short URL https://goo.gl/xVY56d
problem & observation https://xiaolaba.wordpress.com/2015/05/27/arduino-c-code-and-problem-for-loop-do-while-loop/
further to test, C code
結論是, ATMEGA2560 (或所有 AVR 8 bit MCU CORE),
1) 沒有 8 bit 常數加法指令 ADD, 只有16 bit 的 adwi, 所以這GCC編譯器加法執行用減法指令 subi, 例如 +1 = -(-1) = -(0xff)
2) 只有用 brne 指令, 所以 “小於或等於" 或 “不大於" 並沒有使用對應指令, <= 無法執行, 除非比較 n+1, 因此 <=0xfe, 編碼試驗 <= (0xfe+1), 實際與 0xff 比較, 無法用 8 bit counter 配合 for-loop 達成 0 至 255 的計數 (256), 需要改用16 bit counter 或棄用 for-loop, 改用 do-while-loop, 並且 i++ 與 ++i 並沒有分別, 實際編碼都是先加值, 再比較. 所以用 asm 編寫 8 bit for-loop, 與C編譯器生成的編碼嚴重分歧. 也解釋了C程序跑飛的實際原因. 到底有甚麼法方法呢 ? 還是 GCC 配 AVR8 就只能這樣 ?
/*
test routine to understand what is the for-loop and code generation
2015-05-28 xiaolaba
AVR8
arduino 0022. GCC version ?
*/
void setup() {
// initialize the digital pin as an output.
// Pin 13 has an LED connected on most Arduino boards:
//pinMode(13, OUTPUT);
}
void loop() {
/*
//C code, works fine
byte i =0;
do {
i++;
__asm ("nop");
__asm ("pop r21");
} while (i != 10) ;
//C code, works fine
i =0;
do {
i++;
__asm ("nop");
__asm ("pop r23");
} while (i != 0) ;
__asm ("nop");
i =0;
do {
i++;
__asm ("nop");
__asm ("pop r23");
} while (i != 0);
//C code, works fine
for (int i = 0; i < 512; i += 2) { // OK
__asm ("nop");
__asm ("pop r24");
}
*/
//C code, not working but why !!!
for (byte i = 0; i < 0xff; i++) {
__asm ("nop");
__asm ("pop r25");
}
for (byte i = 0; i <= 0xfe; ++i) {
__asm ("nop");
__asm ("pop r26");
}
for (byte i = 0; i <= 0x10; ++i) {
__asm ("nop");
__asm ("pop r27");
}
for (byte i = 0; i != 0xff ; ++i) {
__asm ("nop");
__asm ("pop r28");
}
}
this ASM code, generated for such loop test 0000012e : 12e: 80 e0 ldi r24, 0x00 ; 0 130: 00 00 nop 132: 9f 91 pop r25 134: 8f 5f subi r24, 0xFF ; 255 136: 8f 3f cpi r24, 0xFF ; 255 138: d9 f7 brne .-10 ; 0x130 13a: 80 e0 ldi r24, 0x00 ; 0 13c: 00 00 nop 13e: af 91 pop r26 140: 8f 5f subi r24, 0xFF ; 255 142: 8f 3f cpi r24, 0xFF ; 255 144: d9 f7 brne .-10 ; 0x13c 146: 80 e0 ldi r24, 0x00 ; 0 148: 00 00 nop 14a: bf 91 pop r27 14c: 8f 5f subi r24, 0xFF ; 255 14e: 81 31 cpi r24, 0x11 ; 17 150: d9 f7 brne .-10 ; 0x148 152: 80 e0 ldi r24, 0x00 ; 0 154: 00 00 nop 156: cf 91 pop r28 158: 8f 5f subi r24, 0xFF ; 255 15a: 8f 3f cpi r24, 0xFF ; 255 15c: d9 f7 brne .-10 ; 0x154 15e: 08 95 ret
rem This is asm.bat, uses avr-objdump to obtian assembler listing rem change buildxxxxxxxxxxxxxx.tmp, every time arduino IDE has a different path rem change xxxx.xxx.xxxx.elf, every project has own project name set build=build7733546659278691547.tmp set elf_file=loop_test.cpp.elf set input=C:\DOCUME~1\user\LOCALS~1\Temp\%build%\%elf_file% set output=%elf_file%.txt avr-objdump -d %input% > %output%
.
