below in figure 2.

Value

Byte 0 1

msg.data[0] Switch OFF Switch On

msg.data[1] Square wave Sine Wave

msg.data[2] 10 Hz 1 Hz

Figure 2: protocol

2. DAC Function

Figure 3: DAC function code

In order to generate two waveforms at different frequency, our group used a while loop to

generate the required waveforms in real-time in unison with a microsecond counter. We set the

frequency by changing the time period of the waveforms. The time period would be 0.1 second

for the frequency of 10 Hz while the time period would be 1 second for the frequency of 10 Hz.

Our group used the following methods to generate the sinusoid waves and square waves.

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For the square wave, the voltage changed every half time period. To accomplish this, we used the

modulus operator to calculate the remainder and determine how far into a cycle the loop was.

The time unit is set in microsecond since we are looking for the most accurate waveform after the

delay. When the frequency is set at 1 Hz, the voltage is equal to 3.3 volts if the reminder is less or

equal to the 0.5 seconds. We can also regard it as the first half part of one time period. The

voltage is equal to 0 volts when the remainder is greater than the 0.5 second. The principle also

works when the frequency is set at 10 Hz, but instead we need to compare the value with 0.05

seconds.

For the sinusoid wave, since the range of the sine wave is from -1 to 1, we add 1 in the function

in order to shift the wave above the x-axis. So, the range of the sine wave is changed to 0 to 2

volts and then we produced the 3.3 volts peak to peak sine wave by multiplying a factor of 3.3/2.

3. Log function

Figure 4: log function code

A log function was created to send data back to the computer to confirm proper operation. It

takes formatted string as parameters, which is convenient for us to debug the program if we did

not get the message on the computer.

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Block Diagram

Figure 5: Block Diagram

Flow Chart

Figure 6: Flow Chart

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Verification Testing

1. Description (and sketch) of sensor and relation to overall system

In this project, four groups cooperated together to complete the overall system including the

keyboard subsystem, Digital-to-Analog Converter subsystem, Digital Potentiometer, and

Analog-to-Digital Converter. The keyboard subsystem controls the settings and parameters of the

other three groups. The keypad subsystem sends commands to the DAC subsystem to start/stop

and select the waveform, to the Digital Potentiometer subsystem to select waveform amplitude,

and to the ADC subsystem to start/stop sampling. Also, the keypad subsystem receives user input

from a digital keypad, which is then interpreted into the commands via the mbed’s coding. The

CAN bus, as a media, transfers this information among the various subsystems. For our DAC

group, the physical integrated circuit is showed below:

Figure 7: Physical circuit for connecting CAN bus with Microcontroller

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2. Module and interface schematics (complete with mbed)

Figure 8: Module and interface schematics (complete with mbed)

3. Method of testing and verification of requirements

For the method of testing, we connected the output of DAC converter chip to the oscilloscope to

verify whether the waveform and frequency matched up with the desired values. The ideal

waveforms and frequencies were sinusoid waves at 1 Hz and 10Hz, and square waves at 1 Hz

and 10 Hz. The requirements for verification were that the error should less than 10%, which

means our frequencies should be in the range from 0.9, Hz to 1.1 Hz for 1 Hz and 9 Hz to 11 Hz

for 10 Hz.

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4. Testing results (plots, tables, etc.)

Figure 9: Sinusoid wave at 1 Hz

Figure 10: Sinusoid wave at 10 Hz

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Figure 11: Square wave at 1 Hz

Figure 12: Square wave at 10 Hz

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5. Percentage Error Analysis

%error for sinusoid wave at 1 Hz: (1 – 0.92) / 1 = 8%

%error for sinusoid wave at 10 Hz: (10 – 9.2) / 10 = 8%

%error for square wave at 1 Hz: (1 – 0.93) / 1 = 7%

%error for square wave at 1 Hz: (10 – 9.3) / 10 = 7%

The percentage errors are all less than 10% which means our results are valid for the following

experiment.

Validation Testing The requirements for our Digital-to-Analog Convertor Subsystem are:

Create square and sinusoidal waves at 1 Hz and 10 Hz spanning 0 to +3.3 V.

Receive commands from the Keyboard subsystem to stop/start and select the waveform.

As the verification test showed above, our DAC subsystem meets the requirements. (Figure 9-12)

we can generate square and sinusoidal waves at 1 Hz and 10 Hz spanning 0 to 3.3 V. Also, we

are able to successfully receive commands from the Keypad Control group to select desired

waveform and frequency. The success of our group also paves the way for the Potentiometer

group in order to let them rearrange the amplitude from 1 V to 5 V. It is significant to acquire

square and sinusoidal waves at 1 Hz and 10 Hz from 1 V to 5 V for the analysis of the black box

since we will get 20 sets of data to figure out what exactly the black box is.

Results from Integrated System

A series of diagnostic tests using the integrated system were carried out on a “black box” circuit

of unknown contents. Based on the response to these tests, the black box’s transfer function was

determined to be approximately:

This equation was determined by examining the output of sine and square waves at frequencies

of 1 Hz and 10 Hz and amplitudes of approximately 1, 2, 3, 4, or 5 V. In actuality, due to

limitations of the DAC equipment and the digipot design, the actual amplitudes of the inputs

were slightly different from the ones listed here. Regardless, the gain of the “black box” was still

easily found from these values. A sample of the graphs used for this analysis is available in the

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appendix of this report, with the average values of the input waves listed in the graphs (Figure 16

and 17).

