Blink

1. blink.ino

1. So you are no doubt familiar with the well known blink.ino:

   Code

   // the setup function runs once when you press reset or power the board
   void setup() {
   // initialize digital pin LED_BUILTIN as an output.
   pinMode(LED_BUILTIN, OUTPUT);
   }
   
   // the loop function runs over and over again forever
   void loop() {
   digitalWrite(LED_BUILTIN, HIGH);  // turn the LED on (HIGH is the voltage level)
   delay(1000);                      // wait for a second
   digitalWrite(LED_BUILTIN, LOW);   // turn the LED off by making the voltage LOW
   delay(1000);                      // wait for a second
   }
   

   There is a lot of abstraction happening here, where are the ports/registers?

   Let's now turn this into an embedded program!

2. Create blink.c

1. Create a new directory called embeddedC and a child directory blink

   Terminal

   $ mkdir -p ~/embeddedC/blink && cd embeddedC/blink
   

2. Create a new file inside the the blink directory called blink.c

   Terminal

   $ touch blink.c
   

3. Now it's time start wrighting out the program:

   Code

   #include <avr/io.h> // provides macros and definitions for accessing registers and hardware-specific functions of AVR microcontrollers
   
   #define F_CPU 16000000UL
   
   #include <util/delay.h> // provides delay functions, including _delay_ms() for millisecond delays
   
   #define BLINK_DELAY_MS 2000
   
   int main (void)
   {
       // Arduino digital pin 13 (pin 5 of PORTB) for output
       DDRB |= 0b00100000; // PORTB5
   
       while(1) {
           // turn LED on
           PORTB |= 0b00100000; // PORTB5
           _delay_ms(BLINK_DELAY_MS);
   
           // turn LED off
           PORTB &= ~ 0b00100000; // PORTB5
           _delay_ms(BLINK_DELAY_MS);
       }
   }
   

   Explanation bitwise operations

   • |= operator is used to set (turn on) specific bits without changing others. It performs a bitwise OR operation with the current value of a register, setting only the bits t correspond to 1 in the mask.

     • // Initial value: all bits are 0
       uint8_t register_value = 0b00000000; 
           // Set bit 2
       register_value |= (1 << 2); // 0b00000100
           // Result: register_value = 0b00000100 (Bit 2 is now set)
       

       • &= ~ to clear (turn off) a specific bit, we use the &= operator combined with the bitwise NOT ~. This combination creates a mask where only the desired bit is set to 0, le all other bits are 1. The &= operation then turns off the targeted bit without affecting others

     • // Initial value: all bits are 0
       uint8_t register_value = 0b00000000; 
           // Set bit 2
       register_value &= ~(1 << 2); // 0b11111011
           // Result: register_value = 0b00000000 (Bit 2 is now set)
       

3. Compiling: Object file

4. We need to compile the code so that it can be uploaded to the atmega328p chip:

   • we are going to be using the avr-gcc compiler to first create the object file:

   Terminal

   $ avr-gcc -Os -mmcu=atmega328p -DF_CPU16000000UL -c -o blink.o blink.c`
   

   Command Breakdown

   • -Os

     • -mmcu=atmega328p configures the compilation process to generate code compatible with the ATmega328P's specific architecture, instruction set, and memory layout

     • -DF_CPU=16000000UL the -D option defines a macro (F_CPU) that will be used in the code, similar to adding #define F_CPU 16000000UL (16MHz) at the top of `blink.

     • -c flag tells avr-gcc to compile the source file into an object file (.o) rather than a full executable

     • -o specifies the output file for the compilation. -o allows you to name the output file, which in this case is blink.o

5. Exploring the object file, run the following the command avr-objdump -d -S blink.o:

   Terminal

   $ avr-objdump -d -S blink.o
   

   output

   Output

   blink.o:     file format elf32-avr
   
   Disassembly of section .text.startup:
   
   00000000 <main>:
   0:    25 9a           sbi     0x04, 5 ; 4
   2:    2d 9a           sbi     0x05, 5 ; 5
   4:    2f ef           ldi     r18, 0xFF       ; 255
   6:    87 ea           ldi     r24, 0xA7       ; 167
   8:    91 e6           ldi     r25, 0x61       ; 97
   a:    21 50           subi    r18, 0x01       ; 1
   c:    80 40           sbci    r24, 0x00       ; 0
   e:    90 40           sbci    r25, 0x00       ; 0
   10:   01 f4           brne    .+0             ; 0x12 <main+0x12>
   12:   00 c0           rjmp    .+0             ; 0x14 <main+0x14>
   14:   00 00           nop
   16:   2d 98           cbi     0x05, 5 ; 5
   18:   2f ef           ldi     r18, 0xFF       ; 255
   1a:   87 ea           ldi     r24, 0xA7       ; 167
   1c:   91 e6           ldi     r25, 0x61       ; 97
   1e:   21 50           subi    r18, 0x01       ; 1
   20:   80 40           sbci    r24, 0x00       ; 0
   22:   90 40           sbci    r25, 0x00       ; 0
   24:   01 f4           brne    .+0             ; 0x26 <main+0x26>
   26:   00 c0           rjmp    .+0             ; 0x28 <main+0x28>
   28:   00 00           nop
   2a:   00 c0           rjmp    .+0             ; 0x2c <__zero_reg__+0x2b>
   

   • sbi 0x04, 5: Set bit 5 in the I/O register at address 0x04, typically used to configure a pin as an output. The sbi instruction sets a specific bit in an I/O register.

