Unexpected float behaviour in C with AVR atmega8 - c

Question

I've been trying to figure out why I cannot get a sensible value from multiplying an unsigned int with a float value.
Doing something like 65535*0.1 works as expected but multiplying a float with a uint from memory creates mad values. I have a function that reads an ADC and returns an uin16_t. With this value I am printing it to a 4-digit led-display, which is working fine.
Multiplying the same value with 1.0 returns something different entirely (it's too large for my display so I don't really know what it is).
My code is below but the area of contention is at the bottom in main(). Any help would be great. Thanks
main.c:
#include <avr/io.h>
#include <util/delay.h>
#include <avr/interrupt.h>
#include <stdint.h>
#define BAUD 9600
#include <util/setbaud.h>
#define DISP_BRIGHT_CMD 'z'
#define DISP_RESET 'v'
#define ADC_AVG 3
volatile uint8_t hi,lo;
volatile uint16_t result;
ISR(ADC_vect)
{
lo = ADCL;
hi = ADCH;
MCUCR &= ~_BV(SE); //Clear enable sleep
}
void initSerial(void)
{
// set baud rate
UBRR0H = UBRRH_VALUE;
UBRR0L = UBRRL_VALUE;
// set frame format
UCSR0C |= (0x3 << UCSZ00); // 8n1
// set enable tx/rx
UCSR0B = _BV(RXEN0) | _BV(TXEN0);
}
void initADC(void)
{
// AVCC and ADC0
ADMUX = _BV(REFS0);
// Enable, div128, + 1st setup
ADCSRA |= _BV(ADEN)|_BV(ADSC)|_BV(ADPS2)|_BV(ADPS1)|_BV(ADPS0)|_BV(ADIE);
}
uint16_t readADC(void)
{
uint16_t average=0;
// Start Conversion
ADCSRA |= _BV(ADSC);
for (char i=0;i<ADC_AVG;i++) {
MCUCR |= _BV(SE);
ADCSRA |= _BV(ADSC);
__asm volatile("sleep");
MCUCR &= ~_BV(SE);
result = (hi<<8);
result |= lo;
average += result;
}
average /= ADC_AVG;
return average;
}
void sendByte(char val)
{
while (! (UCSR0A & (1<<UDRE0)) ); //wait until tx is complete
UDR0 = val;
}
/*
* Convert voltage to temperature based on a negative coefficient for MAX6613
*/
uint16_t analogToTemp(uint16_t val)
{
uint16_t temp;
//v = 5 * (val/1023.0);
//temp = (1.8455 - (5.0*(val/1023.0)))/0.01123;
temp = (1.8455 - (5.0*(val/1023.0)))*89;
//temp = val * M_PI;
//v = 5 * ( val/1024);
//temp = (2 - v) * 89;
return temp;
}
void initDisplay()
{
sendByte(DISP_RESET);
sendByte(DISP_BRIGHT_CMD);
sendByte(0);
}
void serialSegments(uint16_t val)
{
// 4 digit display
sendByte(val / 1000);
sendByte((val / 100) % 10);
sendByte((val / 10) % 10);
sendByte(val % 10);
}
int main(void)
{
uint16_t calc=0,sense=0;
DDRB |= _BV(DDB5);
PORTB |= _BV(PORTB5);
initSerial();
initADC();
initDisplay();
sei();
MCUCR |= (1 << SM0); // Setting sleep mode to "ADC Noise Reduction"
MCUCR |= (1 << SE); // Sleep enable
for(;;) {
//PORTB ^= _BV(PORTB5);
if (calc>=9999){ // I can't see the real value. Max val on display is 9999
//if (sense>=330){
PORTB |= _BV(PORTB5);
} else {
PORTB &= ~_BV(PORTB5);
}
sense = readADC();
//calc = sense*1.0; // refuses to calculate properly
calc = analogToTemp(sense); // a bunch of zeroes
//calc = 65535*0.1; // a-ok
serialSegments(calc);
_delay_ms(500);
serialSegments(sense);
_delay_ms(500);
}
return 0;
}
Makefile:
# AVR-GCC Makefile
PROJECT=Temp_Display
SOURCES=main.c
CC=avr-gcc
OBJCOPY=avr-objcopy
MMCU=atmega328p
OSC_HZ=16000000UL
OPTIMISATION=2
PORT=/dev/ttyUSB0
CFLAGS=-mmcu=${MMCU} -std=gnu99 -Wall -O${OPTIMISATION} -DF_CPU=${OSC_HZ} -lm -lc
${PROJECT}.hex: ${PROJECT}.out
${OBJCOPY} -j .text -O ihex ${PROJECT}.out ${PROJECT}.hex
avr-size ${PROJECT}.out
$(PROJECT).out: $(SOURCES)
${CC} ${CFLAGS} -I./ -o ${PROJECT}.out ${SOURCES}
program: ${PROJECT}.hex
stty -F ${PORT} hupcl
avrdude -V -F -c arduino -p m168 -b 57600 -P ${PORT} -U flash:w:${PROJECT}.hex
clean:
rm -f ${PROJECT}.out
rm -f ${PROJECT}.hex
EDIT:
OK, i've simplified the code somewhat
#include <avr/io.h>
#include <util/delay.h>
#include <stdint.h>
#define BAUD 9600
#include <util/setbaud.h>
#define DISP_BRIGHT_CMD 'z'
#define DISP_RESET 'v'
void initSerial(void)
{
// set baud rate
UBRR0H = UBRRH_VALUE;
UBRR0L = UBRRL_VALUE;
// set frame format
UCSR0C |= (0x3 << UCSZ00); // 8n1
// set enable tx/rx
UCSR0B = _BV(TXEN0);
}
void sendByte(char val)
{
while (! (UCSR0A & (1<<UDRE0)) ); //wait until tx is complete
UDR0 = val;
}
void initDisplay()
{
sendByte(DISP_RESET);
sendByte(DISP_BRIGHT_CMD);
sendByte(0);
}
void serialSegments(uint16_t val) {
// 4 digit display
sendByte(val / 1000);
sendByte((val / 100) % 10);
sendByte((val / 10) % 10);
sendByte(val % 10);
}
int main(void)
{
uint16_t i=0,val;
DDRB |= _BV(DDB5);
initSerial();
initDisplay();
for(;;) {
val = (uint16_t)(i++ * 1.5);
serialSegments(i);
_delay_ms(500);
serialSegments(val);
_delay_ms(500);
if (val > 9999){
PORTB |= _BV(PORTB5);
} else {
PORTB &= ~_BV(PORTB5);
}
}
return 0;
}

