1 Analog Interfacing These lecture notes created by Dr. Alex Dean, NCSU.

1

Analog InterfacingAnalog Interfacing

These lecture notes created by Dr. Alex Dean, NCSU

2In These Notes . . . In These Notes . . . • Analog to Digital Converters

– ADC architectures

– Sampling/Aliasing

– Quantization

– Inputs

– M30626 ADC Peripheral

• Digital to Analog Conversion

3Why It’s NeededWhy It’s Needed• Embedded systems often need to measure values of physical parameters• These parameters are usually continuous (analog) and not in a digital form

which computers (which operate on discrete data values) can process

• Pressure– Blood pressure monitor– Altimeter– Car engine controller– Scuba dive computer – Tsunami detector

• Acceleration– Air bag controller– Vehicle stability– Video game remote

• Mechanical strain• Other

– Touch screen controller– EKG, EEG– Breathalyzer

• Temperature– Thermometer (do you have a fever?)– Thermostat for building, fridge, freezer– Car engine controller– Chemical reaction monitor– Safety (e.g. microprocessor processor

thermal management)• Light (or infrared or ultraviolet)

intensity– Digital camera– IR remote control receiver– Tanning bed– UV monitor

• Rotary position– Wind gauge– Knobs

4The Big PictureThe Big Picture

PressureSensor

Analog to Digital

Converter

// Your softwareADC_Code = ad0;V_sensor = ADC_code*V_ref/1023;Pressure_kPa = 250 * (V_sensor/V_supply+0.04);Depth_ft = 33 * (Pressure_kPa – Atmos_Press_kPa)/101.3;

V_sensor ADC_Code

V_ref

V_sensor ADC_Code

Voltages

V_ref

Ground

ADC Output Codes

111..111

000..000000..001

111..110111..101111..100

AirPressure

1. Sensor detects air pressure and generates a proportional output voltage V_sensor

2. ADC generates a proportional digital integer (code) based on V_sensor and V_ref

3. Code can convert that integer to a something more useful

1. first a float representing the voltage, 2. then another float representing pressure,3. finally another float representing depth

5Getting From Analog to DigitalGetting From Analog to Digital• A Comparator is a circuit which compares an analog input voltage

with a reference voltage and determines which is larger, returning a 1-bit number

• An Analog to Digital converter [AD or ADC] is a circuit which accepts an analog input signal (usually a voltage) and produces a corresponding multi-bit number at the output.

0

1

0

1

Vin

Vref

Clock

0Vin0

Vin1

Comparator A/D Converter

6Waveform Sampling and QuantizationWaveform Sampling and Quantization

• A waveform is sampled at a constant rate – every t

– Each such sample represents the instantaneous amplitude at the instant of sampling

– “At 37 ms, the input is 1.91341914513451451234311… V”– Sampling converts a continuous time signal to a discrete time signal

• The sample can now be quantized (converted) into a digital value– Quantization represents a continuous (analog) value with the closest discrete

(digital) value– “The sampled input voltage of 1.91341914513451451234311… V is best

represented by the code 0x018, since it is in the range of 1.901 to 1.9980 V which corresponds to code 0x018.”

time

Dig

ital

val

ue

7Transfer FunctionTransfer Function• The ADC produces a given

output code for all voltages within a specific range

• The ideal transfer function A/D converter is a stair-step function.

• Consider a 2-bit ADC– 0 to 4 V input– LSB = 4/22 = 1 V

• Red line– Truncation– Maximum error is -1 LSB or +

0 LSB

• Blue line– Rounding– Maximum error in conversion

is usually 1/2 bit.– Half as much error if we limit

range to Vref(1-2N/2)

Input Voltage

00

10

11

Ou

tpu

t C

od

e

1 LSB

2.0

V

3.0

V

4.0

V

01

1.0

V

“2.0 V”

“3.0 V”

“0.0 V”

“1.0 V”

8Transfer Function EquationTransfer Function EquationGeneral Equation

n = converted code

Vin = sampled input voltage

V+ref = upper end of input voltage range

V-ref = lower end of input voltage range

N = number of bits of resolution in ADC

2/1

2

refref

Nrefin

VV

VVn

2/12

ref

Nin

VV

n

6762/15

230.3 10

vv

n

Simplification with V-ref = 0 V

floor function: nearest integer I such that I <= XX = I

9A/D – Flash ConversionA/D – Flash Conversion• A multi-level voltage divider is

used to set voltage levels over the complete range of conversion.

• A comparator is used at each level to determine whether the voltage is lower or higher than the level.

