Analog And Digital Interfacing

Network and Systems Laboratorynslab.ee.ntu.edu.tw


Big Picture

Physical world

MSP430

Analog

Interfaci

ng

Other Systems or

ICs

Digital

Communicatio

n


SensorsA device that converts a physical

phenomenon into an electrical signalPhysical phenomenon

light, temperature, humidity, pressure, and etc.Electrical signal

resistance, capacitance, current, voltage, and etc.

Maps a physical phenomenon change to a electrical signal change


Signal Path

Sensors Signal Conditioning

(if necessary)

Convert to voltage

• Amplification• Filtering

• Light• Temperature• Acceleration• Humidity• Pressure• etc.

• Resistance• Capacitance• Current• Voltage• etc.

Analog to Digital

Conversion

(ADC)

10011101

Produce a proper output

voltage level

Converts voltage to

digital number


Sensors We HaveRaw sensors

Produce raw electrical signal

Signal conditioned sensorsHave some signal conditioning circuitProduce analog output

Mostly Voltage Sometime Current


Sensors We HaveDigitalized sensors

Have some signal conditioning circuitConvert analog to digital internallyProvide digital output


Our Focus

Sensors Signal Conditioning

(if necessary)

Convert to voltage

• Amplification• Filtering

• Light• Temperature• Acceleration• Humidity• Pressure• etc.

• Resistance• Capacitance• Current• Voltage• etc.

Analog to Digital

Conversion

10011101

Produce a proper output

voltage level

Converts voltage to

digital number

Sensors

• We are not going to talk about signal conditioning • You can find many sensors that are signal conditioned and provide a proper analog output, or even digitalized•We will focus on how to understand these signal conditioned sensors and get the output we want


Some Raw SensorsTemperature Sensors

Thermistors Temperature-sensitive resistor

RTDs (resistive temperature devices) Temperature-sensitive resistor

Thermocouples Temperature => voltage (mV)

Pressure sensorsPiezoresistive

Resistance change with applied pressure


PhotodiodesLight => current

Generate current proportional to light density


PhotodiodesTwo on Taroko

S1087: for visible rangeS1087-01: for visible to IR(infrared) range


Signal Conditioned Analog OutputProximity sensor

Sharp GP2D120XJ00FAnalog voltage output

AccelerometerADXL330 3-axis accelerometerAnalog voltage output

Industry standard analog outputFlow sensors, pressure sensors, gas sensors,

etc.4~20 mA, 0~5 V, 0~10 V


Proximity SensorSharp GP2D120XJ00FOutput voltage

proportional to the reflection distance

Measure range: 30 cm


AccelerometerAnalog Device Inc. ADXL330

Output voltage proportional to the acceleration

Measurement range: +/- 3.6 g


GyroInvensense ISZ-500

Z axis gyro, ±500 °/sec2.0 mV/°/sec

Invensense IDG-500X/Y axis gyro, ±500 °/sec2.0 mV/°/sec

Invensense IMU-30003-axis gyro, (±250, ±500, ±1000, ±2000) °/secI2C interface


Industry standard analog outputsMany industrial instruments provide analog

outputs4~20 mA current loop

Most commonly usedThe instrument produce a current, range from 4

mA to 20 mAPhysical measured quantity linearly maps to this

current rangeOthers

0 ~ 5 V0 ~ 10 V


4~20 mA current loop Examples

Measure temperature: 0 ~ 100 oC

Measure flow speed: 0 ~ 50 m3/h

Measurements0 oC50 oC100 oC

Current Output4 mA12 mA20 mA

Measurements0 m3/h25 m3/h50 m3/h

Current Output4 mA12 mA20 mA

Usually this range

is user configurab

le


How to Convert to VoltageAdd a resistor


Digitalized SensorsUltrasound sensors

SRF10Interface: I2C

Temperature and Humidity SensorSHT11Interface : manufacturer defined


Digitalized SensorsMagnetometer

SFE MicroMag 3-AxisInterface : SPIApplications: detecting vehicles

Digital compassHitachi HM55BInterface : UART


Digitalized SensorsGPS

Garmin GPS18Interface : UART

Image sensorST VS6451Interface: I2C


Understand The Analog SensorsMost important goal

Obtain the relationship between voltage and the physical phenomenon quantity you want to measure Transfer function

Understand the characteristics and limitation of the sensors


Sensor Characteristics DefinitionsTransfer Function

Relationship between physical quantity and output voltageSensitivity

Ratio of change between physical quantity and output voltage

Accuracy Largest expected error

Linearity How linear the transfer function is

Noise In real world, signal are usually coupled with noise

Resolution minimum detectable signal fluctuation

Bandwidth response times to an instantaneous change in physical

signal


Transfer FunctionRatiometric?

