Module 8 Experiments: Embedded Programming





21/3/17
 Commended this week's tasks a little late due to the delayed finishing of wee 6's works.
First  some reading:
Spent time reading the beautiful tutorial at http://www.ladyada.net/learn/avr/index.html about avr programming.
What is that? It is programming or embedded programming of a microcontroller based circuit ckt44 we have already constructed and tested. The programming is not done directly from the PC. But, from a programmer circuit fab-isp which we have made much earlier. Why the term AVR? Don't know. No official expansion either. Anyway, it refers to the family of Atmel  micrcontrollers and its programming especially as embedded devices. So in our case we have an ATtiny45 based fab-isp programmer programming the Attiny44A based ckt44 circuit.
Here is the connection.
connect At this stage to ensure the correct polarity a black dot each using permanent marker pen are placed near pin 1 of the 2x3 connectors on fab-isp and ckt44 boards.		 Learned a lot more about programming the target board(ckt44) using the programmer (fab-isp) through the above mentioned tutorial. Also spent time reading the explanation about the command avrdude which is the software that writes programs from our machine onto the target chip's flash memory  via the programmer isp. It also performs some associated target programming tasks like setting  the fuses.

Gone through the man pages of avrdude also.
man avrWith the help of these two documents learned some required options of the avrdude command.

For example to know which programmer options are supported the -c option with a dummy argument is used.
Towards the end we have filtered out that usbtiny is our programmer.

Similarly which microcontrollers are programmable and how they could be specified are determined by the -p option with dummy argument.
Determined that 't44' is our target.

Then read further on the fuse settings. It is to program the IC on how it should behave, like its clk source, clk frequency settings, and so on. avrdude is again used. With the -m option an immediate value can be written. The calculation of the fuse values are explained in the tutorial and calculation could be performed here - http://www.engbedded.com/fusecalc/
 Also, noted that both ATtiny45 and ATtiny44A have 4KB of flash memory, 256B of EEPROM, 256B of SRAM. Refer: http://www.microchip.com/wwwproducts/en/ATTINY45. The programs in hexcode we send using avrdude goes to the flash memory and stored there. The microcontroller can only read from the flash memory, it cannot write there. The program is  loaded into the SRAM for execution. The EEPROM could be read and written by the IC, byte by byte. But, writing is to be as per specific procedures to prevent unintentional writing. EEPROM retains its data on power-off.
22/3/17
 Plans to do the programming in assembly language. Reading the instructio set of ATtiny44A, sheet, and avr-instruction-set-mnual.
is t44
avr is
 Once I have spent time reading these I need to do some programming experimentation. First I want to  see that the circuits are not behaving abnormal when fab-ist and t44 boards are connected in series and both ends connected to two USB ports. Checked both the boards and found okay. Placed my fingers on th ICs and no heating up!!

2 usbs
23/3/17
 First checked the connection of t44 via fab-isp again


 
okay.

Once again send the hello-world program to t44.
make program-usbtiny
And using arduino interface verified that it is running.

 sbi 16,3
out DDRA, r16
out PORTA, r16

this code written as led1.asm and issued
avr-gcc led1.asm
result: file format not recognized
renamed it as led1.avr
and same result.
man avr-gcc or man gcc shown that the assembly code extension is .s

[fab@ask embed]$ avr-gcc  led1.s
/tmp/ccOtmOvi.o:(.text+0x2): undefined reference to `DDRA'
/tmp/ccOtmOvi.o:(.text+0x4): undefined reference to `PORTA'
collect2: error: ld returned 1 exit status

