Final Report, Lab 3: Embedded Systems

Digital Potentiometer Subsystem

EE 300W, Section 2, Professor Tim Wheeler

Nicholas Oppman, Joe Mertz, Chris Polasko, Marlon Gil

1

Table of Contents

Table of Contents………...…………………………………………………………………..…1

Abstract…………………………………………………………………………………………..2

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

Rationale…………………………………………………………………………………………4

Implementation…………………………………………………………………………………..6

Verification Testing…………………………………………………………………….10

Validation Testing……………………………………………………………………...11

Results from Integrated Systems…………………………………………………....12

Discussion………………………………………………………………………………………14

Conclusion…………………………………………………………………............................16

Appendices……………………………………………………………………………………..17

Appendix A: Financial Statement…………..………………………………………..17

Appendix B: Gantt Chart……………..……………………...………………………..18

Appendix C: Block Diagrams…………………………………………………………19

Appendix D: Black Box Diagnostics………… ……….……………………………..20

Appendix E: mbed code………………………………………………………………..22

2

Abstract:

Team G Money designed and built a sub-system able to modulate an analog

signal sent from the DAC sub-system with amplitude of 3.3V outputting the specific

voltages requested by the user to the black box. The amplifier uses a standard TL074CN

op-amp that features a voltage divider at the non-inverting terminal in addition to a

digital potentiometer as the feedback resistor, providing the variable gains necessary to

achieve all desired outputs. The digital potentiometer’s resistance is controlled by

hexadecimal commands originating from an mbed device and relayed through a

controller area network (CAN). The mbed serves as the primary tool of communication

to other sub-systems, acting as both the interpreter and the controller of our circuit by

understanding commands sent from other sub-systems, and creating the hex

commands to set the digital potentiometer. The mbed is controlled by C++ coding,

adopting a state machine model, each state outputting a pre-determined hex value

required to achieve a specific output value. Our circuit is powered by a +/- 15V source

and the mbed’s built in 5V source.

3

Introduction:

Today technology is constantly evolving, and improving itself with each new

iteration adding everything from features to reduced size. This level of complexity no

longer allows for a single solution, and instead utilizes the idea of embedded systems,

breaking the main function into many tasks each controlled by a smaller more

manageable sub-systems. Our system will help determine the function of a ‘black box’

by converting a digital signal into an analog signal with set amplitude, scaling this output

to a user requested value and outputting to the black box, the resulting waveform being

converted back to digital to be analyzed this is broken down into four sub-systems DAC,

Digital Potentiometer, Keyboard, and ADC. Team G Money is responsible for providing

the gain to the system. Our sub-system must be able to digitally communicate with the

keyboard sub-system, modulate the DAC subsystem’s output amplitude to 0V no DAC

signal, or to either +1, 2, 3, 4, or 5V which is determined by a user requested command

originating from the Keyboard sub-system. Our sub-system must be able to output

these scaled waveforms to the black box with an accuracy of ± 0.1V.

4

Rationale:

To obtain output voltages from an analog input of 3.3V, the core of our sub-

system utilizes one op-amp that will apply a gain to the input depending on resistor

values. An op-amp was chosen over other methods due to our group’s familiarity with

the device, small footprint, and its simplicity to implement. Previous experience with the

device proved to be very cost effective, allowing for quickened design and

implementation processes. Since an op-amp applies a gain to the input our op-amp

utilizes a voltage divider at the input to scale the original input down to a value much

lower than the lowest required output. This low input voltage can then be amplified meet

all required outputs, including values less than 3.3V. Requiring positive output values,

the input is wired to the non-inverting terminal of the op-amp to achieve a positive gain.

To obtain varying output voltages the sub-system utilizes a digital potentiometer

in series with a fixed resistor connected to ground at the inverting terminal of the op-

amp. The fixed resistor is connected to ground, assuring the input voltage to the

inverting terminal equal to the input at the non-inverting terminal allowing for simpler

calculations while the potentiometer allows for varying feedback resistance, and

variable gain. The digital potentiometer was chosen over a manually adjustable

potentiometer due to its ability to adjust its resistance solely on hexadecimal commands

sent from the mbed device.

We implemented a state machine into the mbed using C++ code, in order to

achieve the required output voltages each state corresponds to the specific

hexadecimal command that sets the digital potentiometer to a value that outputs each

required output voltage. The state machine is very practical to the user, providing a

straightforward approach to achieve the desired outputs. C++ offers a simple way to

implement a state machine, over other languages such as labview.This simple

implementation reduced the time spent on design and implementation, leaving more

time for verification testing on both the circuit and system level, improving accuracy, and

the overall quality for the final product.

