Atmel AVR123: AT90PWM81/161 ADC
Conversion Optimization Versus Temperature
Features
8-bit Atmel
Microcontrollers
• Optimization of ADC conversion results versus temperature
®
• Applicable to the Atmel AT90PWM81/161 when using the internal VREF for the ADC
1 Introduction
Application Note
The AT90PWM81/161 features a 10-bit successive approximation ADC. The ADC
is connected to a 15-channel analog multiplexer, which provides:
• 11 single-ended inputs which are referenced to 0V (GND)
• Two differential voltage input combinations, which come with a programmable
gain stage, providing amplification steps of 14dB (5x), 20 dB (10x), 26 dB (20x),
or 32dB (40x) on the differential input voltage before the A/D conversion. On the
amplified channels, 8-bit resolution can be expected
This application note explains how to re-adjust the ADC conversion results over the
temperature.
Rev. 8270B-AVR-01/12
2 Theory of operation – the ADC conversion
2.1 VREF control
The reference voltage for the ADC (VREF) indicates the conversion range for the ADC.
It can be selected as either:
• AVCC,
• internal 2.56V reference,
• or the voltage present on the external AREF pin.
The internal reference VREF of 2.56V is generated, after multiplication, from the
Bandgap voltage.
2.2 VREF calibration
This internal voltage reference is function of the temperature.
It is calibrated at factory @3V and ambient temperature within accuracy of ±1% of the
2.56V reference voltage. The result of this calibration is stored in the Signature Row.
This Final Test “Amb.VREF” is loaded at address 0x1E (please also see the Atmel
AT90PWM81/161 datasheet).
Still at factory, a reading of the VREF level is achieved at 105°C. This value is also
stored in the Signature Row. This Final Test “Hot VRef” is loaded at address 0x1F.
2.3 ADC conversion
For single ended conversions, the conversion result is:
Read ADC = VIN × 1023 / VREF
where VIN is the voltage on the selected input pin and VREF the selected voltage
reference.
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3 The compensation method
The Atmel AT90PWM81/161 microcontroller offers an embedded temperature sensor.
This feature can be used for runtime compensation of temperature drift in the voltage
reference.
3.1 VREF versus temperature
In following example, we consider the default configuration of BGCRR Register which
shifts the top of the VREF curve to the highest possible temperature. In this
configuration; the higher the temperature is, the higher the VREF.
Figure 3-1. VREF versus temperature.
Vref
Hot Vref
re-calc. Vref
Amb Vref = 2.56V
A
B
25°C
Temp
105°C
Temperature
Notes:
1. Amb VREF is not necessarily strictly equal to 2.56V depending on the calibration (±1%
accuracy).
2. Amb VREF is the calibrated value at Factory = 2.56V @25°C with a ±1% accuracy
Hot VREF is the Read value at Factory @105°C.
3. Amb VREF and Hot VREF are stored in Signature Row during Test operation at Factory and
can be read by Software:
4. Final Test Amb VREF: is loaded in two bytes at Address 0x3D (High Byte) 0x3C (Low Byte).
5. Final Test Hot VREF (only for Read): is loaded in two bytes at Address 0x3F/0x3E.
These constants are the hexadecimal value of the voltage in mV: for instance 0x0A00
represents the Hexadecimal value of 2560mV.
3.2 Temperature measurement
This implementation uses the measurement achieved with the embedded
temperature sensor of the AT90PWM81/161.
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If the temperature sensor has been selected, the temperature measurement formula
is:
Temp (°C) = ((([(ADCH << 8) | ADCL] - (273 + 25-TSOFFSET)) × TSGAIN)/128) + 25
TSGAIN and TSOFFSET are stored in the Signature Row during Test operation at
Factory:
Temperature Sensor Offset:
TSOFFSET is loaded in High Byte of
Address 0x05
Device Temperature Sensor Gain: TSGAIN is loaded in High Byte of Address
0x07 (typical value is 0x80)
3.3 VREF recalculation
Between 25°C and 105°C, VREF curve versus temperature range can be extrapolated
as a straight line (see Figure 3-1).
To improve overall VREF accuracy, the recalculated VREF can be calculated as
following:
Re-calc. VREF = (A × Temp) + B
The known points of the straight line are:
Amb VREF = (A × (25°C)) + B = data stored in Signature Byte
Hot VREF = (A × (105°C)) + B = data stored in Signature Byte
A and B variables can be extracted from these two equations.