.
2020-JAN-24, update.
補記, 解決方案
明確定義, 計數器定義為 short, 只有 8 bit, 對比以上案例的 byte 不同.
short i;
for (i = 0; i <=0xff; i++) {}
生成的 ASM, 確定可以計數 0 – 0xff, 共256次
short i;
for (i = 0; i <=0xff; i++) {
154: 2f 5f subi r18, 0xFF ; 255
156: 3f 4f sbci r19, 0xFF ; 255
158: 81 e0 ldi r24, 0x01 ; 1
15a: 20 30 cpi r18, 0x00 ; 0
15c: 38 07 cpc r19, r24
15e: 11 f6 brne .-124 ; 0xe4
完整源碼,
https://github.com/xiaolaba/ATmega328_usart_print0xff
/*
* UART, dump 0x00 to 0xff to terminal
* hardware, Arduino Nano, Atmega168p/328p, 16MHz XTAL
* compiler, Arduino IDE 1.8.9 or, avr-gcc
* xiaolaba
* 2020-JAN-24
*/
#include avr/io.h
#include util/twi.h
#define F_CPU 16000000L
#define SERIAL_BAUD 115200L
#define UBBRn_V ((F_CPU / 16 + SERIAL_BAUD / 2) / SERIAL_BAUD - 1)
static void usart_putstr(char *s);
static void usart_putchar(char data);
static void usart_printhex(uint8_t v);
static void serial_init(void);
static char HEX_TABLE[]="0123456789ABCDEF";
int main(void)
{
serial_init();
return 0;
}
/** Initializes the serial port and prints "banner. */
// serial port 115200 N81
static void serial_init(void)
{
UBRR0H = UBBRn_V >> 8;
UBRR0L = UBBRn_V;
UCSR0B = _BV(TXEN0);
usart_putstr("xiaolaba\n");
usart_putstr("Arduino Nano, ATmega168, 115200, N81, serial print 0x00 - 0xff\n");
short i;
for (i = 0; i > 4;
usart_putchar(HEX_TABLE[vn]);
vn = v & 0x0F;
usart_putchar(HEX_TABLE[vn]);
}
// ********************************************************************************
// usart Related
// ********************************************************************************
static void usart_putchar(char data) {
// Wait for empty transmit buffer
while ( !(UCSR0A & (_BV(UDRE0))) );
// Start transmission
UDR0 = data;
}
static void usart_putstr(char *s) {
// loop through entire string
while (*s) {
usart_putchar(*s);
s++;
}
}
a.bat, 不開 Arduino IDE, 直接用 avr-gcc 編譯也可以
@echo off ::set mcu=mega324p set mcu=mega168p set main=usart_print0xff.ino set ac=C:\WinAVR-20100110 path %ac%\bin;%ac%\utils\bin;%path% avr-gcc.exe -dumpversion avr-gcc.exe -xc -Os -mmcu=at%mcu% -Wall -g -o %main%.out *.ino ::avr-gcc.exe -O2 -Wl,-Map,%1.map -o %1.out %1.c %2 %3 -mmcu=at%mcu% cmd /c avr-objdump.exe -h -S %main%.out >%main%.lst cmd /c avr-objcopy.exe -O ihex %main%.out %main%.hex avr-size.exe %main%.out del %main%.out ::goto end ::pboot.exe -c1 -b19200 -p%main%.hex ::l.exe -b9600 :end
結果的畫面如下
xiaolabaDIY 