make all
Building file: ../ATCOMMANDS/atcommands.c
Invoking: AVR Compiler
avr-gcc -Wall -Os -fpack-struct -fshort-enums -ffunction-sections -fdata-sections -std=gnu99 -funsigned-char -funsigned-bitfields -mmcu=atmega88 -DF_CPU=11059200UL -MMD -MP -MF"ATCOMMANDS/atcommands.d" -MT"ATCOMMANDS/atcommands.d" -c -o "ATCOMMANDS/atcommands.o" "../ATCOMMANDS/atcommands.c"
Finished building: ../ATCOMMANDS/atcommands.c

Building file: ../1Wire/crc8.c
Invoking: AVR Compiler
avr-gcc -Wall -Os -fpack-struct -fshort-enums -ffunction-sections -fdata-sections -std=gnu99 -funsigned-char -funsigned-bitfields -mmcu=atmega88 -DF_CPU=11059200UL -MMD -MP -MF"1Wire/crc8.d" -MT"1Wire/crc8.d" -c -o "1Wire/crc8.o" "../1Wire/crc8.c"
Finished building: ../1Wire/crc8.c

Building file: ../1Wire/ds18x20.c
Invoking: AVR Compiler
avr-gcc -Wall -Os -fpack-struct -fshort-enums -ffunction-sections -fdata-sections -std=gnu99 -funsigned-char -funsigned-bitfields -mmcu=atmega88 -DF_CPU=11059200UL -MMD -MP -MF"1Wire/ds18x20.d" -MT"1Wire/ds18x20.d" -c -o "1Wire/ds18x20.o" "../1Wire/ds18x20.c"
Finished building: ../1Wire/ds18x20.c

Building file: ../1Wire/onewire.c
Invoking: AVR Compiler
avr-gcc -Wall -Os -fpack-struct -fshort-enums -ffunction-sections -fdata-sections -std=gnu99 -funsigned-char -funsigned-bitfields -mmcu=atmega88 -DF_CPU=11059200UL -MMD -MP -MF"1Wire/onewire.d" -MT"1Wire/onewire.d" -c -o "1Wire/onewire.o" "../1Wire/onewire.c"
Finished building: ../1Wire/onewire.c

Building file: ../common.c
Invoking: AVR Compiler
avr-gcc -Wall -Os -fpack-struct -fshort-enums -ffunction-sections -fdata-sections -std=gnu99 -funsigned-char -funsigned-bitfields -mmcu=atmega88 -DF_CPU=11059200UL -MMD -MP -MF"common.d" -MT"common.d" -c -o "common.o" "../common.c"
Finished building: ../common.c

../main.c:18:19: error: redefinition of ‘tempPeriodTime’
volatile uint16_t tempPeriodTime = 60; //liczba sekund pomiedzy odczytami temperatury
^
In file included from ../main.c:15:0:
../common.h:15:19: note: previous definition of ‘tempPeriodTime’ was here
volatile uint16_t tempPeriodTime = 60; //liczba sekund pomiedzy odczytami temperatury
Building file: ../main.c
Invoking: AVR Compiler
avr-gcc -Wall -Os -fpack-struct -fshort-enums -ffunction-sections -fdata-sections -std=gnu99 -funsigned-char -funsigned-bitfields -mmcu=atmega88 -DF_CPU=11059200UL -MMD -MP -MF"main.d" -MT"main.d" -c -o "main.o" "../main.c"
^
../main.c:19:19: error: redefinition of ‘totalConsumption’
volatile uint16_t totalConsumption = 0;
^
In file included from ../main.c:15:0:
../common.h:16:19: note: previous definition of ‘totalConsumption’ was here
subdir.mk:21: polecenia dla obiektu 'main.o' nie powiodły się
volatile uint16_t totalConsumption = 0;
^
make: *** [main.o] Błąd 1

jak zorganizować projekt aby było przejrzyście i zgodnie ze sztuką ?