With -c option (telling compile but not link) error is gone
[fab@ask embed]$ ll
total 3832
-rw-r--r-- 1 fab fab 2527656 Mar 22 12:25 Atmel-0856-AVR-Instruction-Set-Manual.pdf
-rw-r--r-- 1 fab fab    5656 Mar 21 21:59 embed.pgm.connect.jpg
-rw-r--r-- 1 fab fab  125823 Mar 22 23:35 isp.t44.usb.jpg
-rw-r--r-- 1 fab fab      38 Mar 23 16:06 led1.s
-rw-r--r-- 1 fab fab  188017 Mar 22 11:13 ssho.ebd.is.t44.png
-rw-r--r-- 1 fab fab  206991 Mar 22 12:45 ssho.emb.avr.ins.png
-rw-r--r-- 1 fab fab  840617 Mar 21 19:18 ssho.manavrdude.png
-rw-r--r-- 1 fab fab    4019 Mar 21 20:06 trmnl.avd.part.txt
-rw-r--r-- 1 fab fab    6354 Mar 21 19:51 trmnl.avd.pgmr.txt
[fab@ask embed]$ avr-gcc  led1.s -c
[fab@ask embed]$ ll
total 3836
-rw-r--r-- 1 fab fab 2527656 Mar 22 12:25 Atmel-0856-AVR-Instruction-Set-Manual.pdf
-rw-r--r-- 1 fab fab    5656 Mar 21 21:59 embed.pgm.connect.jpg
-rw-r--r-- 1 fab fab  125823 Mar 22 23:35 isp.t44.usb.jpg
-rw-r--r-- 1 fab fab     564 Mar 23 16:49 led1.o
-rw-r--r-- 1 fab fab      38 Mar 23 16:06 led1.s
-rw-r--r-- 1 fab fab  188017 Mar 22 11:13 ssho.ebd.is.t44.png
-rw-r--r-- 1 fab fab  206991 Mar 22 12:45 ssho.emb.avr.ins.png
-rw-r--r-- 1 fab fab  840617 Mar 21 19:18 ssho.manavrdude.png
-rw-r--r-- 1 fab fab    4019 Mar 21 20:06 trmnl.avd.part.txt
-rw-r--r-- 1 fab fab    6354 Mar 21 19:51 trmnl.avd.pgmr.txt
[fab@ask embed]$

[fab@ask embed]$ avr-gcc  led1.s -mmcu=attiny44 -c
aslo produced same result

the assembler seems to produce same result
[fab@ask embed]$ avr-as led1.s

Three small .s pgm written intende to do the same function of lighting the LED.
First one is producing error on compiling
Now compiling and loading the second one led1.2.s




But the LED is not lighted.
Checking next program

Again writing okay.
No LED glowing.
Requested my colleague Tanvir to check my board with his code.
Board is okay.
Here is the general pattern of commands I tried during this stage of the debugging process.
avr-as led1.4.s -o led1.4.o
avr-objcopy -O ihex led1.4.o led1.4.hex
avrdude -p t44 -P usb -c usbtiny -U flash:w:led1.4.hex


-c flag no linking
-s removes symbol table information from executable


error of led1.1.s is gone as we enclosed the erroneous code portion quoted

avr-objdump -S led1.6.o //to dump the code of the objectfile


codes genrated are not seemed correct

Happened come across via google this site and the tips by

clawson  at 

http://www.avrfreaks.net/forum/cant-compile-avr-assembly-code-gnulinux

helped to include some settings  and include file information at the beginning of the assembler file. Also changing file name form .s to .S also helped.


some includes are entered.
avr-as is not generating the right code

avr-gcc -mmcu=attiny44a  led1.4.S -o led1.4.elf -nostartfiles
avr-objdump -S led1.4.elf

avr-objcopy -O ihex led1.4.elf led1.4.hex


tried avra also after installin it, but seems not giving the desired result.

Finally the LED is glowing!!
code:

#define __SFR_OFFSET 0
#include <avr/io.h>
.org 0
rjmp main
 
main:
ldi r16,0b00001000
out DDRA, r16
out PORTA, r16

loop:  rjmp loop

(one observed possible mistake might be that the r16 bits might not have set the way I intended it.)

steps
1. avr-gcc -mmcu=attiny44a  led1.8.S -o led1.8.elf -nostartfiles
2. avr-objcopy -O ihex led1.8.elf led1.8.hex
3. avrdude -p t44 -P usb -c usbtiny -U flash:w:led1.8.hex


Additional steps:
[fab@ask embed]$ cat led1.8.hex
:0A00000000C008E00ABB0BBBFFCFF5
:00000001FF
[fab@ask embed]$


[fab@ask embed]$ avr-objdump -S led1.8.elf

led1.8.elf:     file format elf32-avr


Disassembly of section .text:

00000000 <__ctors_end>:
   0:    00 c0           rjmp    .+0          ; 0x2 <main>

00000002 <main>:
   2:    08 e0           ldi    r16, 0x08    ; 8
   4:    0a bb           out    0x1a, r16    ; 26
   6:    0b bb           out    0x1b, r16    ; 27

00000008 <loop>:
   8:    ff cf           rjmp    .-2          ; 0x8 <loop>
[fab@ask embed]$
24/3/17
 Let us now consolidate various codes, commands, and results of yesterday and add some more results, as shown below.