   • ldi r18, 0xFF: Load the constant 0xFF (255 in decimal) into register r18. This sets up a value, often for delays or looping.

   • subi r18, 0x01: Subtracts 1 from r18. This, along with the sbci instructions, forms a delay loop by decrementing registers r18, r24, and r25 until they reach zero.

   • brne .+0: Branch if not equal (loop back if r18, r24, or r25 is non-zero). The program will keep looping here to create a delay.

   • rjmp .+0: Relative jump (infinite loop). This is often used to create a long delay or an idle state. The program will stall here until an external reset or interrupt occurs.

   • cbi 0x05, 5: Clear bit 5 in I/O register 0x05, likely to turn off the LED.

   • sbi/cbi instructions toggle an LED pin on and off.

   • ldi, subi, sbci, and brne form delay loops by decrementing registers until they reach zero.

   • rjmp and nop provide additional delays or idle periods

   Info

   All information can be found here for ISA AVR Instruction Set

4. Compiling: .elf (Executable and Linkable Format) file

6. Now we can make the .elf (Executable and Linkable Format) file, run the following code:

   Terminal

   $ avr-gcc -Os -DF_CPU=16000000 -mmcu=atmega328p -o blink.elf blink.o
   

7. Exploring the object file, run the following the command:

   Terminal

   $ avr-objdump -h -s blink.elf
   

   Output

   
   blink.elf:     file format elf32-avr
   
   Sections:
   Idx Name          Size      VMA       LMA       File off  Algn
   0 .data         00000000  00800100  000000B0  00000124  2**0
                   CONTENTS, ALLOC, LOAD, DATA
   1 .text         000000B0  00000000  00000000  00000074  2**1
                   CONTENTS, ALLOC, LOAD, READONLY, CODE
   2 .comment      00000011  00000000  00000000  00000124  2**0
                   CONTENTS, READONLY
   3 .note.gnu.avr.deviceinfo 00000040  00000000  00000000  00000138  2**2
                   CONTENTS, READONLY
   4 .debug_aranges 00000020  00000000  00000000  00000178  2**3
                   CONTENTS, READONLY, DEBUGGING
   5 .debug_info   000006af  00000000  00000000  00000198  2**0
                   CONTENTS, READONLY, DEBUGGING
   6 .debug_abbrev 000005b6  00000000  00000000  00000847  2**0
                   CONTENTS, READONLY, DEBUGGING
   7 .debug_line   0000007c  00000000  00000000  00000dfd  2**0
                   CONTENTS, READONLY, DEBUGGING
   8 .debug_str    00000208  00000000  00000000  00000e79  2**0
                   CONTENTS, READONLY, DEBUGGING
   Contents of section .text:
   0000 0c943400 0c943e00 0c943e00 0c943e00  ..4...>...>...>.
   ...
   Contents of section .comment:
   ...
   Contents of section .note.gnu.avr.deviceinfo:
   ...
   Contents of section .debug_aranges:
   ...
   Contents of section .debug_info:
   ...
   Contents of section .debug_abbrev:
   ...
   Contents of section .debug_line:
   ...
   Contents of section .debug_str:
   ...
   

5. Compiling: hex file

8. Now we need to create the .hex file to be burned to the atmega328p:

   Terminal

   avr-objcopy.exe -O ihex blink.elf blink.hex
   

   Command Breakdown

   • avr-objcopy is a tool that copies and converts object files. In AVR development, it’s commonly used to convert compiled .elf files into formats suitable for uploading to microcontrollers, such as Intel HEX
   • -O ihex option specifies the output format. -O stands for "output format," and ihex refers to Intel HEX format.

9. Now you can look inside the hex code:

   Terminal

   $ avr-objdump.exe -h -s blink.hex`
   

   Output

   blink.hex:     file format ihex
   
   Sections:
   Idx Name          Size      VMA       LMA       File off  Algn
   0 .sec1         000000B0  00000000  00000000  00000000  2**0
                   CONTENTS, ALLOC, LOAD
   Contents of section .sec1:
   0000 0c943400 0c943e00 0c943e00 0c943e00  ..4...>...>...>.
   0010 0c943e00 0c943e00 0c943e00 0c943e00  ..>...>...>...>.
   0020 0c943e00 0c943e00 0c943e00 0c943e00  ..>...>...>...>.
   0030 0c943e00 0c943e00 0c943e00 0c943e00  ..>...>...>...>.
   0040 0c943e00 0c943e00 0c943e00 0c943e00  ..>...>...>...>.
   0050 0c943e00 0c943e00 0c943e00 0c943e00  ..>...>...>...>.
   0060 0c943e00 0c943e00 11241fbe cfefd8e0  ..>...>..$......
   0070 debfcdbf 0e944000 0c945600 0c940000  ......@...V.....
   0080 259a2d9a 2fef87ea 91e62150 80409040  %.-./.....!P.@.@
   0090 e1f700c0 00002d98 2fef87ea 91e62150  ......-./.....!P
   00a0 80409040 e1f700c0 0000ebcf f894ffcf  .@.@............
   

