Follow-up Comment #2, bug #34423 (project avr-libc): Thank you for your post. Yes, you're rigth. It should work, but doesn't and I wonder why. (The compiler options are as default in AVRStudio 5). Here's wider description of my problem.
I am writing a simple project with Maxim digital thermometer ds18b20 in Atmel AVRStudio 5 using AVRlibc. The main goal is to understand how gcc works, that's why any suggestions are welcome. Uploaded sources are available at http://students.mimuw.edu.pl/~kg277366/thermometer.zip with default AVRStudio makefile. The built files are included as well. The main problem with this project is the fact that the MSB of temperature is incorrect. GCC uses register r27 with undefined value instead of r14 containing MSB. The part of thermometer.lss file which shows this code disassembly is attached at the end of ds18b20.h file and below. Runtime tests confirm wrong value of MSB. _crc_ibutton_update function comes from AVRlibc util/crc16.h file. According to my tests, loss of MSB occurs only when my ds_read_temp_crc function is inlined and AVRlibc _crc_ibutton_update function is called. That's why I guess GCC has problem with _crc_ibutton_update function. Of course I don't exclude error in my source code. I'll be thankful for any ideas explaining this weird compiler behavior. static int16_t ds_read_temp_crc() { int16_t temp; uint8_t i, data, crc; if (!ds_reset()) 2e2: 0e 94 82 00 call 0x104 ; 0x104 <ds_reset> 2e6: 88 23 and r24, r24 2e8: 09 f4 brne .+2 ; 0x2ec <main+0x118> { return DS_RESET_TEMP; } 2ea: 7b c0 rjmp .+246 ; 0x3e2 <main+0x20e> // skip ROM ds_write_byte(0xCC); 2ec: 8c ec ldi r24, 0xCC ; 204 2ee: 0e 94 9e 00 call 0x13c ; 0x13c <ds_write_byte> // read scratch pad ds_write_byte(0xBE); 2f2: 8e eb ldi r24, 0xBE ; 190 2f4: 0e 94 9e 00 call 0x13c ; 0x13c <ds_write_byte> // LSB of temp data = ds_read_byte(); temp = (int16_t)data; 2f8: 0e 94 c6 00 call 0x18c ; 0x18c <ds_read_byte> 2fc: 08 2f mov r16, r24 ; LSB stored in r16 2fe: 80 e0 ldi r24, 0x00 ; 0 crc = _crc_ibutton_update(0, data); 300: 60 2f mov r22, r16 302: 0e 94 41 00 call 0x82 ; 0x82 <_crc_ibutton_update> 306: 18 2f mov r17, r24 // MSB of temp data = ds_read_byte(); temp |= (int16_t)(data) << 8; 308: 0e 94 c6 00 call 0x18c ; 0x18c <ds_read_byte> 30c: e8 2e mov r14, r24 ; MSB stored in r14 30e: 81 2f mov r24, r17 crc = _crc_ibutton_update(crc, data); 310: 6e 2d mov r22, r14 312: 0e 94 41 00 call 0x82 ; 0x82 <_crc_ibutton_update> 316: d8 2e mov r13, r24 for (i = 0; i < 7; i++) { data = ds_read_byte(); crc = _crc_ibutton_update(crc, data); } 318: 17 e0 ldi r17, 0x07 ; 7 31a: 0e 94 c6 00 call 0x18c ; 0x18c <ds_read_byte> 31e: 68 2f mov r22, r24 320: 8d 2d mov r24, r13 322: 0e 94 41 00 call 0x82 ; 0x82 <_crc_ibutton_update> 326: d8 2e mov r13, r24 328: 11 50 subi r17, 0x01 ; 1 32a: b9 f7 brne .-18 ; 0x31a <main+0x146> if (crc) { return DS_CRC_ERROR; } 32c: 88 23 and r24, r24 32e: 09 f0 breq .+2 ; 0x332 <main+0x15e> 330: 5b c0 rjmp .+182 ; 0x3e8 <main+0x214> 332: a0 e0 ldi r26, 0x00 ; r27:r26 contains: 0x?? 00 334: ed 01 movw r28, r26 ; r29:r28 contains: 0x?? 00 336: 10 e0 ldi r17, 0x00 ; r17:r16 contains: 0x00 LSB return temp; 338: 0c 2b or r16, r28 ; r17:r16 := r17:r16 OR r29:r28, where r29 is undefined instead of MSB 33a: 1d 2b or r17, r29 // temp in r17:r29 33c: b5 e0 ldi r27, 0x05 ; 5 33e: 00 35 cpi r16, 0x50 ; 80 340: 1b 07 cpc r17, r27 342: 09 f4 brne .+2 ; 0x346 <main+0x172> 344: 53 c0 rjmp .+166 ; 0x3ec <main+0x218> 346: e0 e1 ldi r30, 0x10 ; 16 348: 00 31 cpi r16, 0x10 ; 16 34a: 1e 07 cpc r17, r30 34c: 09 f4 brne .+2 ; 0x350 <main+0x17c> 34e: 4e c0 rjmp .+156 ; 0x3ec <main+0x218> _______________________________________________________ Reply to this item at: <http://savannah.nongnu.org/bugs/?34423> _______________________________________________ Message sent via/by Savannah http://savannah.nongnu.org/ _______________________________________________ AVR-libc-dev mailing list AVR-libc-dev@nongnu.org https://lists.nongnu.org/mailman/listinfo/avr-libc-dev