Answer 1

Unsuffixed floating point constant are of type double not float.
Use f suffix to have a float literal, e.g., 0.1f
This can make a huge overhead as MCUs like atmega8 don't have a floating point unit and all floating point operations have to be implemented in firmware by the implementation.
With small devices like atmega8 one usually try to avoid using float operations as without a FPU they are very expensive in CPU cycles.
Now there is no reason an implementation would no correctly translate an expression like:
calc = sense * 1.0;
when calc and sense are of uint16_t type.

Answer 2

Not exactly your code, maybe close enough, maybe not.
First off when I display the output and compare these to lines:
val = (unsigned int)(i++ * 1.5);
...
val = i+(i>>1); i++;
the result is the same. Disassembling shows a few things as well. First off avr-gcc
avr-gcc --version
avr-gcc (GCC) 4.3.4
Copyright (C) 2008 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
uses floats not doubles, so the comments about 1.5F vs 1.5 are quite valid in general but not relevant here. Second it is producing floating point values, single precision, and is doing floating point math, the compiler did not take a shortcut there, it converts to float, does the multiply then converts back.
Using my hex display routine and your decimal display routine (modified to output on a serial terminal), here again it produces the same output, the floating point math does not appear to be the problem.
I started this task to see if the floating point performance was a killer, and it is, but the timing changes depending on how I test it. it took as much as 157 times longer for the code with the floating point compared to fixed point. If I leave in the call to serialSegments(), but have that call a dummy routine instead of the serial port it is 3 times slower for the float. Also I built two different ways and pulled in a libc/m which used a different set of floating point routines, the floating point routines chosen by the C compiler were 7 times slower than the libc/libm.a sitting in the /usr/lib64/avr/lib/ directory. Once you add the waiting on serial port and other delays you may not notice the difference in timing so this experiment while demonstrating that the float is quite painful, is probably not the smoking gun even if your code is timing sensitive, we are talking a few milliseconds.
In addition to the code below I tried this as well:
for(i=0;i<9999;i++)
{
vala = (unsigned int)(i * 1.5);
valb = i+(i>>1); i++;
if(vala!=valb)
{
hexstring16(i);
hexstring16(vala);
hexstring16(valb);
}
}
No fail. I limited to 9999 because serialSegments() only chops up decimals from 0 to 9999. Now your loop goes beyond that to 65535, but you would see that cause problems without the float though, right?
avr.c
#define UCSRA (*((volatile unsigned char *)(0xC0)))
#define UDR (*((volatile unsigned char *)(0xC6)))
#define TCCR0A (*((volatile unsigned char *)(0x44)))
