Electronic – ATmega2560 assembly pin read and compare logic problem

assemblyatmegaatmelatmel-studio

I apologize if my question was vague.

Program: Atmel Studio 6

I am trying to make a program that enters an loop which it will only break out of when a pin goes low.

;Set PA0 high before entering loop checking for low
CBI DDRA,DDA0       ;PA0 to input
SBI PINA0,0         ;PA0 pull-up enable
;PA0 is now input & high

chklow: inc r16     ; Increase r16
in r18,PORTA0       ; Copies PINA0 to r18
cpi r18,0           ; Compares r18 to 0
brne chklow         ; Branch if PA0 is not low
nop             ; Loop exit (do nothing)

What happens is that when I set PINA0, PORTA0 goes high. I guess this is good, it's like I pressed a button attached to PINA0.

Now I enter the loop, and I have my I/O view box up, so I see that PORTA0 is set. This is the rightmost box in PORTA.

But "in r18,PORTA" doesn't make r18 become '1'… it remains 0.

And so the Z-bit is set to 1 because the compare succeeded, and the branch doesn't occur.

What's wrong? I want the loop to continue indefinitely until I make PORTA0 low.

EDIT: I changed it now, using your advice:

CBI DDRA,DDA0           ;PA0 to input
SBI PORTA0,0            ;PA0 pull-up enable


chklow:
in r18,PORTA        ; Copies PINA0 to r18
cpi r18,0x00        ; Compares r18 to 0
brne chklow         ; Branch if PA0 is not low
nop             ; Loop exit (do nothing)

So what I changed was I copied the entire PORTA byte, instead of trying to copy the PORTA0 bit, which isn't possible (at least using my method), and now it works. Thanks 😉

Also, I know this is a dirty solution, because it fails if ANY bit in Pin A is set, but I can refine that later, this is a working prototype 😀

Best Answer

Couple things that make me frown:

SBI PINA0,0         ;PA0 pull-up enable

According to the datasheet:

Three I/O memory address locations are allocated for each port, one each for the Data Register – PORTx, Data Direction Register – DDRx, and the Port Input Pins – PINx. The Port Input Pins I/O location is read only, while the Data Register and the Data Direction Register are read/write. However, writing a logic one to a bit in the PINx Register, will result in a toggle in the corresponding bit in the Data Register. In addition, the Pull-up Disable – PUD bit in MCUCR disables the pull-up function for all pins in all ports when set.

To enable the pull-up you should write to PORTA not to PINA0. Also notice the difference PINA0 and PINA in your code. You cannot read a single bit from an IO port, you must read the whole byte. Then you can check one or more bits in the byte read using AND, SBRC or SBRS.

nop             ; Loop exit (do nothing)

Don't use nop to "do nothing", because the microcontroller will go forward to the next instruction in flash which can be anything, including an instruction from a previous program that was in flash if it wasn't overwritten. Anything can happen from there.

Related Solutions

Electronic – Does anyone have any examples or suggestions utilizing Atmel’s ATMEGA2560 and external interrupt’s

@ LoneTech: I figured this site was to help people. Not put them down for lack of information, but to provide it for them if the need is there. Also, Interrupts can be used to interrupt other code without hurting the speed of your main loop. If your button is supposed to change a process and your main loop is controlling a wave or something. INT0 could be used. You also don't know what other operations this individual plans on using it for.

@ Michael: This code took me a while to track down and understand as well. I have written code for AVRs for 5+ years now and for PC 13+ years. Its amazing how little documentation is there at your fingertips. I ran across a nice tutorial and finally it clicked.

void InitINT0()
{
    // Enable INT0 External Interrupt
    EIMSK |= 1<<INT0;

    // Falling-Edge Triggered INT0 - This will depend on if you
    // are using a pullup resistor or a pulldown resistor on your
    // button and port
    MCUCR |= 1<<ISC01;
}

int main(void)
{
    //Init Timer
    InitINT0();

    //Which Interrupt pin you need to enable
    //can be found in the datasheet, look at the pin
    //configuration, usually within the first 5 pages
    //track down INT0 - which is PORTD pin 0.
    //This needs to be an input.
    DDRD &= 0;

    // Enable Interrupts
    sei();

    //Givin that PORTD0 is the INT0, your button should
    //be hooked up to it.

    while(1)
    {
        //Do your operation
    }
}

// External Interrupt 0 ISR
ISR(INT0_vect) 
{
    while (/*Button is down*/) //0 is the pin for your button
    { 
        //Pause your operation , this will pause everything though
        //because Interrupts take priority over pretty much everything
        //This includes other interrupts and your main loop
    }
}

Electronic – c18 assembly inside function problem. unidentified label (bit names)

For the commented out C part, you don't appear to have have any ending semicolons (e.g. first line should be EECON1bits.EEPGD = 0;) , so the compiler certainly won't like that.

For the inline assembly, IIRC the register names hould b e usable, but I think you have to specify operands fully (e.g. instead of movwf EECON2,A you do movwf EECON2,0)
It's possible the bit names must be specified by number instead of name too, for example instead of bcf EECON1,EEPGD,A you do bcf EECON1, 7, 0 (EEPGD is bit 7 of EECON1) The A at the end is not right anyway, this should be 1 or 0 (or W or F in MPASM)
It's been a while since I used the inline assembly for anything like this so memory is fuzzy, but try it and let us know how it goes.

Best Answer

Related Solutions

Electronic – Does anyone have any examples or suggestions utilizing Atmel’s ATMEGA2560 and external interrupt’s

Electronic – c18 assembly inside function problem. unidentified label (bit names)

Related Topic