File
 Code
 Compiling commands
 avr-objdump result
 Remarks
led1.1.s
 #define __SFR_OFFSET 0
#include <avr/io.h>

ldi r16,"(1<<PA3)"
out DDRA, r16
out PORTA, r16
  avr-gcc -c led1.1.s
or
avr-as led1.1.s
    0:    00 e0           ldi    r16, 0x00    ; 0
   2:    00 b9           out    0x00, r16    ; 0
   4:    00 b9           out    0x00, r16    ; 0

 Not the desired
led1.2.s
 sbi 16,3
out DDRA, r16
out PORTA, r16
  avr-gcc -c led1.2.s
or
avr-as led1.2.s		    0:    83 9a           sbi    0x10, 3    ; 16
   2:    00 b9           out    0x00, r16    ; 0
   4:    00 b9           out    0x00, r16    ; 0

 Not the desired
led1.3.s
 sbi DDRA,3
sbi PORTA,3

 avr-gcc -c led1.3.s
or
avr-as led1.3.s
    0:    03 9a           sbi    0x00, 3    ; 0
   2:    03 9a           sbi    0x00, 3    ; 0

 Not the desired
led1.5.s
 .org 0
rjmp main
main:
sbi DDRA,3
sbi PORTA,3
loop:  rjmp loop
 avr-gcc -c led1.5.s
or
avr-as led1.5.s
 00000000 <main-0x2>:
   0:    00 c0           rjmp    .+0          ; 0x2 <main>
00000002 <main>:
   2:    03 9a           sbi    0x00, 3    ; 0
   4:    03 9a           sbi    0x00, 3    ; 0
00000006 <loop>:
   6:    00 c0           rjmp    .+0          ; 0x8 <loop+0x2>
 Not the desired
led1.6.s
 #define __SFR_OFFSET 0
#include <avr/io.h>
.org 0
rjmp main
ldi r16,"(1<<PA3)"
out DDRA, r16
out PORTA, r16
loop:  rjmp loop
 avr-gcc -c led1.6.s
or
avr-as led1.6.s		 00000000 <loop-0x8>:
   0:    00 c0           rjmp    .+0          ; 0x2 <loop-0x6>
   2:    00 e0           ldi    r16, 0x00    ; 0
   4:    00 b9           out    0x00, r16    ; 0
   6:    00 b9           out    0x00, r16    ; 0
00000008 <loop>:
   8:    00 c0           rjmp    .+0          ; 0xa <loop+0x2>

 Not the desired
led1.4.S
 #include <avr/io.h>
.org 0
rjmp main
main:
sbi 16,3
out DDRA, r16
out PORTA, r16
loop:
rjmp loop
 [fab@ask embed]$ avr-gcc -c led1.4.S
In file included from led1.4.S:1:0:
/usr/avr/include/avr/io.h:581:6: warning: #warning "device type not defined" [-Wcpp]
 #    warning "device type not defined"
      ^~~~~~~

or
avr-as led1.4.S
(No warning)
 00000000 <main-0x2>:
   0:    00 c0           rjmp    .+0          ; 0x2 <main>

00000002 <main>:
   2:    83 9a           sbi    0x10, 3    ; 16
   4:    00 b9           out    0x00, r16    ; 0
   6:    00 b9           out    0x00, r16    ; 0

00000008 <loop>:
   8:    00 c0           rjmp    .+0          ; 0xa <loop+0x2>
[f
 Not the desired
led1.7.S
 #define __SFR_OFFSET 0
#include <avr/io.h>
.org 0
rjmp main
main:
SBI 16,3
out DDRA, R16
out PORTA, R16
loop:
rjmp loop
 [fab@ask embed]$ avr-gcc -c led1.7.S
In file included from led1.7.S:2:0:
/usr/avr/include/avr/io.h:581:6: warning: #warning "device type not defined" [-Wcpp]
 #    warning "device type not defined"
      ^~~~~~~

or
avr-as led1.7.S
(No warning)
 00000000 <main-0x2>:
   0:    00 c0           rjmp    .+0          ; 0x2 <main>

00000002 <main>:
   2:    83 9a           sbi    0x10, 3    ; 16
   4:    00 b9           out    0x00, r16    ; 0
   6:    00 b9           out    0x00, r16    ; 0