The mbed was chosen over more complex devices such as the MyDAQ as our

primary means of digital communication due to its functionality and ease to implement.

Compared to the multi functioning MyDAQ , the mbed also functions as a power source

offering both 3.3V and 5V, while being designed to excel at sending and receiving

commands to and from multiple devices, a feature crucial to meet our requirement for

sub-system communication. Offering both sources reduced the number of instruments

needed for testing, powering parts of our circuit and functioning as the 3.3V input.

Moreover costing around $55 it is also far more cost effective when compared to the

MyDAQ costing around $200. Compared to the cost of the other parts and devices used

5

in our implementation, using the mbed significantly reduced our production costs.

Finally a CAN device was implemented as a transfer device that relays the hexadecimal

commands generated by the mbed to the digital potentiometer due to its ability to

transfer data bi-directionally. Communication between sub-systems could also be done

using a similar method, implementing an mbed device in the keyboard sub-system. This

will generate commands, transferred through CAN devices to the other meds found in

each sub-system. Like the mbed in ours these will control the functions of that particular

sub-system. The first command sent from the keyboard sub-system will determine and

establish communication the appropriate sub-system, the following commands will

dictate what actions that subsystem will do. This connection will be cutoff only when a

user inputs the value pertaining to the main menu, this command will restart the

keyboard, such that its next command will determine another sub-system to

communicate with, and also be sent to the receiving sub-system telling it to stay put and

continue its previous function. This allows the user to individually adjust each sub-

system functions to get the outputs they desire.

6

Implementation:

Our sub-system must be able to output a range of required voltages from a fixed

3.3V analog input from the DAC. To scale the input our sub-system utilizes one op-amp,

this will apply a gain to the input and provide various outputs depending on resistor

values. However, the op-amp will only scale the input with a minimum factor of one, to

accommodate our op-amp utilizes a voltage divider at the input to scale the original

input down to a value much lower than the lowest required output which we chose to be

.5V.

Using a simple voltage division equation, the appropriate resistor values are

determined:

𝑉𝑜𝑢𝑡 = 𝑉𝑖𝑛 ∗ (𝑅2/(𝑅1 + 𝑅2))

Setting Vout= .5V, and Vin= 3.3V, we find R2 = 7500 Ω and R1= 42000 Ω

The lower input voltage now allows us to scale the input to all required voltages,

including values less than 3.3V. We are also required to only output positive voltages,

meaning we must have a positive gain, this is achieved by wiring the voltage divided

input to the non-inverting terminal of the op-amp, creating a simple non-inverting op-

amp.

To vary the output voltages the sub-system features a digital potentiometer in

series with a fixed resistor connected to ground at the inverting terminal of the op-amp.

The digital potentiometer offers a varying feedback resistance, and thus has the ability

to produce a variable gain. The fixed resistor is connected to ground, to assure the input

voltage to the inverting terminal equal to the input at the non-inverting terminal making

calculations simpler.

The fixed resistance value was determined by solving the inverting op-amp gain

equation:

0 = ((𝑉𝑖𝑛(−𝑡𝑒𝑟𝑚𝑖𝑛𝑎𝑙))/𝑅3) + ((𝑉𝑖𝑛(−𝑡𝑒𝑟𝑚𝑖𝑛𝑎𝑙) − 𝑉𝑜𝑢𝑡)/𝑅𝑝𝑜𝑡)

When the potentiometer is maxed we will obtain the largest gain. The maximum

resistance for the digital potentiometer was determined by using a DMM, and found to

be 44694 Ω . So plugging in the related values Vin=.5V, Vout= 5V, and Rpot= 44694 Ω.

We calculate the fixed resistance to be R3= 4966 Ω.

7

To find the required potentiometer resistances that will achieve the other required

output voltages, we plug in the input voltage, fixed resistance and desired output

voltages and resolve the previous equation for Rpot. Which we found to be:

Rpot resistance (Ω) Vout (V)

4966 Ω 1 V

14898 Ω 2 V

24830 Ω 3 V

34762 Ω 4 V

44694 Ω 5 V

Table 1: Rpot resistance for various outputs

Setting the digital potentiometer’s wiper to achieve these values is accomplished

by sending a hexadecimal value to the potentiometer. These values were determined by

using the positioning equation given on the potentiometer datasheet:

Figure 1:Wiper Position Calculation

Our circuit connects PB directly to ground, knowing this we only needed to use

the second equation for Rwb. Plugging in the max resistance Rab, appropriate

constants, and the different values of Rpot, we solved for the appropriate hex value Dn

required for each case which we found to be:

Required Voltage Corresponding Hex Value

8

1V 0xF6

2V 0xC4

3V 0x8C

4V 0x55

5V 0x1C

Table 2: Required hex value to position wiper

The hexadecimal command is generated by an LPC1768 mbed microcontroller.