3.4 ADC measurement compensation
The ADC measurement result can be compensated with following formula:
Compensated ADC = Read ADC × Re-calc. VREF / 2.56
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4 Hardware configuration
4.1 AT90PWM81/161
Five parts have been tested. Their Fuse configuration is:
Extended = FD
High = D9
Low = CC
4.2 STK521
XTAL:
8MHz
Monitoring of VREF:
AREF output / VSS
UART software output: PB0 connected to RS232 interface
4.3 Hyperterminal
UART Bitrate = 19200 bit/s
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5 Software configuration
See Chapter 8: Code example (compiled with IAR).
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6 Result of temperature measurement
The chart in Figure 6-1 confirms that, over the five tested parts, the difference of
temperature measurement is + 4°C.
Figure 6-1. Temperature monitoring gap.
Temp monitoring gap
4.000
3.000
2.000
Part 1
1.000
Part 2
Part 3
0.000
-1.000
5
15
25
35
45
55
65
75
85
95
105
115
125
Part 4
Part 5
-2.000
-3.000
-4.000
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7 Results of VREF recalculation
The chart in Figure 7-1 provides the accuracy (%) of the recalculated VREF versus the
real VREF over the temperature range:
= (VREF Recalculated – VREF output monitoring) / VREF output monitoring
Figure 7-1. Accuracy vs. temperature.
Accuracy vs Temperature
1.00
0.80
0.60
Part 1
0.40
Part 2
Part 3
Part 4
0.20
Part 5
0.00
5
15
25
35
45
55
65
75
85
95
105
115
125
-0.20
-0.40
These typical results confirm that over a temperature range of [+5°C to +125°C]; the
accuracy of the recalculation is better than 1%.
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8 Code example
8.1 C Main function
//! //! Copyright (c) 2009 Atmel.
//! This program uses a loop to:
//! - monitor the temperature sensor
//! - calculate the VREF which should be equal to the real internal VREF
/
//_____I N C L U D E S _________________________________________________
#include "config.h"
#include "iopwm81.h"
#include "my_print.h"
//_____D E C L A T A T I O N S ___________________________________________
#define HIGHBYTE(v) ((unsigned char) (((unsigned int) (v)) >> 8));
#define LOWBYTE(v) ((unsigned char) (v));
int main(void)
{
unsigned char gain, offset, temp, vref_amb_low, vref_hot_low,vref_amb_high,
vref_hot_high,result;
unsigned int vref_recalc, vref_amb, vref_hot, n;
char g, i;
float a, b;
PORTB= 0x00;
DDRB=0xC7;
ADCSRA |= 0x80;/* ADEN=1 */
ADMUX |=0x80; /*Vref=2.56V */
ADMUX &= ~0x2F;
ADMUX |=0x0C; /* MUX to Temp sensor */
ADCSRB =0x80; /* ADC High speed + free running*/
ADCSRA |=0x04; /* prescaler /16 */
ADCSRA |= (1<<ADSC); /* first conversion */
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while (SPMEN==1);
asm("LDI R17,$00");/* Beginning of LPM sequence to read the Temp. sensor
OFFSET in Signature Row */
asm("LDI R16,$05");
asm("MOV R31,R17");/* */
asm("MOV R30,R16");/* ;move address to z pointer (R31=ZH R30=ZL)*/
SPMCSR=0x21;
asm("LPM");/* ;Store program memory*/
asm("MOV R16, R0");/* ;Store return value (1byte->R16 register)*/
asm("OUT 0x1B, R16");/* ;Store return value (1byte->R16 register)*/
while (SPMEN==1);
offset=GPIOR2; /* return of Temp. sensor OFFSET in Signature Row */
asm("LDI R17,$00") ;/*Beginning of LPM sequence to read the Temp. sensor GAIN
in Signature Row */
asm("LDI R16,$07");
asm("MOV R31,R17");/* */
asm("MOV R30,R16");/* ;move adress to z pointer (R31=ZH R30=ZL)*/
SPMCSR=0x21;
asm("LPM");/* ;Store program memory*/
asm("MOV R16, R0");/* ;Store return value (1byte->R16 register)*/
asm("OUT 0x1A, R16");/* ;Store return value (1byte->R16 register)*/
while (SPMEN==1);
gain=GPIOR1; /* return of Temp. sensor GAIN in Signature Row */
asm("LDI R17,$00");/*Beginning of LPM sequence to read the Vref. Amb.(low Byte)
in Signature Row */
asm("LDI R16,$3C");
asm("MOV R31,R17");/* */
asm("MOV R30,R16");/* ;move adress to z pointer (R31=ZH R30=ZL)*/
SPMCSR=0x21;
asm("LPM");/* ;Store program memory*/
asm("MOV R16, R0");/* ;Store return value (1byte->R16 register)*/
asm("OUT 0x1A, R16");/* ;Store return value (1byte->R16 register)*/
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while (SPMEN==1);
vref_amb_low=GPIOR1; /* return of Vref. Amb. Low Byte in Signature Row */
asm("LDI R17,$00") ;/*Beginning of LPM sequence to read the Vref. Amb.(High
Byte) in Signature Row */
asm("LDI R16,$3D");
