How to get output from an ATTINY12L microcontroller

Question

So far I have only worked with ATmega32 chips, programming them in C. Now I have to work with an ATTINY12, which can only be programmed in ASM.

As a proof of concept, I wrote a simple program that turns on a LED on PB0. I can compile the code and flash the microcontroller successfully, but the LED does not turn on.

I verified that the circuit works as intended and I tried 2 different microcontroller to rule out a hardware issue. It looks like I am misunderstanding something in the source code I have assembled based on a couple of tutorials I've found.

#include <avr/io.h>
.org 0x00 
.global main

main:
    ldi R16,0b00000001
    out DDRB,R16           // Set PB0 as output pin

    ldi R16,0b00000001
    out PORTB,R16          // Set PB0 to high

loop:
    rjmp loop              // loop forever
.end

I compiled the source with Microchip Studio and flashed the chip with AVRDUDE. I also added LEDs to PB1-PB5 for debugging, but none of them showed any sign of life.

The program seems so simple and straightforward. What am I missing?

Edit: After a couple of very helpful comments, I found another weird behavior. When I try to set a single bit on PORTB using sbi PORTB, 1, which should set PB0 to high, the compiler throws an error operand out of range: 56. This does not make sense, because PORTB is supposed to have 6 bits (0-63). This indicates to me, that my main problem lies in the way I use PORTB in my code.

The same applies to sbi DDRB, 1, only that the compile error reads operand out of range: 55.

Answer 1

After countless hours of deep dives into datasheets and old source code I found the underlying problem.

PORTB, DDRB and other constants point to address locations in the I/O space of the controller. For the ATTiny12 controller, those are defined in iotn12.h included via avr/io.h:

/* Data Direction Register, Port B */
#define DDRB    _SFR_IO8(0x17)

/* Data Register, Port B */
#define PORTB   _SFR_IO8(0x18)

_SFR_IO8 is defined in sfr_defs.h. It basically implements a switch for different architectures to either use on offset of 0x20 or not. Somehow this switch fails for the ATTiny12 and sets the offset to 0x20, although no offset should be used.

This results in PORTB and DDRB pointing at the wrong locations in memory. As long as data is only written via out from a register to those locations, the compiler doesn't complain, but when sbi or cbi is used, the compiler throws an operand out of range error, because those commands can't be used to manipulate those memory locations.

So, when using PORTB and DDRB or even _SFR_IO8(0x18) directly with out, the compiler doesn't complain but the program obviously doesn't work, because values are written to the wrong location.

How to solve the issue? You can override the switch by defining the offset yourself in your ASM code, before avr/io.h is included: #define __SFR_OFFSET 0.

Once this offset is set, the program works as intended.

Answer 2

I am not that much of an ASM-Dev, but maybe your issue is in the code as well.

I had a look at the Datasheet Datasheet Page 16 - there the ISR-Vectors are "mounted".

Maybe you have to put a "rjmp" to your "main" at 0x00 - but as I said: I don't know much about ASM.

So I Googled for a bit and came across GitHub.

You could try the following ASM-Code:

#include <avr/io.h>
.org 0x00
  rjmp main
  RETI
  RETI
  RETI
  RETI
  RETI
main:
  ldi R16,0b00000001
  out DDRB,R16 
  out PORTB,R16
loop:
  rjmp loop

And as I said in the comments: Make sure, your reset is inactive (Pull-Up to VCC). Also, make sure the fuses are set in a way, that the internal RC-Oscillator is to be used.

You could also: 1. Flash the IC via programmer, 2. disconnect the programmer, 3. power cycle IC to rule out any issue with your programmer holding the IC in "serial programming" mode.

Answer 3

Have you tried measuring the pin voltage with a meter instead of an LED?

What's the circuit you are connecting to PB0? How did you verify it works at intended? LEDs have two terminals.

• You may have the LED backwards.
• You may be connecting both sides of the LED to Vcc and neither to ground.
• Vcc may not be high enough to turn on the LED.
• Your ATTINY may not be able to carry enough current to turn on the LED, in which case you need a transistor to amplify the current.
• You may have neglected a current-limited resistor and burned out the LED.