#define TCCR0B (*((volatile unsigned char *)(0x45)))
#define TCNT0 (*((volatile unsigned char *)(0x46)))
#define TCCR1A (*((volatile unsigned char *)(0x80)))
#define TCCR1B (*((volatile unsigned char *)(0x81)))
#define TCNT1L (*((volatile unsigned char *)(0x84)))
#define TCNT1H (*((volatile unsigned char *)(0x85)))
void dummy ( unsigned int );
void uart_putc ( unsigned char c )
{
while(1) if(UCSRA&0x20) break;
UDR=c;
}
void hexstring16 ( unsigned int d )
{
unsigned int rb;
unsigned int rc;
rb=16;
while(1)
{
rb-=4;
rc=(d>>rb)&0xF;
if(rc>9) rc+=0x37; else rc+=0x30;
uart_putc(rc);
if(rb==0) break;
}
uart_putc(0x0D);
uart_putc(0x0A);
}
#ifdef SEGMENTS
void sendByte(char val)
{
uart_putc(0x30+val);
}
void serialSegments(unsigned int val) {
// 4 digit display
dummy(val / 1000);
dummy((val / 100) % 10);
dummy((val / 10) % 10);
dummy(val % 10);
}
//void serialSegments(unsigned int val) {
//// 4 digit display
//sendByte(val / 1000);
//sendByte((val / 100) % 10);
//sendByte((val / 10) % 10);
//sendByte(val % 10);
//uart_putc(0x0D);
//uart_putc(0x0A);
//}
#else
void serialSegments(unsigned int val)
{
dummy(val);
}
//void serialSegments(unsigned int val)
//{
//hexstring(val);
//}
#endif
int main(void)
{
unsigned int i,val;
volatile unsigned int xal,xbl,xcl;
volatile unsigned int xah,xbh,xch;
hexstring16(0x1234);
TCCR1A = 0x00;
TCCR1B = 0x05;
xal=TCNT1L;
xah=TCNT1H;
for(i=0;i<9999;)
{
val = (unsigned int)(i++ * 1.5);
//serialSegments(val);
//hexstring16(val);
dummy(val);
}
xbl=TCNT1L;
xbh=TCNT1H;
for(i=0;i<9999;)
{
val = i+(i>>1); i++;
//serialSegments(val);
//hexstring16(val);
dummy(val);
}
xcl=TCNT1L;
xch=TCNT1H;
xal|=xah<<8;
xbl|=xbh<<8;
xcl|=xch<<8;
hexstring16(xal);
hexstring16(xbl);
hexstring16(xcl);
hexstring16(xbl-xal);
hexstring16(xcl-xbl);
return 0;
}
dummy.s
.globl dummy
dummy:
ret
vectors.s
.globl _start
_start:
rjmp reset
reset:
rcall main
1:
rjmp 1b
.globl dummy
dummy:
ret
Makefile
all : avrone.hex avrtwo.hex
avrone.hex : avr.c dummy.s
avr-as dummy.s -o dummy.o
avr-gcc avr.c dummy.o -o avrone.elf -mmcu=atmega328p -std=gnu99 -Wall -O2 -DSEGMENTS
avr-objdump -D avrone.elf > avrone.list
avr-objcopy avrone.elf -O ihex avrone.hex
a vrtwo.hex : avr.c vectors.s
avr-as vectors.s -o vectors.o
avr-as dummy.s -o dummy.o
avr-gcc -c avr.c -o avrtwo.o -mmcu=atmega328p -std=gnu99 -Wall -O2 -nostartfiles
avr-ld vectors.o avrtwo.o -o avrtwo.elf libc.a libm.a
avr-objdump -D avrtwo.elf > avrtwo.list
avr-objcopy avrtwo.elf -O ihex avrtwo.hex
clean :
rm -f *.hex
rm -f *.elf
This was all run on an arduino pro mini (atmega328p).

Answer 3

Multiplication 65535*0.1 works because it is optimized and precalculated by the compiler, so it translates to 6554.
calc and sense variables are of type uint16_t, and your function analogToTemp() is of that same type and returns that type too. Your runtime calculation inside this function is with uint16_t. It should be done with float, and then truncated to just integer part and casted to the uint16_t result of the function.