asm("MOV R31,R17");/* */
asm("MOV R30,R16");/* ;move adress to z pointer (R31=ZH R30=ZL)*/
SPMCSR=0x21;
asm("LPM");/* ;Store program memory*/
asm("MOV R16, R0");/* ;Store return value (1byte->R16 register)*/
asm("OUT 0x1A, R16");/* ;Store return value (1byte->R16 register)*/
while (SPMEN==1);
vref_amb_high=GPIOR1; /* return of Vref. Amb. High Byte in Signature Row */
asm("LDI R17,$00");/*Beginning of LPM sequence to read the Vref. Hot(low Byte) in
Signature Row */
asm("LDI R16,$3E");
asm("MOV R31,R17");/* */
asm("MOV R30,R16");/* ;move adress to z pointer (R31=ZH R30=ZL)*/
SPMCSR=0x21;
asm("LPM");/* ;Store program memory*/
asm("MOV R16, R0");/* ;Store return value (1byte->R16 register)*/
asm("OUT 0x1A, R16");/* ;Store return value (1byte->R16 register)*/
while (SPMEN==1);
vref_hot_low=GPIOR1; /* return of Vref. Hot Low Byte in Signature Row */
asm("LDI R17,$00");/*Beginning of LPM sequence to read the Vref. Hot(High Byte)
in Signature Row */
asm("LDI R16,$3F");
asm("MOV R31,R17");/* */
asm("MOV R30,R16");/* ;move adress to z pointer (R31=ZH R30=ZL)*/
SPMCSR=0x21;
asm("LPM");/* ;Store program memory*/
asm("MOV R16, R0");/* ;Store return value (1byte->R16 register)*/
asm("OUT 0x1A, R16");/* ;Store return value (1byte->R16 register)*/
while (SPMEN==1);
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vref_hot_high=GPIOR1; /* return of Vref. Hot High Byte in Signature Row */
vref_hot= (vref_hot_high * 256) + vref_hot_low;
vref_amb = (vref_amb_high * 256) + vref_amb_low;
a=(vref_hot - vref_amb)*100;
a = a / 0x50;
b= vref_amb - ((a * 0x19)/100);
while(1)
{
while (ADIF == 0);
ADCSRA |=0x10; /* reset ADIF */
temp =ADCL;
result =(ADCH<<8);
result = result | temp;
result = result - (273+25-offset);
temp = gain / 128;
result = result * temp;
temp = result +25;
putchar(0x54); putchar(0x3D);print_hex(temp);/* T=... temperature measurement in
hex format*/
for(i=1;i<100;i++); putchar(0x0D); for(i=1;i<100;i++); putchar(0x0A);
vref_recalc = (((a * temp)/100) + b); /* Vref. recalculated versus the temperature
measurement */
putchar(0x41);putchar(0x3D);
print_hex(vref_amb_high);
print_hex(vref_amb_low);
for(i=1;i<100;i++); putchar(0x0D); for(i=1;i<100;i++); putchar(0x0A); /* A=... Vref
Amb. in hex format*/
putchar(0x48);putchar(0x3D);
print_hex(vref_hot_high);
print_hex(vref_hot_low);
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for(i=1;i<100;i++); putchar(0x0D); for(i=1;i<100;i++); putchar(0x0A); /* H=... Vref
Hot. in hex format*/
putchar(0x52);putchar(0x3D); /* R=... Vref recalculated in hex format*/
temp=HIGHBYTE(vref_recalc);
print_hex (temp);
temp=LOWBYTE(vref_recalc);
print_hex (temp);
for(i=1;i<100;i++); putchar(0x0D); for(i=1;i<100;i++); putchar(0x0A); /* R=... Vref
Recalculated in hex format*/
for(n=1;n<10000;n++)
{
for(i=1;i<100;i++);/* Delay to improve display in Hyperterminal */
}
ADMUX |=0x0C;
ADCSRA |= (1<<ADSC); /* Starts a new conversion on Temp. sensor */
}
}
8.2 C Software UART function
#include "my_print.h"
void print_hex(unsigned char n)
{
unsigned char i;
i=n>>4;
if(i>9) putchar(i-0x0A+'A');
else putchar(i+'0');
i=n&0x0F;
if(i>9) putchar(i-0x0A+'A');
else putchar(i+'0');
}
void print_int(unsigned int n)
{
print_hex(n>>8);
print_hex(n);
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8.3 Assembler Soft_uart.s90
// include the register definitions for the used AT90PWM81/161
mcu
#include "iopwm81.h"
PUBLIC putchar
PUBLIC soft_uart_init
;***** Pin definitions
TxD EQU
0
;Transmit pin is PDx
;***** Global register variables
#define
#define
bitcnt r16
temp
r17
;bit counter
;temporary storage register
#define
Txbyte r18
;Data to be transmitted
RSEG CODE:CODE:NOROOT(1)
;*********************************************************************
;*
;* "putchar"
;*
;* This subroutine transmits the byte stored in the "Txbyte" register
;* The number of stop bits used is set with the sb constant
;*
;* Number of words
:14 including return
;* Number of cycles
:Depens on bit rate
;* Low registers used
:None
;* High registers used
:2 (bitcnt,Txbyte)
;* Pointers used
:None
;*
;*********************************************************************
sb EQU 1
putchar:
cli
;ldi
mov
ldi
14
;Number of stop bits (1, 2, ...)
bitcnt, (9+sb)
;1+8+sb (sb is # of stop bits)
r18, r16
r16, (9+sb) ;1+8+sb (sb is # of stop bits)
com
Txbyte
;Inverte everything
sec
;Start bit
putchar0:
brcc
cbi