Output voltage is a ratio of supply

voltage

Ratiometric?Output voltage is a

ratio of supply voltageADXL330 accelerometer

From ADXL330 datasheet

It means when supply voltage Vs is 3 V, if the acceleration is 0 g, the output voltage will be 1.5 V (typical)

It means when supply voltage Vs is 3 V, if the acceleration increase by 1 g, the output voltage will increase 330 mV (typical)

Transfer function: Voltage (V) = 1.5 + (0.3 * acceleration (g) )


Transfer FunctionProximity sensor: Sharp GP2D120XJ00F

Transfer functionVoltage (V) = f(Distance (cm))Not a linear function• Check datasheet

• if the manufacturer has provided

• Table mapping• Approximations

• By a few straight lines• Curve fitting


Transfer FunctionPhotodiode: S1087, S1087-01

From Taroko Schematic

Step 1: find the relationship

between light level and current

Step 2: find the relationship

between current and the voltage at

ADC4(V = IR)

Transfer functionVoltage (V) = f(light level(lx))Calculate by yourself


SensitivityADXL330 accelerometer

Sharp GP2D120XJ00FNot a constanst

Photodiode: S1087, S1087-01(Current)/(light level)


LinearityPhotodiode: S1087, S1087-01

ADXL330 accelerometer

Sharp GP2D120XJ00F

GOOD

GOOD

BAD


BandwidthHow fast the next valid output ready

Proximity sensor: Sharp GP2D120XJ00F

ADXL330 accelerometer

When power up, you have to wait for 52.9ms to get a first valid output. You have to wait another 47.9ms to get the second one.

For the module we will use, the filter capacitor is 0.1 uF. It means the accelerometer module can have at most 50 different read-outs in one second


Analog to Digital Conversion

Periodically read the voltage of the signalConvert to a digital number

Represent the voltage of the signalAt the sampling time

T


Analog to Digital Conversion


ADC takes two inputsVoltage reference (dynamic range)

Range of voltage that it can measure It has a limited range

Sensor signalResolution

How many bits the ADC can outputSample Rate

How many sample it can takes in one secondFor MSP430F1611, it is about 200 ksps

That’s why we need signal conditioning to produce a proper

voltage output

Analog to Digital Converter (ADC)


ResolutionYou heard 10-bit ADC, 12-bit ADC, 16-bit

ADCWhat are they?Number of bits the ADC can output

• This is a 4-bit ADC [0,0.1) V -> 0000 [0.1,0.2) V -> 0001 [0.2,0.3) V -> 0010 [1.5,1.6) V -> 1111

• If it is a 6-bit ADC [0,0.025) V -> 000000 [0.025,0.05) V -> 000001 [0.05,0.075) V -> 000010 [1.575,1.6) V -> 111111


1 bit 2 possible values 2 bits 4 possible values 8 bits 256 possible values 12 bits 4096 possible values 16 bits 65356 possible values : : N bits 2N possible values

Discretization

} Quantization error


Dynamic RangeRange of signal amplitudes which the ADC

can resolve

Max possible value

Min possible value


Voltage reference Input voltage compares to the voltage reference

Ratio of (input voltage)/(voltage reference) determines the output number

Two voltage references Positive voltage reference (Vref+) Negative voltage reference (Vref-)

• In this figure, Vref+ = 1.6V; Vref- = 0 V [0,0.1) V -> 0000 [0.8,0.9) V -> 1000 [1.5,1.6) V -> 1111

• If we change the Vref+ = 3.2V; Vref- remains 0 V, then [0,0.2) V -> 0000 [1.6,1.8) V -> 1000 [3.0,3.2) V -> 1111