PS. dlaczego po zmianie znaczników z code na syntax zginęły wartości includów ?

bo przy kompilacji mam taką listę błędów

../main.c:18:19: error: redefinition of ‘tempPeriodTime’

In file included from ../main.c:15:0:
../common.h:15:19: note: previous definition of ‘tempPeriodTime’ was here
volatile uint16_t tempPeriodTime = 60;

17:53:54 **** Incremental Build of configuration Release for project usart ****
make all
Building file: ../main.c
Invoking: AVR Compiler
avr-gcc -Wall -Os -fpack-struct -fshort-enums -ffunction-sections -fdata-sections -std=gnu99 -funsigned-char -funsigned-bitfields -mmcu=atmega88 -DF_CPU=11059200UL -MMD -MP -MF"main.d" -MT"main.d" -c -o "main.o" "../main.c"
Finished building: ../main.c

Building target: z_uart.elf
Invoking: AVR C Linker
avr-gcc -Wl,-Map,z_uart.map -mmcu=atmega88 -o "z_uart.elf" ./MKUART/mkuart.o ./ATCOMMANDS/atcommands.o ./1Wire/crc8.o ./1Wire/ds18x20.o ./1Wire/onewire.o ./common.o ./main.o
./main.o:(.bss.totalConsumption+0x0): multiple definition of `totalConsumption'
makefile:53: polecenia dla obiektu 'z_uart.elf' nie powiodły się
./ATCOMMANDS/atcommands.o:(.bss.totalConsumption+0x0): first defined here
./main.o:(.data.currentHeatState+0x0): multiple definition of `currentHeatState'
./ATCOMMANDS/atcommands.o:(.data.currentHeatState+0x0): first defined here
./main.o:(.data.previousHeatState+0x0): multiple definition of `previousHeatState'
./ATCOMMANDS/atcommands.o:(.data.previousHeatState+0x0): first defined here
./main.o:(.data.tempPeriodTime+0x0): multiple definition of `tempPeriodTime'
./ATCOMMANDS/atcommands.o:(.data.tempPeriodTime+0x0): first defined here
collect2: error: ld returned 1 exit status
make: *** [z_uart.elf] Błąd 1

17:53:55 Build Finished (took 508ms)

17:57:13 **** Incremental Build of configuration Release for project usart ****
make all
Building file: ../main.c
Invoking: AVR Compiler
avr-gcc -Wall -Os -fpack-struct -fshort-enums -ffunction-sections -fdata-sections -std=gnu99 -funsigned-char -funsigned-bitfields -mmcu=atmega88 -DF_CPU=11059200UL -MMD -MP -MF"main.d" -MT"main.d" -c -o "main.o" "../main.c"
../main.c: In function ‘main’:
../main.c:166:25: error: ‘totalConsumption’ undeclared (first use in this function)
sendTotalConsumption(totalConsumption);
^
../main.c:166:25: note: each undeclared identifier is reported only once for each function it appears in
subdir.mk:21: polecenia dla obiektu 'main.o' nie powiodły się
../main.c:195:11: error: ‘currentHeatState’ undeclared (first use in this function)
switch (currentHeatState) {
^
../main.c:196:8: error: ‘PERMANENT’ undeclared (first use in this function)
case PERMANENT :
^
../main.c:197:27: error: ‘previousHeatState’ undeclared (first use in this function)
if(currentHeatState != previousHeatState) {
^
../main.c:206:8: error: ‘VARY’ undeclared (first use in this function)
case VARY :
^
../main.c:217:24: error: ‘OFF’ undeclared (first use in this function)
currentHeatState = OFF;
^
../main.c:239:18: error: ‘tempPeriodTime’ undeclared (first use in this function)
if (seconds % tempPeriodTime == 0)
^
make: *** [main.o] Błąd 1

17:57:13 Build Finished (took 205ms)

czyli common musi byc załączony

Bo to są DEKLARACJE zmiennych

zaś w pliku d_led.c masz bez extern i to są DEFINICJE tych zmiennych

nie zwróciłeś uwagi w Bluebooku na to jak naciskam mocno na to aby czytelnik zrozumiał czym różni się DEKLARACJA od DEFINICJI ? .... brak zrozumienia tego zagadnienia będzie powodował, że wciąż będzie się miało problemy ze zrozumieniem różnych części kodu w C

nie musiałem robić extern
Wystarczyło ,ze w common.h