rjmp
putchar1
PORTB,TxD
putchar2
;If carry set
;
send a '0'
;else
putchar1:
sbi
nop
PORTB,TxD
;
putchar2:
rcall UART_delay
send a '1'
;One bit delay
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rcall UART_delay
lsr
dec
brne
Txbyte
bitcnt
putchar0
sei
ret
;Get next bit
;If not all bit sent
;
send next
;else
;
return
;*********************************************************************
;*
;* "UART_delay"
;*
;* This delay subroutine generates the required delay between the bits
when
;* transmitting and receiving bytes. The total execution time is set
by the
;* constant "b":
;*
;*
3•b + 7 cycles (including rcall and ret)
;*
;* Number of words
:4 including return
;* Low registers used
:None
;* High registers used
:1 (temp)
;* Pointers used
:None
;*
;*********************************************************************
b EQU 63
UART_delay:
;ldi
temp,b
nop
nop
ldi
r17,b
UART_delay1:
dec
temp
brne UART_delay1
ret
;***** Program Execution Starts Here
;***** Test program
soft_uart_init:
sbi
sbi
ret
forever:
PORTB,TxD
DDRB,TxD
ldi
r18,0x55
rcall putchar
rjmp forever
;Init port pins
;'U'
END
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9 Application note revision history
Please note that the page numbers are referring to this document. The revision
reference in this section is to the document revision.
9.1 Rev. 8270B – 01/12
1. AT90PWM161 is added.
9.2 Rev. 8270A – 09/10
1. Initial version.
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10 Table of contents
Features ............................................................................................... 1
1 Introduction ...................................................................................... 1
2 Theory of operation – the ADC conversion ................................... 2
2.1 VREF control.......................................................................................................... 2
2.2 VREF calibration .................................................................................................... 2
2.3 ADC conversion................................................................................................... 2
3 The compensation method ............................................................. 3
3.1 VREF versus temperature ..................................................................................... 3
3.2 Temperature measurement................................................................................. 3
3.3 VREF recalculation ................................................................................................ 4
3.4 ADC measurement compensation ...................................................................... 4
4 Hardware configuration................................................................... 5
4.1 AT90PWM81/161 ................................................................................................ 5
4.2 STK521................................................................................................................ 5
4.3 Hyperterminal ...................................................................................................... 5
5 Software configuration .................................................................... 6
6 Result of temperature measurement.............................................. 7
7 Results of VREF recalculation .......................................................... 8
8 Code example................................................................................... 9
8.1 C Main function.................................................................................................... 9
8.2 C Software UART function ................................................................................ 13
8.3 Assembler Soft_uart.s90 ................................................................................... 14
9 Application note revision history ................................................. 16
9.1 Rev. 8270B – 01/12........................................................................................... 16
9.2 Rev. 8270A – 09/10........................................................................................... 16
10 Table of contents ......................................................................... 17
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