使用 AVR atmega8 在 C 中出现意外的浮动行为
我一直试图弄清楚为什么我无法通过将无符号整数与浮点值相乘来获得合理的值。
执行类似 65535*0.1 的操作可以按预期工作，但是将浮点数与内存中的 uint 相乘会产生疯狂的值。我有一个读取 ADC 并返回 uin16_t 的函数。我将这个值打印到 4 位 LED 显示屏上，该显示屏工作正常。
将相同的值乘以 1.0 会返回完全不同的结果（它对于我的显示来说太大了，所以我真的不知道它是什么）。
我的代码如下，但争用区域位于 main() 的底部。任何帮助都会很棒。谢谢
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
编辑： 好的，我稍微简化了代码
#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;
}
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;
}
如果你对这篇内容有疑问，欢迎到本站社区发帖提问 参与讨论，获取更多帮助，或者扫码二维码加入 Web 技术交流群。
绑定邮箱获取回复消息
由于您还没有绑定你的真实邮箱，如果其他用户或者作者回复了您的评论，将不能在第一时间通知您！
发布评论
评论（3）
无后缀浮点常量的类型为
double，而不是float。使用
f后缀获得float文字，例如0.1f这可能会产生巨大的开销，因为像 atmega8 这样的 MCU 没有浮动点单元和所有浮点运算必须通过实现在固件中实现。
对于像 atmega8 这样的小型设备，人们通常会尽量避免使用浮点运算，因为如果没有 FPU，它们的 CPU 周期会非常昂贵。
实现没有理由无法正确翻译如下表达式：
calc = sense * 1.0;现在，当
calc和sense为时， >uint16_t类型。Unsuffixed floating point constant are of type
doublenotfloat.Use
fsuffix to have afloatliteral, e.g.,0.1fThis 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
floatoperations 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
calcandsenseare ofuint16_ttype.不完全是你的代码，也许足够接近，也许不是。
首先，当我显示输出并将它们与行进行比较时：
结果是相同的。拆解也能看出一些东西。首先，avr-gcc
使用浮点数而不是双精度数，因此关于 1.5F 与 1.5 的评论一般来说是相当有效的，但在这里不相关。其次，它生成浮点值，单精度，并且正在进行浮点数学运算，编译器没有采取捷径，它转换为浮点数，进行乘法，然后再转换回来。
使用我的十六进制显示例程和您的十进制显示例程（修改为在串行终端上输出），这里再次产生相同的输出，浮点数学似乎不是问题。
我开始这个任务是为了看看浮点性能是否是杀手，确实如此，但时间会根据我的测试方式而变化。与定点相比，使用浮点的代码花费的时间要多 157 倍。如果我保留对serialSegments()的调用，但让其调用虚拟例程而不是串行端口，那么浮点数的速度会慢3倍。另外，我构建了两种不同的方法，并引入了 libc/m，它使用了一组不同的浮点例程，C 编译器选择的浮点例程比 /usr/lib64 中的 libc/libm.a 慢 7 倍/avr/lib/ 目录。一旦添加了串行端口上的等待和其他延迟，您可能不会注意到时间上的差异，因此这个实验虽然证明了浮动非常痛苦，但即使您的代码对时间敏感，也可能不是确凿的证据，我们正在谈论一些毫秒。
除了下面的代码之外，我还尝试了这个：
for(i=0;i<9999;i++)
{
vala = (无符号整数)(i * 1.5);
valb = i+(i>>1);我++；
if(vala!=valb)
{
十六进制字符串16(i);
hexstring16(vala);
hexstring16(valb);
}
没有失败
。我限制为 9999，因为serialSegments() 只截取从 0 到 9999 的小数。现在你的循环超出了这个范围，达到了 65535，但是你会发现如果没有浮动，这会导致问题，对吗？
avr.c
dummy.s
Vectors.s
Makefile
这一切都在 arduino pro mini (atmega328p) 上运行。
Not exactly your code, maybe close enough, maybe not.
First off when I display the output and compare these to lines:
the result is the same. Disassembling shows a few things as well. First off avr-gcc
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
dummy.s
vectors.s
Makefile
This was all run on an arduino pro mini (atmega328p).