00000008 <loop>:
   8:    00 c0           rjmp    .+0          ; 0xa <loop+0x2>

 Not the desired
led1.8.S #define __SFR_OFFSET 0
#include <avr/io.h>
.org 0
rjmp main
 main:
ldi r16,0b00001000
out DDRA, r16
out PORTA, r16
loop:  rjmp loop
 [fab@ask embed]$ avr-gcc -c led1.8.S
In file included from led1.8.S:2:0:
/usr/avr/include/avr/io.h:581:6: warning: #warning "device type not defined" [-Wcpp]
 #    warning "device type not defined"
      ^~~~~~~

or
avr-as led1.8.S
(No warning)		 00000000 <main-0x2>:
   0:    00 c0           rjmp    .+0          ; 0x2 <main>
00000002 <main>:
   2:    08 e0           ldi    r16, 0x08    ; 8
   4:    00 b9           out    0x00, r16    ; 0
   6:    00 b9           out    0x00, r16    ; 0
00000008 <loop>:
   8:    00 c0           rjmp    .+0          ; 0xa <loop+0x2>
 Not the desired
led1.8.S #define __SFR_OFFSET 0
#include <avr/io.h>
.org 0
rjmp main
 main:
ldi r16,0b00001000
out DDRA, r16
out PORTA, r16
loop:  rjmp loop		 avr-gcc -mmcu=attiny44a  led1.8.S -o led1.8.elf -nostartfiles
avr-objcopy -O ihex led1.8.elf led1.8.hex 		00000000 <__ctors_end>:
   0:    00 c0           rjmp    .+0          ; 0x2 <main>
00000002 <main>:
   2:    08 e0           ldi    r16, 0x08    ; 8
   4:    0a bb           out    0x1a, r16    ; 26
   6:    0b bb           out    0x1b, r16    ; 27
00000008 <loop>:
   8:    ff cf           rjmp    .-2          ; 0x8 <loop>		 Right.
LED lights

See the capital .S extension also
led1.7.S #define __SFR_OFFSET 0
#include <avr/io.h>
.org 0
rjmp main
main:
SBI 16,3
out DDRA, R16
out PORTA, R16
loop:
rjmp loop 		 avr-gcc -mmcu=attiny44a  led1.7.S -o led1.7.elf -nostartfiles
 00000000 <__ctors_end>:
   0:    00 c0           rjmp    .+0          ; 0x2 <main>
00000002 <main>:
   2:    83 9a           sbi    0x10, 3    ; 16
   4:    0a bb           out    0x1a, r16    ; 26
   6:    0b bb           out    0x1b, r16    ; 27
00000008 <loop>:
   8:    ff cf           rjmp    .-2          ; 0x8 <loop>
 LED not lighted
led1.8.S #define __SFR_OFFSET 0
#include <avr/io.h>
.org 0
rjmp main
 main:
ldi r16,0b00001000
out DDRA, r16
out PORTA, r16
loop:  rjmp loop 		avr-gcc -c  -mmcu=attiny44a  led1.8.S
avr-objcopy -O ihex led1.8.o led1.8.hex		 00000000 <main-0x2>:
   0:    00 c0           rjmp    .+0          ; 0x2 <main>

00000002 <main>:
   2:    08 e0           ldi    r16, 0x08    ; 8
   4:    0a bb           out    0x1a, r16    ; 26
   6:    0b bb           out    0x1b, r16    ; 27

00000008 <loop>:
   8:    00 c0           rjmp    .+0          ; 0xa <loop+0x2>
 Right.
LED glows.
led1.9.S #include <avr/io.h>
.org 0
rjmp main
main:
ldi r16,0b00001000
out DDRA, r16
out PORTA, r16
loop:  rjmp loop
 avr-gcc -c  -mmcu=attiny44a  led1.9.S 00000000 <main-0x2>:
   0:    00 c0           rjmp    .+0          ; 0x2 <main>

00000002 <main>:
   2:    08 e0           ldi    r16, 0x08    ; 8
   4:    0a bf           out    0x3a, r16    ; 58
   6:    0b bf           out    0x3b, r16    ; 59

00000008 <loop>:
   8:    00 c0           rjmp    .+0          ; 0xa <loop+0x2>

 Wrong code
First line is needed.
led1.1.S
 #define __SFR_OFFSET 0
#include <avr/io.h>

ldi r16,"(1<<PA3)"
out DDRA, r16
out PORTA, r16		 avr-gcc -c  -mmcu=attiny44a led1.1.S
00000000 <.text>:
   0:    00 e0           ldi    r16, 0x00    ; 0
   2:    0a bb           out    0x1a, r16    ; 26
   4:    0b bb           out    0x1b, r16    ; 27