About Voltage ReferenceVoltage reference can by internal or external

Many ADCs have internal voltage reference integrated

Unstable Voltage reference will affect the accuracy of the ADC

Minimum, MaximumIf input voltage > Vref+, always output 1111If input voltage < Vref-, always output 0000

Voltage Reference cannot exceed power supply voltage


Dynamic RangeToo low

Good resolutionSaturation

Too highPoor resolutionNo saturation


Detectable Voltage ChangeVoltage reference + Resolution

Define the detectable voltage changeDetectable voltage change = ((Vref+) –

(Vref-))/(2^resolution)

ExamplesVref = 1.6V, Resolution = 4-bit

Vref = 3.2V, Resolution = 4-bit

Vref = 1.6V, Resolution = 6-bit

1.6/(2^4) = 0.1 V

3.2/(2^4) = 0.2 V

1.6/(2^6) = 0.025 V


Sample And Hold

There is a sample-and-hold circuit before A/D conversion

Mostly integrated in the ADC chip When no conversion, switch S1 is open When a conversion start

S1 closed Input signal charge C1 S1 open, C1 holds the value of input

signal A/D conversion

Sample-and-hold time Time between S1 close and re-open If it is too short

C1 will not fully charged (error)


ADC ClockADC needs a clock

For sample-and-hold and the A/D conversion


ADC on Taroko12-bit on-chip ADC in MSP430Voltage reference

Internal Vref: 1.5V, 2.5VNo external Vref on TarokoUser configurable combination for Vref+ and

Vref-Sample rate

Approximate 200 kspsUser configurable sample-and-hold timeUser configurable clock sources


An Example: ADXL330ADXL330 3-axis accelerometer

Transfer function: V = 1.5 + (0.3 * g)


An Example: ADXL330Taroko ADC setting

Resolution: 12-bitVoltage reference

Vref+ = 2.5 V Vref- = 0 V

Conversion formula NADC is the output number

For our setting

refref

refbitnADC VV

VVinN )12(

5.2

)4095(05.2

0)4095(

VinVinN ADC


An Example: ADXL330Transfer function: V = 1.5 + (0.3 * g)Conversion formula

When the acceleration is 0 gV = 1.5NADC = 2457When the acceleration is 1.5 gV = 1.95NADC = 3194When NADC = 1784, what is the acceleration?

When NADC = 2635, what is the acceleration?

5.2

)4095(05.2

0)4095(

VinVinN ADC


Real World DesignIn practical design, there are always errors

Noises Power supply noise Digital circuit noise RF noise Devices noise

Devices Tolerances 5% resistor, 1% resistor 10% capacitor etc.

Temperature drift Devices characteristics change when temperature change

Two very simple methods to deal with two very common errors


First Type of ErrorsNoise

DynamicADXL330 at 0 g (1.5V)

Sample at S1 -> ok Sample at S2 -> it is 0.333 g Sample at S3 -> it is -0.333 g

SolutionAverage

(S1 + S2 + S3)/3 = (1.5+1.6+1.4)/3 = 1.5

1.5V1.4V

1.6V

S1

S2

S3


How Many SamplesHow many samples needed to average

Depends on your requirementSuggestion: factor of 2 (2N)

Take 10 samples and average

Take 16 samples and average

Total += Si; //(i=1,…,10)Result = Total/10;

Total += Si; //(i=1,…,16)Result = (Total>>4);

Division => slow

Bit shift => fast


Cautions! Is the Total large enough to hold the summation

Max possible Total = 4096*16 = 65536 Total should be at least 16-bit unsigned int

IMPORTANT!! int in IAR for MSP430 is 16-bit long

Total += Si; //(i=1,…,16)Result = (Total>>4);


Second Type of ErrorsOffset

Static For example: ADXL330

For A accelerometer, 0 g output maybe 1.5V For B accelerometer, 0 g output maybe 1.54V

From ADXL330 datasheet

30% of ADXL330, 0 g output is

1.53V

4% of ADXL330, 0 g output is 1.5V


CalibrationMaintain a calibration constant, adjust the offset

errorADXL330 with calibration

Transfer function: V = (1.5 + Ccal) + (0.3 * g) Ccal is calibration constant

For A accelerometer, 0 g output is 1.5V Ccal = 0

If NADC = 2879, acceleration should be 0.859 g

For B accelerometer, 0 g output is 1.54V Ccal = 0.04

If NADC = 2879, acceleration should be ?? G


Calibration ReferenceCalibration needs a reference

Precision of the reference decide the precision of the calibration

How do you calibrate the accelerometer ADXL330??