• The series of comparator outputs are encoded to a binary number in digital logic (a priority encoder)

• Components used– 2N resistors– 2N-1 comparators

• Note – This particular resistor divider

generates voltages which are not offset by ½ bit, so maximum error is 1 bit

– We could change this offset voltage by using resistors of values R, 2R, 2R ... 2R, 3R (starting at bottom)

1/8 V

2/8 V

3/8 V

4/8 V

5/8 V

6/8 V

7/8 V +

-

Encoder

+

-

+

-

+

-

+

-

+

-

+

-

1V

Vin

ComparatorsR

R

R

R

R

R

R

R

3

1

1

1

0

0

0

0

10ADC - Dual Slope IntegratingADC - Dual Slope Integrating• Operation

• Input signal is integrated for a fixed time

• Input is switched to the negative reference and the negative reference is then integrated until the integrator output is zero

• The time required to integrate the signal back to zero is used to compute the value of the signal

• Accuracy dependent on Vref and timing

• Characteristics• Noise tolerant (Integrates

variations in the input signal during the T1 phase)

• Typically slow conversion rates (Hz to few kHz)

21

00

11T

ref

T

in dtVC

dtVC

1

2

T

TVV refin

Slope proportionalto input voltage

11ADC - Dual Slope IntegratingADC - Dual Slope Integrating

Control Logic

Counter

Start of ConversionStatus

Clock

Analog Input (Va)

Digital Output 12

+

-

Comparator output

+

--Vreference

Comparator

Integrator

12ADC - Successive Approximation ConversionADC - Successive Approximation Conversion• Successively approximate input

voltage by using a binary search and a DAC

• SA Register holds current approximation of result

• Set all DAC input bits to 0

• Start with DAC’s most significant bit

• Repeat – Set next input bit for DAC to 1

– Wait for DAC and comparator to stabilize

– If the DAC output (test voltage) is smaller than the input then set the current bit to 1, else clear the current bit to 0

Vo

ltag

e

TimeStart of Conversion

Test voltage(DAC output)

T1 T2 T3 T4 T5 T6000000

111111

100000

100100

AnalogInput

know

1xx

xxx,

try

1100

00

100110

know

xxx

xxx,

try

100

000

know

10x

xxx,

try

1010

00

know

100

xxx,

try

100

100

know

100

1xx,

try

100

110

know

100

11x,

try

100

111

know

100

110.

Do

ne.

13A/D - Successive ApproximationA/D - Successive ApproximationConverter Schematic

D/A Converter

SuccessiveApproximation

RegisterStart of ConversionStatus

Clock

Analog Input

Digital Output 12

+

-

Comparator outputConverter Schematic

14ADC Performance MetricsADC Performance Metrics• Linearity measures how well the transition voltages lie on

a straight line.

• Differential linearity measure the equality of the step size.

• Conversion time:between start of conversion and generation of result

• Conversion rate = inverse of conversion time

15Sampling ProblemsSampling Problems• Nyquist criterion

– Fsample >= 2 * Fmax frequency component

– Frequency components above ½ Fsample are aliased, distort measured signal

• Nyquist and the real world– This theorem assumes we have a perfect filter with “brick wall”

roll-off

– Real world filters have more gentle roll-off

– Inexpensive filters are even worse (e.g. first order filter is 20 dB/decade, aka 6 dB/octave)

– So we have to choose a sampling frequency high enough that our filter attenuates aliasing components adequately

16QuantizationQuantization• Quantization: converting an analog value (infinite resolution or

range) to a digital value of N bits(finite resolution, 2N levels can be represented)

• Quantization error– Due to limited resolution of digital representation– <= 1/(2*2N)– Acoustic impact can be minimized by dithering (adding noise to input signal)

• 16 bits…. too much for a generic microcontroller application?– Consider a 0-5V analog signal to be quantized– The LSB represents a change of 76 microvolts– Unless you’re very careful with your circuit design, you can expect noise of

of at least tens of millivolts to be added in– 10 mV noise = 131 quantization levels. So log2 131 = 7.03 bits of 16 are

useless!

17Inputs Inputs • Multiplexing

– Typically share a single ADC among multiple inputs

– Need to select an input, allow time to settle before sampling

• Signal Conditioning– Amplify and filter input signal

– Protect against out-of-range inputs with clamping diodes

18Sample and Hold DevicesSample and Hold Devices• Some A/D converters require

the input analog signal to be held constant during conversion, (eg. successive approximation devices)

• In other cases, peak capture or sampling at a specific point in time necessitates a sampling device.