Code is written to the microcontroller that designates where to position the wiper on the

digital potentiometer.

The code allows communication between the CAN bus, the mbed, and the digital

potentiometer. Information sent from the keypad is received through the CAN bus and

the code that is compiled on the mbed microcontroller will interpret the information

received. The data received from the CAN bus has a specific identification number that

designates which portion of the system it is trying to communicate with. The designation

for our portion of the embedded system was the number 2. It also has a number that will

designate which value of gain to apply if the keypad is communicating with our circuit.

The following excerpts from our code will interpret this received data:

CANMessage msg;

This establishes a variable “msg” that the code will recognize as the data

received on the CAN bus.

if(can1.read(msg)&&(msg.id==2))

This if statement will check if there is a message received on the CAN bus and

also if that message contains the identification number, 2, meaning the keypad is

communicating with our circuit.

If both of these statements are true then the code within the if statement will

execute. Contained in this code is the case structure that allows the value of the digital

potentiometer to be changed by the keypad.The keypad will send data to the CAN bus

that will designate which case to execute. Each case sets the wiper of the potentiometer

at a different position changing the feedback resistance and in turn, the gain. This

portion of code is as follows:

9

switch (*msg.data)

// cases for voltage that the subsystem outputs

case '1':

cs = 0; - Select the digital potentiometer chip

spi.write(0x13); - Write to the command bit of the potentiometer chip

spi.write(0x1C); - Position the wiper of the potentiometer

cs = 1; - Deselect the potentiometer chip

break; - Exit the case structure

The “*msg.data” is the portion of data received on the CAN bus that designates

which case to execute. Within each case there is a line to select the digital

potentiometer chip, a line to write to the command bit, and a line to implement each

change in potentiometer resistance value. The line writing to the command bit is telling

the digital potentiometer to implement the change in wiper position on the chip. Then

the calculated hex value for the specific wiper position is written to the chip for each

case. After the wiper position is set, the chip is then deselected and the switch

statement ends.

10

Verification Testing:

Figure 2: Schematic of System

In order to test our circuit design we used the 3.3V output from the mbed

microcontroller to simulate the input voltage received from the digital to analog

converter. We also powered our operational amplifier with +/- 15V. The results from the

initial testing are shown in Figure below. Once the resistance value for each gain was

calculated and the corresponding wiper position value was found we tested our circuit.

We implemented the value onto the digital potentiometer chip and measured the value

of resistance being executed. After some minor circuit debugging we were able to

obtain each desired resistance value across the digital potentiometer. We then tested

the gain. The case structure was coded onto the microcontroller and we integrated our

circuit with the keypad in order to test the code. The keypad sent commands to our

circuit to change the gain from the input signal of 3.3V to values of 1,2,3,4, or 5V. Each

case was executed and the actual values were recorded and are shown in the chart

below. The values obtained from testing passed the verification test because each value

was within 0.1V of the desired value. This testing verified that our implementation of the

circuit design and code, was now ready to be integrated into the entire system.

Table 3: Testing Results

11

Validation Testing:

The gain circuit was integrated into the system and we began testing. The results

obtained from these tests almost exactly matched our subsystem testing results and

each value was within 2% error of the required value.

Required Voltage Actual Voltage Percent Error

1V 0.98V 2.00%

2V 1.99V 0.50%

3V 2.96V 1.30%

4V 3.97V 0.75%

5V 5.00V 0.00%

Table 4: Accuracy and Percent Error Calculations

The gain portion accurately amplified the square or sine wave that was received

from the digital to analog converter. Our team did many different testing runs, producing

sine and square waves of different frequencies and amplitude. Before running these

signals through the black box to obtain the transfer function, we used an oscilloscope to

view the signals generated by our system. After analyzing the resultant signals on the

oscilloscope we deduced that each portion of the system was functioning properly.We

could change shape, frequency, and amplitude and each change was measured

accurately using the oscilloscope. The accuracy of these produced signals is extremely

important because then our results obtained from the output of the black box will be

more accurate and easier to quantify as a transfer function. After testing the entire

integrated system and analyzing the results, our design was validated.

