Sunday, February 23, 2014

avr-gcc adafruit trinket example

I've been interested in learning more about programming microcontrollers and saw that Adafruit has a low cost board (called trinket) that is very convenient since it can be programmed over USB. When I first got my hands on this, I was making sketches for it using the Arduino IDE, but wanted to dive in at a lower level and be able to write C code and reprogram the Attiny85 chip that is on board. There are many different ways to do this, but here is the way I went about doing it.

Install the following packages
sudo apt-get update
sudo apt-get install gcc-avr gdb-avr binutils-avr avr-libc avrdude

You should now have an environment that you can compile and link code for Atmel microprocessors. The last package (avrdude) is what we will use to program the chip. Here is an example of how to blink the LED on pin 1 of the trinket (the red LED).

// blink.c
#include <avr/io.h>
#include <util/delay.h>

// prototype
void wait_sec(int seconds);

int main (void)
{
   // direction register
   DDRB |= 0x03; // pin 0 and pin1 as output
   while(1)
   {
      PORTB |= 0x02; // LED ON
      wait_sec(1); // sleep
      PORTB &= 0x01; // LED OFF
      wait_sec(1); // sleep
   }
   return 0;
}

// sleeps the specified amount of seconds
void wait_sec(int seconds)
{
   int i;
   for(i=0; i<seconds; i++)
      _delay_ms(1000);
}

You can compile the code with avr-gcc. At the time of writing this post, I'm using avr-gcc version 4.7.2. For this contrived example, I will not create a makefile, but it's usually good practice to do so.

avr-gcc -I. -I. -g -mmcu=attiny85 -DF_CPU=8000000UL -Os -fpack-struct -fshort-enums -funsigned-bitfields -funsigned-char -Wall -Wstrict-prototypes -Wa,-ahlms=blink.lst -c blink.c -o blink.o
avr-gcc -Wl,-Map,myproject.out.map -mmcu=attiny85 -lm  -o myproject.out blink.o 
avr-objcopy -j .text                    \
  -j .data                       \
  -O ihex myproject.out myproject.hex
avr-objcopy -j .eeprom                  \
  --change-section-lma .eeprom=0 \
  -O ihex myproject.out myproject.ee.hex
avr-objcopy: --change-section-lma .eeprom=0x0000000000000000 never used

If things went smoothly, you should now have a file called myproject.hex. This is the binary we will program the chip with using avrdude. To program the trinket, press the reset button so the red LED blinks which signifies it's in bootloader mode. Execute the following to upload and program the chip.
sudo avrdude -c usbtiny -p attiny85 -U flash:w:myproject.hex

avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.00s

avrdude: Device signature = 0x1e930b
avrdude: reading input file "myproject.hex"
avrdude: input file myproject.hex auto detected as Intel Hex
avrdude: writing flash (116 bytes):

Writing | ############################                       | 55% 0.03savrdude: 6 retries during SPI command
Writing | ################################################## | 100% 0.07s



avrdude: 116 bytes of flash written
avrdude: verifying flash memory against myproject.hex:
avrdude: load data flash data from input file myproject.hex:
avrdude: input file myproject.hex auto detected as Intel Hex
avrdude: input file myproject.hex contains 116 bytes
avrdude: reading on-chip flash data:

Reading | ################################################## | 100% 0.01s



avrdude: verifying ...
avrdude: 116 bytes of flash verified

avrdude: safemode: Fuses OK

avrdude done.  Thank you.
Once the chip is programmed, you should see the blink code running.
video

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