How do you calibrate a temperature sensor?

Acceleration => voltage

To calibrate, you want to know the voltage output at 0 g. But how to

make 0 g?

You must produce a precise 0 g (or 1 g) acceleration

Temperature => voltage

To calibrate, you want to know the voltage output at

certain temperature (25

oC).

You must produce a precise temperature


Digital CommunicationsSending bits between each other

Serial communicationSending bits one by oneUART, SPI, I2C, USB, etc.

Parallel communicationSending multiple bits at one timeEthernet

A B10011101

A B1 0 0 1 1 1 0 1

A B

1

111

1

00

0

Our focus


AgreementsIn order to communicate, two parties must at

least agree on:Logic level

MSP430 RS232 (COM port)

Output data Start, stop Output data valid time (clock)

0V -> logic low (0)

3.3V -> logic high (1)-12V -> logic high

(1)12V -> logic low

(0)

What number it represent?

1 1 1 10 0 0 0Read 1 11 100 00

2x Read

A B

For digital, wired

communication


ProtocolsChips to Chips

SPI: Serial Peripheral Interface I2C: Inter Integrated CircuitManufacturer definedUART: Universal Asynchronous Receiver Transmitter

Systems to systems (usually)Define logic level

RS-232: old traditional RS-485: popular in industrial control system

Define output data UART: Universal Asynchronous Receiver Transmitter

Define both USB

A BUAR

TUAR

TRS232

or RS485

RS232 or

RS485


SPIMaster–Slave modeSynchronous protocolAll transmissions are referenced to a common clockClock generated by the master (MCU)Four main signalsMaster Out Slave In (MOSI): data from master to slaveMaster In Slave Out (MISO): data from slave to masterSerial CLocK (SCLK or SCK): clockChip Select (CS): select particular peripheral when

multiple peripherals are connected to master


Read data at clock edge

Data Register Exchange Master transfer a byte to slave, push slave to transfer a byte

back to master Master Read and Write

Simultaneously Master write only

Ignore the byte it receive Master read only

Master must transfer a dummy byte in order to initiate a slave transmission

SPI TransmissionMOSI

CLK

Slave read

1 1 1

1

0 0 0 0

MISO

CLK

Master read

1 0 0

0

1 1 0 1

CSCS


I2CTwo wires, multiple devices

SDA (serial data)SCL (serial clock)

I2C transmissionSTART: SDA is pulled low while SCL stays highTransfer:

SDA sets the transferred bit while SCL is low (blue) data is read when SCL rises (green)

STOP: SDA is pulled high while SCL stays high

Pull-up resisto

r


I2CAcknowledge (ACK) and Not Acknowledge

(NACK)Upon the transmission of the eighth data bitTransmitter releases the SDAMaster then generates an additional clock

pulse on SCLTriggers the receiver to acknowledge the byte

by pulling SDA low


I2CAddressing

7-bit addressDirection bit

If 0: master write to slave If 1: master read from slave

10-bit address

Clock stretchingA slave may hold the clock line (SCL) low after

receiving (or sending) a bit, indicating that it is not yet ready to process more data

10-bit address

7-bit address


ComparisonSPI I2C

Four wires Two wires

Full duplex Half duplex

Higher throughput (then I2C) Lower throughput

Synchronous protocol Synchronous protocol

No slave acknowledgment Acknowledgment

Simple Complicated


Manufacturer Defined ProtocolsThe ideals are similar

Feed a clockStart, stopRead/write the data

Read datasheetMany timesGet use to those timing diagrams

We will use the temperature/humidity sensor SHT11 on Taroko as an example


ImplementationsHow to implement these protocols?