   Info

   • Sections

     • Idx: Index of the section, here it’s 0 because there’s only one section.
     • Name (.sec1): The name of this section, which is likely used for the program code.
     • Size (000000B0): The size of this section in hexadecimal (0xB0, or 176 bytes).
     • VMA (Virtual Memory Address): The address in memory where this section will be loaded, here it starts at 0x00000000.
     • LMA (Load Memory Address): Where the section is loaded in flash memory, matching the VMA.
     • File Offset (00000000): The offset in the file where this section’s data starts.
     • Alignment (2**0): Specifies that there’s no special alignment needed.

   • sec1

     • 0000 0c943400 0c943e00 0c943e00 0c943e00 ..4...>...>...>.
       • Address (0000): This is the address in memory where the data on this line starts.
       • (0c943400 0c943e00 0c943e00 0c943e00): These are the machine code instructions. Each group of 8 hex characters (4 bytes) represents an instruction
         • 0c943400
           • 0c94: This is a JMP or RJMP (Relative Jump) instruction.
           • 3400: The address to jump to
         • 0c943e00
           • Each 0c94 prefix also represents an RJMP or similar instruction.
           • 3e00 is the destination address of the jump.
       • (..4...>...>...>.): This shows an ASCII interpretation of the hex bytes, useful in some cases for debugging, though often unreadable

6. Uploading Blink

10. Now its time to upload the code:

    • Uni machine:

    Terminal

    avrdude -C "C:\ProgramData\arduino-ide-v2\Local\Arduino15\packages\arduino\tools\avrdude\6.3.0-arduino17\etc\avrdude.conf" -c arduino -p atmega328p -P COM6 -b 115200 -U flash:w:blink.hex
    

    • Your personal machine:

    Terminal

    $ avrdude -C "C:\Users\YOURUSERNAME\AppData\Local\Arduino15\packages\arduino\tools\avrdude\6.3.0-arduino17\etc\avrdude.conf" -c arduino -p atmega328p -P COM6 -b 115200 -U flash:w:blink.hex
    

    Output

    

    Command Breakdown

    • -Cis the path to the configuration file
    • -c specifies the programmer
    • -p specifies the part no, ie the chipset
    • -P is the port, COM# (Windows) or /dev/ttyACM# (Linux)
      • Windows getting the correct port run:
        • $ wmic path Win32_SerialPort get caption, name, deviceID
      • Linux use dmesg or lsusb
    • -b specifies the baudrate, overrides default RS-232
    • -U memory operation, ie, flash is the memtype in which mode, r,w,v and then the file

11. You should now see the BUILT-IN LED flashing on the board every 2 seconds.

12. To make life easier going forward we should use a Makefile:

    Code

    MCU = atmega328p
    F_CPU = 16000000UL
    CC = avr-gcc
    CFLAGS = -Os -mmcu=$(MCU) -DF_CPU=$(F_CPU)
    OBJCOPY = avr-objcopy
    AVRDUDE = avrdude
    AVRDUDECONFIG = "C:\ProgramData\arduino-ide-v2\Local\Arduino15\packages\arduino\tools\avrdude\6.3.0-arduino17\etc\avrdude.conf"
    PORT = COM6
    BAUD = 115200
    PROGRAMMER = -c arduino -p $(MCU) -P $(PORT) -b $(BAUD)
    
    all: blink.hex
    
    blink.hex: blink.elf
            $(OBJCOPY) -O ihex -R .eeprom $< $@
    
    blink.elf: blink.o
            $(CC) $(CFLAGS) -o $@ $<
    
    blink.o: blink.c
            $(CC) $(CFLAGS) -c -o $@ $<
    
    upload: blink.hex
            $(AVRDUDE) -C $(AVRDUDECONFIG) $(PROGRAMMER) -U flash:w:blink.hex
    
    clean:
            rm -f blink.o blink.elf blink.hex
    

    Info

    If you are on your machine the AVRDUDECONFIG file path will be this format:

    • AVRDUDECONFIG = "C:\Users\YOURUSERNAME\AppData\Local\Arduino15\packages\arduino\tools\avrdude\6.3.0-arduino17\etc\avrdude.conf"

13. Now every time we modify blink.c you can use the Makefile

    • make clean
    • make
    • make upload