The ADC of the AVR1

8/9/2019 The ADC of the AVR1

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The ADC of the AVRAnalog to Digital Conversion

Most real world data is analog. Whether it be temperature,

pressure, voltage, etc, their variation is alwas analog in nature. !or e"ample, the

temperature inside a boiler is around #$$%C. During its light&up, the temperature

never approaches directl to #$$%C. 'f the ambient temperature is ($$%C, it will

start increasing graduall to ()$%C, )$$%C and thus reaches #$$%C over a period

of time. This is an analog data.

*ignal Ac+uisition rocess

-ow, we must process the data that we have received. ut analog signal

processing is +uite inefficient in terms of accurac, speed and desired output.

/ence, we convert them to digital form using an Analog to Digital Converter 0ADC1.

*ignal Ac+uisition rocess'n general, the signal 0or data1 ac+uisition process has 2 steps.

'n the Real World, a sensorsenses an phsical parameter and converts

into an e+uivalent analog electrical signal.

!or efficient and ease of signal processing, this analog signal is converted

into a digital signal using an Analog to Digital Converter (ADC).
http://maxembedded.wordpress.com/2011/06/18/sensor-fundamentals/http://maxembedded.wordpress.com/2011/06/18/sensor-fundamentals/


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This digital signal is then fed to the Microcontroller (MCU)and is

processed accordingl.

ADC ins 3 ATM45A67829

'nterfacing *ensors'n general, sensors provide with analog output, but a MC: is a digital one. /ence

we need to use ADC. !or simple circuits, comparator op&amps can be used. ut

even this won;t be re+uired if we use a MC:. We can straightawa use the inbuilt

ADC of the MC:. 'n ATM45A67829,


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Right now, we are concerned about the 8 channel 10 bit resolutionfeature.

8 channelimplies that there are # ADC pins are multiple"ed together. >ou

can easil see that these pins are located across


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The ADC of the AVR converts analog signal into digital signal at some regular

interval. This interval is determined b the cloc fre+uenc. 'n general, the ADC

operates within a fre+uenc range of )$/F to 9$$/F. ut the C: cloc

fre+uenc is much higher 0in the order of M/F1. *o to achieve it, fre+uenc division

must tae place. The prescaler acts as this division factor. 't produces desired

fre+uenc from the e"ternal higher fre+uenc. There are some predefined division

factors 3 9, (, #, 67, 29, 7(, and 69#. !or e"ample, a prescaler of 7( implies

!GADC B !GC:87(. !or !GC: B 67M/F, !GADC B 67M87( B 9)$/F.

-ow, the maHor +uestion is? which fre+uenc to selectI


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ADC Voltage Reference ins

The ADC needs a reference voltage to wor upon. !or this we have a three pins

AR4!, AVCC and 5-D. We can suppl our own reference voltage across AR4!

and 5-D. !or this, choose the irst o,tion. Apart from this case, ou can either

connect a capacitor across AR4! pin and ground it to prevent from noise, or ou

ma choose to leave it unconnected. 'f ou want to use the VCC 0J)V1, choose

the second o,tion.


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'nput Channel and 5ain *elections

Thus, to initialiFe ADM:, we writeADMUX = (1


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ADC rescaler *elections

Assuming TAE fre+uenc of 67M/F and the fre+uenc range of )$/F&9$$/F,

we choose a prescaler of 69#.

Thus, !GADC B 67M869# B 69)/F.

Thus, we initialiFe ADC*RA as follows.ADCSRA = (1


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ADC Data Registers 0ADEAR B 61

>ou can ver well see the the effect of ADEAR bit 0in ADM: register1. :pon

setting ADEAR B 6, the conversion result is left adHusted.

*!'


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These options are will be discussed in the posts related to timers. Those who have

prior nowledge of timers can use it. The rest can leave it for now, we won;t be

using this anwa.

ADC 'nitialiFationThe following code segment initialiFes the ADC.1

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!"#a#c$"%"&()' // AREF = Acc ADMUX = (1


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EDR Connections

-ow suppose we want to displa the corresponding ADC values in an ECD. *o, we

also need to connect an ECD to our MC:. Read this post to now about ECD

interfacing.

*ince it is an EDR, it senses the intensit of light and accordingl change its

resistance. The resistance decreases e"ponentiall as the light intensit increases.

*uppose we also want to light up an E4D whenever the light level decreases. *o,

we can connect the E4D to an one of the 5'< pins, sa C$.

-ote that since the ADC returns values in between $ and 6$92, for dar conditions,

the value should be low 0below 6$$ or 6)$1 whereas for bright conditions, the value

should be +uite high 0above N$$1.

-ow let;s write the complete code.

4"ample CodeTo learn about ECD interfacing, view thispost. >ou can tpe, compile and build it in

AVR *tudio ). View thispage to now how. To now about the '8< port operations

in AVR, view thispage.I

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?"%cl.#e


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?"%cl.#e Blc#@B

?#e*"%e RES ,00?#e*"%e RRES ,00

// "%"&"al"e a#c!"#a#c$"%"&()' // AREF = Acc ADMUX = (1


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3"le(1) ' a#c$res.l&0 = a#c$rea#(0); // rea# a#c al.e a& PA0 a#c$res.l&1 = a#c$rea#(1); // rea# a#c al.e a& PA1

// c!%#"&"!% *!r le# &! l!3

"*(a#c$res.l&0 < RES 99 a#c$res.l&1 < RRES) PRC = 0:01; else PRC = 0:00;

// %!3 #"spla6 !% lc#

"&!a(a#c$res.l&0> "%&$.**er> 10); lc#$!&!:6(12>0); lc#$p.&s("%&$.**er);

"&!a(a#c$res.l&0> "%&$.**er> 10);

lc#$!&!:6(12>1); lc#$p.&s("%&$.**er); $#ela6$s(,0); --

*ensor CalibrationCalibration means lining our real world data with the virtual data. 'n the problem

statement given earlier, ' have mentioned that the E4D should glow if the light

intensit reduces. ut whenshould it start to glowI The MC:8code doesn;t now

b itself. >ou get the readings from the sensor continuousl in between $ and 6$92.

*o, the +uestion is how do we know that below such and such level the LED

should glow?

This is achieved b calibration. >ou need to phsicall set this value. What ou do

is that ou run the sensor for all the lighting conditions. >ou have the ADC values

for all these levels. -ow, ou need to phsicall see and chec the conditions

ourself and then appl a threshold. elow this threshold, the light intensit goes

sufficientl down enough for the E4D to glow.


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The potentiometer connected in the circuit is also for the same reason. -ow, b the

basic nowledge of electronics, ou could easil sa that upon changing the pot

value the ADC value changes. Thus, for various reasons 0lie poor lighting

conditions, ou are unable to distinguish between bright and dar conditions, etc1,

ou can var the pot to get desired results.

This is wh ' have given the two thresholds 0RT/R4* anf ET/R4*1 in the

beginning of the code.

*o, this is all with the ADC. ' hope ou enHoed reading this. 7lease ,ost the

coents belo! or an# suggestion9 doubt9 clariication9 etc:

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The ADC of the AVR1