 Wrong.
Seems ldi r16,"(1<<PA3)" didn't work
led1.10.S
 #define __SFR_OFFSET 0
#include <avr/io.h>

ldi r16,(1<<PA3)
out DDRA, r16
out PORTA, r16

 avr-gcc -c  -mmcu=attiny44a led1.10.S 00000000 <.text>:
   0:    08 e0           ldi    r16, 0x08    ; 8
   2:    0a bb           out    0x1a, r16    ; 26
   4:    0b bb           out    0x1b, r16    ; 27
 Right.
Glows.
So removing quote mark helped.
led1.2.S
 #define __SFR_OFFSET 0
#include <avr/io.h>
sbi 16,3
out DDRA, r16
out PORTA, r16
 avr-gcc -c  -mmcu=attiny44a led1.2.S 00000000 <.text>:
   0:    83 9a           sbi    0x10, 3    ; 16
   2:    0a bb           out    0x1a, r16    ; 26
   4:    0b bb           out    0x1b, r16    ; 27

 No. What's wrong?
led1.3.S
 #define __SFR_OFFSET 0
#include <avr/io.h>
sbi DDRA,3
sbi PORTA,3
 avr-gcc -c  -mmcu=attiny44a led1.3.S
 00000000 <.text>:
   0:    d3 9a           sbi    0x1a, 3    ; 26
   2:    db 9a           sbi    0x1b, 3    ; 27

 Right.
Glows.



Now to continue with other experiments

24/3/17: Now to light the LED when th button is pressed.
File
Code

 avr-objdump result
 Remarks
led2.1.S
 #define __SFR_OFFSET 0
#include <avr/io.h>

cbi DDRA,7
    cbi PINA,7   

loop: 
    in r16,PINA
    andi r16,0x80
    breq  loop

    sbi DDRA,3
    sbi PORTA,3
 00000000 <loop-0x4>:
   0:    d7 98           cbi    0x1a, 7    ; 26
   2:    cf 98           cbi    0x19, 7    ; 25

00000004 <loop>:
   4:    09 b3           in    r16, 0x19    ; 25
   6:    00 78           andi    r16, 0x80    ; 128
   8:    01 f0           breq    .+0          ; 0xa <loop+0x6>
   a:    d3 9a           sbi    0x1a, 3    ; 26
   c:    db 9a           sbi    0x1b, 3    ; 27

 LED stays ON.
Looking at the code the reason is the branch to loop back is actually not taking place!!
Feels the assembly to hex code conversion could have been done manually.
Else, there is a scope for a simple one, as far what I come across. The reason is the intermediate code generated is not resolving the address conversion. So a dummy linking is required. The posting by kroylar at  stackoverflow link was of great help to get rid off this.
Subsequently the file  linker.x as below is created.
SECTIONS
{
  . = 0x0;
  .text : { *(.text) }
}
led2.1.S #define __SFR_OFFSET 0
#include <avr/io.h>

    cbi DDRA,7
    cbi PINA,7   

loop: 
    in r16,PINA
    andi r16,0x80
    breq  loop

    sbi DDRA,3
    sbi PORTA,3		 00000000 <loop-0x4>:
   0:    d7 98           cbi    0x1a, 7    ; 26
   2:    cf 98           cbi    0x19, 7    ; 25

00000004 <loop>:
   4:    09 b3           in    r16, 0x19    ; 25
   6:    00 78           andi    r16, 0x80    ; 128
   8:    e9 f3           breq    .-6          ; 0x4 <loop>
   a:    d3 9a           sbi    0x1a, 3    ; 26
   c:    db 9a           sbi    0x1b, 3    ; 27

 Now the branching issue is solved!!
Commands issued.

avr-gcc -c  -mmcu=attiny44a led2.1.S
avr-ld  -Tlinker.x led2.1.o -o led2.o


LED goes off

Now button pressed and the LED glows. The expected result.



