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cembeddedavr

ATMega128 Output Flicker on Startup

I'm using an ATMega128 micro and have all of my pin inits set to output and set to low under my main section of code:

PORTB=0x00;
DDRB=0xFF;

However on startup, the output associated with PORTB.0 flicks high for a split second (I've caught it on the scope) and it seems the other outputs are doing the same. Seems like it goes LOW-HIGH-LOW. I've done some reading that it could be caused by the tri-state to output switch during startup, so I've set the PUD register to 1 before the pin inits and then back to 0 after and still no luck. Does anyone have any other ideas to keep that output off during startup? It doesn't always occur either which is what has me stumped.

Solution

  • The fundamental problem is a hardware issue - lack of a pull-down resistor on the GPIO so that it is floating when in the reset-default high-impedance input state.

    The best you can do in software is to initialise the GPIO at the earliest opportunity immediately after the reset. To do this in CodeVisionAVR you need to use a customised startup.asm in your project as described in section 4.18 of the CoadeVisionAVR compiler manual:

    enter image description here...

    Where I suggest you initialise PORTB and DDRB as follows:

                 LDI    R16,  0x00       
             OUT    PORTB, R16 
             LDI    R16,  0xFF       
             OUT    DDRB, R16

    immediately before step 2, i.e. the first four instructions. The amount of time the GPIO will be left floating will possibly be too small for the relay to react if it is a mechanical relay. You may still have a problem for a solid state relay. The length of any pulse may depend on the power-supply rise time; if it is slow, you may get a longer pulse.