Hardware There are SPI, I2C peripherals on MSP430F1611 You need to properly configure (setting registers) the

modulesSoftware

Use GPIO Generate clock Read data: set the pin to input Write data: set the pin to output

Things to considerTiming: some devices cannot operate too fastCheck devices datasheet

// use P1.0 as clock pinP1SEL &= ~(0x01); // GPIOP1DIR |= 0x01; // output

// clock P1OUT |= 0x01; // high P1OUT &= ~(0x01); // low

// add some delay between clock P1OUT |= 0x01; // high (delay some time) P1OUT &= ~(0x01); // low


In GeneralYou want to interface some digital ICs to MSP430

CC2420 radio chip (SPI)8 Mbit flash memory (SPI)SHT11 sensor (manufacturer defined, similar to

I2C)Typical process

1.How to interface? Connections: two wires, three wires, four wires, etc.

2.How to communicate? Available commands Timing diagram: start, transfer, (ack), stop

3.How to configure the IC Setting the registers on the IC

4.Start reading/writing data from/to the IC


An Example: SHT11The temperature/humidity sensor on TarokoHow to Interface

two wires bi-direction

SHT11 datasheet

1. Use a GPIO pin as clock (SCK), it is always output direction

2. Use another GPIO as DATA, dynamic setting it to input(read) or output(write) direction


An Example: SHT11How to start

What are the commands available


An Example: SHT11Timing diagram

Pull-up

Data pin in output

direction

Set data pin to input direction,

then SHT11 controls the DATA

line


An Example: SHT11Configure device

If you don’t understand what is the meaning, check datasheet


TimingCheck the timing

requirement carefullyThe device won’t

work at all if you exceed the limit


UARTUART: Universal Asynchronous Receiver Transmitter

Two wires: Rx, TxFull duplexAsynchronous

No common clock required

Without common clockHow do they communicate??

ARx

Tx

BTx

Rx


Universal data ratesTwo devices agree on same data rate

Baudrate: 1200, 2400, 4800, 9600, …, (bits per second)

A(1 MHz system

clock)

B(8 MHz system

clock)UART

UART

Baudrate

generator

Baudrate

generator

96009600

Baudrate: 9600 bps

Length of a bit = 1/9600 (seconds)


UART DataSend one byte at a timeData

One start bit Pull-down the line

7 or 8 bits data One or two stop bit

Pull-up the line for one or two slotsSimple error checking: parity (optional)

Even parity: If the data has odd number of 1, parity bit = 1 (make it even); else parity bit = 0

Odd parity: If the data has even number of 1, parity bit = 1 (make it odd); else parity bit = 0

Four parameters for UART communication Baudrate, data-bit, parity, stop-bit We wrote: 9600 8N1,

Means: baudrate=9600, 8-bit data, no parity, 1 stop bit

This bit became parity bit when parity checking

is enabled


RS232Since the 1960sCable lengths of up to 25 metersSignal level

logic high -> -5 to -15V (typically -12V)logic low -> +15 and +1V (typically +12V)

Interface Signal Function 25-pin 9-pin Direction

Tx Transmitted data 2 3 From DTE to DCE

Rx Received data 3 2 To DTE from DCE

RTS Request to send 4 7 From DTE to DCE

CTS Clear to send 5 8 To DTE from DCE

DTR Data terminal ready 20 4 From DTE to DCE

DSR Data set ready 6 6 To DTE from DCE

DCD Data carrier detect 8 1 To DTE from DCE

RI Ring indicator 22 9 To DTE from DCE

FGFrame ground (chassis)

1 - Common

SG Signal ground 7 5 Common


RS485Two-wire, half-duplex

Multipoint serial connection

Signal levelDifference between the wires’ voltages

A – B > 0.2V => logic high (0) A – B < -0.2V => logic low (1)

Why it is popular in industrial applicationsOnly two wire35 Mbit/s up to 10 m and 100 kbit/s at 1200 m


USB Serial ICThat’s what we have on Taroko

FT232BL (www.ftdichip.com)To communicate with PCIt simulate a serial communication on PC

through USB interface Virtual com port


Applications

MSP430 U

AR

T USB serial

IC (FT232

)

PC or other

systems

RS232 IC

RS485 IC

RS485 IC

Other systems

CO

M

PO

RT

US

B

All you need to do is properly configure UART for both sides

(baudrate, data-bit, parity, stop-bit), and start sending/receiving

data

Documents

Analog And Digital Interfacing