24/3/17
File
 Code

 avr-objdump result
 Remarks
led2.2.S
 #define __SFR_OFFSET 0
#include <avr/io.h>

    cbi DDRA,7 ; PA7 for  button input
    cbi PINA,7   ; clear the input
    sbi DDRA,3 ; PA3 to led
  
loop: 
    cbi PORTA,3 ; led off
poll:
    in r16,PINA ; reading input
    andi r16,0x80 ; checking input
    breq  loop
  
    sbi PORTA,3; led on
    rjmp poll
 00000000 <loop-0x6>:
   0:    d7 98           cbi    0x1a, 7    ; 26
   2:    cf 98           cbi    0x19, 7    ; 25
   4:    d3 9a           sbi    0x1a, 3    ; 26

00000006 <loop>:
   6:    db 98           cbi    0x1b, 3    ; 27

00000008 <poll>:
   8:    09 b3           in    r16, 0x19    ; 25
   a:    00 78           andi    r16, 0x80    ; 128
   c:    e1 f3           breq    .-8          ; 0x6 <loop>
   e:    db 9a           sbi    0x1b, 3    ; 27
  10:    fb cf           rjmp    .-10         ; 0x8 <poll>
[fab
 To light the LED as far as the button is kept pressed.
Compilation and loading.
avr-gcc -c  -mmcu=attiny44a led2.2.S
avr-ld  -Tlinker.x led2.2.o -o led2.o
avr-objcopy -O ihex -S led2.o
avrdude -p t44 -P usb -c usbtiny -U flash:w:led2.o

Result: OK
led2.3.S
 #define __SFR_OFFSET 0
#include <avr/io.h>
;Default =  button released ->led on; Button pressed -> led off

    cbi DDRA,7 ; PA7 for  button input
    cbi PINA,7   ; clear the input
    sbi DDRA,3 ; PA3 to led
   
   
loop: 
    sbi PORTA,3; led on
poll:
    in r16,PINA ; reading input
    andi r16,0x80 ; checking input
    breq  loop ; button not pressed
  
    cbi PORTA,3; led off
    rjmp poll

 00000000 <loop-0x6>:
   0:    d7 98           cbi    0x1a, 7    ; 26
   2:    cf 98           cbi    0x19, 7    ; 25
   4:    d3 9a           sbi    0x1a, 3    ; 26

00000006 <loop>:
   6:    db 9a           sbi    0x1b, 3    ; 27

00000008 <poll>:
   8:    09 b3           in    r16, 0x19    ; 25
   a:    00 78           andi    r16, 0x80    ; 128
   c:    e1 f3           breq    .-8          ; 0x6 <loop>
   e:    db 98           cbi    0x1b, 3    ; 27
  10:    fb cf           rjmp    .-10         ; 0x8 <poll>

 Button released ->led on; Button pressed -> led off
Compilation and loading.
avr-gcc -c  -mmcu=attiny44a led2.3.S
avr-ld  -Tlinker.x led2.3.o -o led2.o
avr-objcopy -O ihex -S led2.o
avrdude -p t44 -P usb -c usbtiny -U flash:w:led2.o

Result: OK
led.toggle.S
 #define __SFR_OFFSET 0
#include <avr/io.h>
;Buuton to toggle LED

    cbi DDRA,7 ; PA7 for  button input
    cbi PINA,7   ; clear the input
    sbi DDRA,3 ; PA3 to led
    clr r17; for toggling
     
poll:
    in r16,PINA ; reading input
    andi r16,0x80 ; checking input
    breq  poll ; button not pressed
  
   
    com  r17
   breq ledon
   
    cbi PORTA,3; led off
    rjmp poll
   
ledon:  
    sbi PORTA,3; led on
    rjmp poll

 00000000 <poll-0x8>:
   0:    d7 98           cbi    0x1a, 7    ; 26
   2:    cf 98           cbi    0x19, 7    ; 25
   4:    d3 9a           sbi    0x1a, 3    ; 26
   6:    11 27           eor    r17, r17

00000008 <poll>:
   8:    09 b3           in    r16, 0x19    ; 25
   a:    00 78           andi    r16, 0x80    ; 128
   c:    e9 f3           breq    .-6          ; 0x8 <poll>
   e:    10 95           com    r17
  10:    11 f0           breq    .+4          ; 0x16 <ledon>
  12:    db 98           cbi    0x1b, 3    ; 27
  14:    f9 cf           rjmp    .-14         ; 0x8 <poll>

00000016 <ledon>:
  16:    db 9a           sbi    0x1b, 3    ; 27
  18:    f7 cf           rjmp    .-18         ; 0x8 <poll>

 To toggle the LED.

You can't read PORTA.
A register is complemented after every button press.


Result: Ok