Based on the transfer function of the black box circuit, it was inferred that the circuit was likely

an inverting op amp with a fractional gain of 0.1, with a positive offset of 1 volt achieved by the

application of 1 volt to the non-inverting terminal. A possible diagram of this configuration is

available below:

Figure 13: Schematic of Black Box

Discussion

Our system needs to receive the command from Keyboard system through CAN bus; then we select

MCP2551 (High-Speed CAN Transceiver) as our receiver because it can be driven by 5 volt with

low current standby operation and high transmission rate up to 1 Mb/s. In the implementation, we

use the ticker function to repeatedly check the CAN bus message each second and if there is a

message in the CAN bus with the correct ID we extract the commands from the message. In addition,

we implemented a logging system and simple LED indicator for debugging. The CAN bus is really

effective. Whenever there is a message on the CAN bus, the LED on our mbed will flash and, if the

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message is sent to our group, we will use the logging system to send the command and our system’s

current status back to the terminal on the PC through USB serial port. This logging system helps us

to debug our system efficiently. Compared to the CAN bus, there is also an alternative called SPI

(Serial Peripheral Interface) bus that could have been used in the project. SPI bus is a synchronous

serial data link which is used for short distance, and single master communication. There is one

master device communicating in master/slave mode with other slave devices in order to transfer

information. However, the disadvantage of using the SPI is that there are many existing variations,

making it difficult to find development tools like host adapters that support those variations. Also,

the cost of CAN bus is cheaper than the SPI bus. Considered the factors above, CAN bus is the first

pick for our project.

We optimized accuracy and smoothness of the output signal. Originally we delay milliseconds to

simulate a time counter; then we change the interval to microseconds, which vastly improve the

amplitude accuracy. But there is a tradeoff between the accuracy of the amplitude and the accuracy

of the frequency. On the one hand, the higher delay resolution, the higher amplitude accuracy. On the

other hand, the higher delay resolution, the lower frequency accuracy. Through large amount of

testing, we find that setting time interval to 1000 microseconds is the equilibrium point.

During the lab time, our team members read the assignment handout first. Then for the diagnostics

system block diagram, we clarified that our DAC subsystem received command from the keyboard

subsystem and sent out the analog signal wave through CAN bus. We made consensus on time

schedule and planned to try to finish this lab in 5 weeks. We separated work on this project and had

extra group meeting besides the regular lab time in order to make forward steps prior to the time

schedule. Our group also communicated well with the other groups. We came up with a way to

follow the order through the CAN bus by the keypad control group. Instead of broadcasting the

message with unique commands for each group, each group is assigned a unique ID and each

message only carries the commands to one group. Our group ID is 2 and therefore we only care

about the message with group ID of 2. As for communicating with other groups, they all have good

habits on recording the lab’s progress on the notebook. This is the greatest think we can improve on

and we could better perform if we had done this lab again.

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Value Statement

Throughout this lab3, we had a taste of system engineering, which is a crucial part in our future

careers. Since each subsystem may communicate with other groups, we develop a high quality

protocol so that groups can implement their subsystem independently by strictly following the

protocol.

From this assignment, we improved our programming style by separating the code into different files

so that as the code increases in complexity, others can still easily read and continue working on the

code. After having to solve the dilemma between signal accuracy and signal smoothness, we will pay

more attention to the chip’s specification to make sure we can select the correct chip to avoid any

design conflict in our next project. Furthermore, in our DAC subsystem, the log system that detects

message on the CAN bus as well as tracking our system’s current status, and sends the log

information back to PC through serial port, reduces our debug time vastly during the implementation

and whole system’s integration.

Conclusion

It has been shown that our DAC subsystem can successfully receive commands from the keyboard sy

stem and produce desired output signal. After integration and verification test, the diagnostic suit wa

s able to consistently operate. The cumulative error, the amplitude of the signal that sent to the black

box deviate from the requirements, draws attention to us. In conclusion, this embedded system impro

ves our skills in system engineering design and communication among team members.

Appendix

Name lists Prices per Unit Quantities Costs

Breadboard $35.00 1 $35.00

CAN bus (MCP2551) $2.11 2 $4.22

DAC convertor (MCP4725) $4.95 1 $4.95

Microcontroller (LPC1768) $49.95 1 $49.95

1-kΩ resistors $0.10 2 $0.20

Power Supply Cables $3.59 3 $10.77

Jumper Wires $6.50 1 $6.50

Engineer Time $35.00 3x20hrs $2100

Total Costs: $2211.59

Figure 14: Bill of Materials for the DAC Subsystem

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Stages Week 1 Week 2 Week 3 Week 4 Week 5 Week 6 Week 7

Team Inventory

Subsystem Block

Diagram

mbed Code Flow Chart

Testing Plan

Build

Integration and Testing

Black Box Diagnostics

Final Report

Figure 15: Gantt chart

Figure 16: Response to listed wave (frequencies are identical)

Figure 17: Response to listed wave (frequencies are identical)

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400 500

Sine wave, amp = 2.47, f = .881 Hz

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20 25 30

Square Wave, f = 9.1 Hz, Amp = 2.34