• This function is accomplished by a sample and hold device as shown to the right:

• These devices are incorporated into some A/D converters

Analog InputSignal

Samplingswitch

HoldCapacitor

OutputSignal

19M30626 ADC PeripheralM30626 ADC Peripheral• For details see M16C/62P Hardware Manual (A/D

Converter)• 10 bit successive approximation converter (bits=1), can

operate in 8 bit mode (bits=0). • Input voltage: 0 to VCC

• Reference voltage applied to VREF pin– Can be disconnected with VCUT bit to save power

• Input Multiplexer: 26 input channels– Select one of three groups using adgsel1:0

• P10: 0 0 • P0: 1 0• P2: 1 1

– Select one of the eight input channels in a group with ch2:0– Or select ANEX0 or ANEX1 with opa0=1 or opa1=1

20Input Multiplexer DetailInput Multiplexer Detail

21ADC Conversion SpeedADC Conversion Speed

• Rates– With S/H: 28 AD cycles for 8 bits, 33 for 10 bits (smp = 1)

– Without S/H: 49 AD cycles for 8 bits, 59 for 10 bits (smp = 0)

• ADC clock generation– Can select AD = fAD, fAD/2, fAD/3, fAD/4, fAD/6, fAD/12 (use cks2-0)

– fAD= 24 MHz (see next page)

• Frequency restrictions– AD <= 10 MHz

– AD >= 1 MHz (using S/H)

– AD >= 250 kHz (not using S/H)

AD

22CPU Clock SystemCPU Clock System

12 MHz ceramic or crystal oscillator here

PLL doubles frequency

24 MHz

24 MHz

24 or 12 MHz

3 MHz

0.75 MHz

32.768 kHz crystal oscillator here

1024 Hz

23M30626 A/D Converter OverviewM30626 A/D Converter Overview

24Conversion ModesConversion Modes• For details see M16C/62P Hardware Manual (A/D Converter)• Common operation details

– Code starts conversion(s) by setting adst = 1– Conversion stops…

• When complete (ADC sets adst=0 as indicator) – in one-shot or single sweep mode• Code can also stop (set adst = 0) – primarily for repeat modes

– Result is in result register (16 bits) for that channel (AD0-AD7, 0x03c0-0x03cf)• Modes

– One-shot conversion of a channel (0)• Generates interrupt if ADIC register’s interrupt level is > 0

– Repeated conversion of a channel (1)• No interrupt generated, can read result register instead

– Single sweep mode (2)• Converts a set of channels once: Channels 0-1, 0-3, 0-5 or 0-7

– Repeat sweep mode 0 (3)• Converts a set of channels repeatedly: Channels 0-1, 0-3, 0-5 or 0-7

– Repeat sweep mode 1 (7)• Converts a set of channels repeatedly: Channels 0, 0-1, 0-2 or 0-3

• Control Registers– ADCON0 (0x03d6), ADCON2 (0x03d4), ADCON1 (0x03d7)

25QSK62P Board Analog InputsQSK62P Board Analog Inputs• P10_0 (AN0) – Potentiometer (R135)

• P10_1 (AN1) – Thermistor (RT101)– A thermistor’s resistance depends

on its temperature

– A voltage divider with a thermistor produces a voltage (VTh) which varies with temperature

– The thermistor circuit on the QSK produces thesevoltages based upon theinput temperature

• P10_2 – P10_7 – Unused

• Port P0 – Unused

• Port P2 – Unused

deg C deg F VTh-5 23 4.2580 32 3.2775 41 2.546

10 50 1.99315 59 1.57320 68 1.2525 77 130 86 0.805535 95 0.652840 104 0.532345 113 0.4365

26Checklist for Using ADCChecklist for Using ADC• Configure

– I/O pin• set to input

– ADC• AD clock speed• resolution• mode• sample and hold• trigger mode• connect reference voltage

– Interrupt, if used• Set ADIC to non-zero value• enable interrupts (FSET I)

– Set interrupt vector in sect30.inc to ADC ISR• Use

– Select channel if it has changed (or hasn’t been set yet)• ch2-0 bits in ADCON0• select port group

– Start conversion– (Wait until done)

• polling• interrupt

– Get data and use• read from correct AD result register

27DemonstrationsDemonstrations• Polled one-shot conversion mode

– Repeatedly convert input voltage and display value

– Configure ADC

– Main loop• Start conversion of channel 0

• Wait until it’s done

• Convert result to text and display on LCD

• Interrupt-driven one-shot conversion to fill buffer– Convert input voltage until input buffer is full, then stop