12

Results from Integrated System:

Appendix D shows graphs made using the data that was imported to Excel after

being recorded in the ADC subsystem. This data was recorded after running the

system for inputs that the black box could receive. We made several observations after

analyzing the graph in Figures. From the graph, we concluded that the output does not

depend on frequency as changing it from 1Hz to 10Hz does not show a shift in phase or

a change in amplitude from the graphs in Appendix D. When changing the voltage of

the input, the output also change when it is the same type of wave and frequency. We

were able to determine a transfer function after seeing how much the output change by

observing that the output was inverted from the input and the peak to peak voltage also

decreased. We observed that the output’s max was one and that the output was the

input multiplied by a gain that was less than one. After analyzing the gain of the

maximum value of the input and the minimum value of the output for each wave from

the data that was imported from the ADC subsystem, we concluded that the gain was

−1

8. From the previous observations, we concluded that the black box had the following

transfer function:

𝑉𝑜 = 1 −1

8∗ 𝑉𝑖𝑛

Where𝑉𝑜is the output of the black box, and 𝑉𝑖𝑛is the input to the black box.

In order to get this transfer function, the team realized that the black box needs to

be a Differential Amplifier, similar to the one shown in Figure 3. By applying the

following equation:

𝑉𝑜 = 𝑉2 ∗ (𝑅𝑔

𝑅2+𝑅𝑔) ∗ (

𝑅1+𝑅𝑓

𝑅1) − 𝑉1 ∗ (𝑅𝑓/𝑅1)

By setting𝑅𝑓

𝑅1=

1

8 and, 𝑉2 ∗ (

𝑅𝑔

𝑅2+𝑅𝑔) ∗ (

𝑅1+𝑅𝑓

𝑅1) = 1, 𝑉1= 𝑉𝑖𝑛 and 𝑉2 = 15 V, the transfer

function above is realized. 15 V is what powers the black box so having a gain of 1

15gives the 1V. These gains are then achieved implementing the appropriate resistor

resistance values in the gain formula for an op-amp.

13

Figure 3: Differential Amplifier

http://en.wikipedia.org/wiki/Differential_amplifier

14

Discussion:

In our approach to building the module our design was effective, not only meeting

all requirements, but utilizing very few parts: one CAN, an op-amp, one digital

potentiometer, and a single mbed device. Not only was our design able to keep the

integrity of the product by meeting all requirements, the minimum of parts allowed for

simpler design and implementation. Cutting the costs of the overall project by reducing

the number of materials needed and reducing the to work hours to design and produce.

The approach to building the DigiPot circuit was also effective. Using the digital

potentiometer as the feedback resistor on the op-amp design proved to be functional

applying all gains necessary to output the required voltages. The simplicity of our design

reduced breadboard clutter allowing for easier debugging, this also makes the core of

our sub-system easy to replicate and thus reducing costs to produce.

Our method to communication from sub-system to the other subsystems was

also effective. Having each sub-system have its own mbed to generate commands

allowed each sub-system to take only the specific commands from its mbed. Reducing

the probability of one sub-system taking multiple commands at once and

malfunctioning. The CAN devices were also an effective method to transfer commands

from within and between sub-systems, sending commands from the keyboard sub-

system to accurately establish communication with one of the various sub-systems, and

sending commands which allow each sub-system to function correctly. The wire

connections and connections between the input and outputs were also straight forward

and effective to establish, it only required us to connect our operational amplifier to a

power supply that supplied 15V and -15V, the CAN transceiver to the other subsystem's

transceivers, the input from the DAQ subsystem and output the signal to the black box

to our subsystem. This simplicity also allowed for simpler debugging. We also chose to

share a common ground which reduced the noise between sub-systems giving better

accuracy in results The mbed and DigiPot were effective because the code that

controlled the subsystem and was easy to modify, requiring only to change a few lines

of code. However, our method of debugging was not effective, despite the simplicity of

our circuit, we spent two lab sessions debugging the DigiPot attempting to move the

circuit to another spot on the breadboard, rewiring the circuit, and changing different

parts of the code. During those two sessions, our output voltage of the subsystem was

not changing when changing the resistance value in the code. We ultimately overcame

this obstacle, by realizing the op-amp was not powered by the correct voltages and

changing the voltage powering the operational amplifier to these values, we also added

a while loop in the code to have our circuit continue to function even without constant

input. The CAN bus was useful as a means to communicate with our subsystem

because the code for the microcontroller only required to add two lines of code in order

15

for the CAN bus to communicate with our mbed. SPI is an alternate method to using

the CAN bus but would have not have been as appropriate since the keyboard only

need to communicate with one subsystem at a time and SPI is more suited to

communicating with multiple subsystems at once.