– Configure ADC and interrupt controller

– Main code• Start conversion of channel 0

• Wait until all conversions are done

• Stop

28One Shot ADC Mode – Demonstration CodeOne Shot ADC Mode – Demonstration Codevoid adc_init(void) { pd10_0 = INPUT; // To use channel 0

on P10 group as input/* clock selection: 24 MHz system clock,

6 MHz phiAD -> divide by 4 cks bits are 0 0 0 */// ADCON 0 ch0 = 0; // channel 0 ch1 = 0; ch2 = 0; md0 = 0; // one shot md1 = 0; cks0 = 0; // divide clock by 4// ADCON 1 trg = 0; // SW trigger md2 = 0; // one shot bits = 1; // 10-bit conversion cks1 = 0; // divide clock by 4\ vcut = 1; // connect reference voltage// ADCON 2 smp = 1; // use sample and hold adgsel0 = 0; // select port P10 group adgsel1 = 0; cks2 = 0; // divide clock by 4}

#include "qsk_bsp.h" #include "stdio.h"#include "string.h"

#define INPUT (0)unsigned position_code;

void main(void){

char buf[9];mcu_init(); // Initialize MCUadc_init(); // ADC tooInitDisplay(); // LCD too

while (1) { adst = 1; while (adst)

; position_code = ad0 & 0x03ff; sprintf(buf, "%d", position_code); DisplayString(LCD_LINE1,“ "); DisplayString(LCD_LINE1, buf); // display position}

}

29Repeated ADCRepeated ADC• The microcontroller performs repeated A/D conversions,

and can read data whenever needed

adcon0 = 0x88;adcon1 = 0x28;adcon2 = 0X01;adst = 1; // Start a conversion here

• Then in your procedureTempStore = ad0 & 0x03ff;

30Using ADC ValuesUsing ADC Values• The ADC gives an integer representing the input voltage relative to the reference

voltages• Several conversions may be needed

– For many applications you will need to compute the approximate input voltage• Vin = …

– For some sensor-based applications you will need to compute the physical parameter value based on that voltage (e.g. pressure) – this depends on the sensor’s transfer function

– You will likely need to do additional computations based on this physical parameter (e.g. compute depth based on pressure)

• Data type– It’s likely that doing these conversions with integer math will lead to excessive loss of

precision, so use floating point math• This requires using the floating point library (math.h).

– AFTER you have the application working, you can think about accelerating the program using fixed-point math (scaled integers). We cover this in ECE 561.

• Sometimes you will want to output ASCII characters (to the LCD, for example). You will need to convert the floating point number to ASCII via successive division or by using sprintf.

31Digital to Analog ConversionDigital to Analog Conversion• May need to generate an analog voltage or current as an output signal

– Audio, motor speed control, LED brightness, etc.

• Digital to Analog Converter equation– n = input code

– N = number of bits of resolution of converter

– Vref = reference voltage

– Vout = output voltage

– Vout = Vref * n/(2N)

0

1

0

1Vout

Vref

D/A Converter

32M30626 DACsM30626 DACs• Two eight-bit DACs

– DA0 is on port 9 bit 3– DA1 is on port 9 bit 4

• Initialization– Configure port direction bit to input! This disables the digital logic output for that bit.– Enable DAC with enable bit DA0E or DA1E

• Use– Write output values to data register DA0 or DA1

33AppendixAppendix

34One Shot-Setting Control InterruptsOne Shot-Setting Control Interruptsadic = 0X01; /* 00000001; /* Enable the ADC interrupt

||||||||______interrupt priority select bit 0 |||||||_______interrupt priority select bit 1 ||||||________interrupt priority select bit 2 |||||_________interrupt request bit ||||__________reserved */

_asm (" fset i") ; // globally enable interrupts adst = 1; // Start a conversion here while (1){} // Program waits here forever}

#pragma INTERRUPT ADCInt // compiler directive telling where

// the ADC interrupt is locatedvoid ADCInt(void){

TempStore = ad0 & 0x03ff;// Mask off the upper 6 bits of// variable leaving only the

result} // in the variable itself

35Setting Control Registers & InterruptSetting Control Registers & InterruptTo use interrupts, the ADC interrupt vector needs to point to the function. The

interrupt vector table is near the end of the startup file “sect30.inc”. Insert the function label “_ADCInt” into the interrupt vector table at vector 14 as shown below.

. . ..lword dummy_int ; DMA1(for user)(vector 12).lword dummy_int ; Key input interrupt(for user)(vect 13).glb _ADCInt.lword _ADCInt ; A-D(for user)(vector 14).lword dummy_int ; uart2 transmit(for user)(vector 15).lword dummy_int ; uart2 receive(for user)(vector 16)

. . ..

#pragma INTERRUPT ADCInt // compiler directive telling where

// the ADC interrupt is locatedvoid ADCInt(void){

TempStore = ad0 & 0x03ff; // Mask off the upper 6 bits of// variable leaving only the

result} // in the variable itself