During the integration of our design, we made two modifications to our code. On

Table 1, there is a column for the adjusted Rpot values determined through testing.

Testing the code, we found that changing the hex value of the D value for the DigiPot by

one (hex) would change the output voltage by a value .02 V and adjustments to the hex

found on Table 2. This slight modification to the hex value, generated all required output

voltages to within +/- .05 V, providing better accuracy than what was required. The other

change we made after testing our subsystem with the keyboard subsystem. The

keyboard subsystem used “9” to go back to the menu in their subsystem. After testing,

our subsystem would not remember the value that the keyboard gave it. This was

because when or subsystem received a value that was not “1” through “5” it would go to

a default value. In order to fix this, we added a while case in the code, shown in

Appendix E, so that when “9” was pressed, it would remember the value and not

change its function.

In this lab, we had a problem involving all team members. Due to being limited to

one computer and workspace. This hindered progress, and limited our work efficiency.

To fix this issue for future labs, we will take time at the beginning to organize an attack

strategy and plan our task and resources, such that every member has a task and

always has something to do. For example some tasks including updating code could be

done outside the lab, leaving the computer free for more testing and research. We will

also have more open communications with the other teams, this will allow us to track

each other’s progress and plan accordingly. It also opens opportunity for more

debugging ideas and can help our efficiency. Although there are some tasks that only

our team could do, it would be beneficial to all of our teams in our system if we were

able to coordinate how our systems would communicate and how to design our

subsystem to better integrate with the other subsystems.

16

Conclusion

Our team, G Money, has created a subsystem modulates a given signal,

receives commands from the keyboard, and outputs the waveform to the black box.

Using the CAN bus and the mbed microcontroller, our subsystem is able to read the

command sent by the keyboard and modulate the signal created by the DAQ using the

non-inverting amplifier made using the DigiPot and the operational amplifier. Our

subsystem is able to modulate the amplitude of the signal to 1, 2, 3, 4, or 5V with an

accuracy 0.1V depending on the command and output it to the black box. In

conjunction with the other teams in our system, our subsystem was integrated with the

other subsystems in order to find the transfer function of the black box. The transfer

function was found after analyzing and comparing the output of the black box to the

input. To improve our subsystem for future work, we could design it so that it does not

output a signal if it has not received a command or if the the keyboard sends a

command to the the ADC subsystem to stop sampling to reduce power consumption

when the system is not gathering data.

17

Appendices:

Appendix A: Financial Statement (Bill of Materials):

18

Appendix B: Gantt Chart:

19

Appendix C: Block Diagrams:

Figure 4: Block Diagram of entire system

Figure 5: Block Diagram of DigiPot Subsystem.

20

Appendix D: Black Box Diagnostics:

Figure 6: 1Hz Sine wave at 4Vpp

Figure 7:1Hz Square wave at 3Vpp

Figure 8:10Hz Sine wave at 5Vpp

21

Figure 9:10Hz Square wave at 2Vpp

22

Appendix E: mbed code:

#include "mbed.h"

SPI spi(p11, p12, p13); // mosi, miso, sclk

DigitalOut cs(p14);

CAN can1(p30, p29);

int main()

// Chip must be deselected

cs = 1;

// Setup the spi for 8 bit data, high steady state clock,

// second edge capture, with a 1MHz clock rate

spi.format(8,3);

spi.frequency(1000000);

CANMessage msg;

while(1)

if(can1.read(msg)&&(msg.id==2))

switch (*msg.data)

// cases for voltage that the subsystem outputs

case '1':

cs = 0;

spi.write(0x13);

spi.write(0x1C);

cs = 1;

break;

case '2':

cs = 0 ;

spi.write(0x13);

spi.write(0x55);

cs = 1;

break;

case '3':

cs = 0;

spi.write(0x13);

spi.write(0x8C);

cs = 1;

23

break;

case '4':

cs = 0 ;

spi.write(0x13);

spi.write(0xC4);

cs = 1;

break;

case '5':

cs = 0 ;

spi.write(0x13);

spi.write(0xF6);

cs = 1;

break;

// needed to keep value after keypad moves to ADC subsystem

case '9':

break;

default:

cs = 0 ;

spi.write(0x13);

spi.write(0x00);

cs = 1;

break;