Electronic – Turn on and let LED stay on if value is over threshold in main while-loop

atmegaavrcled

I'm totally stuck with this rather trivial (?) problem, i've got this pseudo code that i will convert to C (i'm programming a ATmega8 MCU):

Include libraries
Threshold = 40 //Celsius

While true
    Temp = Read_temp_from_sensor()

    If Temp > Threshold
         Turn on LED
    Else
         Turn off LED

Is there a way to make the LED stay on for the duration of the While-loop but only as long as the temperature read from the sensor is higher than the threshold value?

I'm guessing that i don't want to evaluate the values in the If/Else block at every cycle, am i on the right track here?

Best Answer

This is exactly what you do - you evaluate if/else at every cycle. Your code is basically ok, it just has one flaw - if the temperature is near the threshold your LED will blink rapidly as temperature readings will be above 40 and below 40 a little bit. You might not want that. To avoid this you have to implement hysteresis - have some margin temperature has to overcome to either turn LED on or off. Something like this:

Threshold = 40 //Celsius
Hysteresis = 1
While true
    Temp = Read_temp_from_sensor()

    If Temp > Threshold + Hysteresis
         Turn on LED
    Else If Temp < Threshold - Hysteresis
         Turn off LED

You might want to consider sleeping the microcontroller in between loop cycles, using interrupts form the watchdog to wake it up in equal time intervals.

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The first thing one does in any microcontroller project is blink an LED; keep it blinking and give it a name ("Blinky"), too. It's the heart beat of your widget, and will always work so long as your program doesn't get stuck.
Commit to the local version control repository.
Next is to fly through the entire program in pseudo/crapcode, based on your program flow diagrams or whatever planning methods you've subscribed to. If something doesn't work or you just don't like it, comment it out but keep it there so you know to fix it later. If you don't know how to do something, write down what it's supposed to do in comments.
Commit to the local version control repository.
Time to fill in the blanks! Implement one function at a time, for example the timer, and test it. Your program should always compile and work as expected. I like to implement any user interface connections at this point, like UART -> RS232 -> PC links or LCD displays. Don't forget about Blinky.
Commit to the local version control repository.
Try to break your code with rigorous test routines; debug. Have others review your code; debug. Run your widget through its design parameters regularly, like temperature variations; debug.
Commit to the local version control repository.
Disconnect Blinky if you're a heartless fool, and ship.

AVRFreaks has an excellent tutorial written by Dean Camera (aka. abcminuser) called Modularizing C Code: Managing large projects. You may also be interested in reading State Machines for Microprocessors by James Wagner.

Electronic – How to use AVR ADC with multiplier

int can only be 0-255

That's not correct. In the C programming language, an int is always at least 16 bits wide, even on 8-bit micros. A char is 8 bits wide.

But when using inttypes.h, I strongly recommend using the explicit types like uint8_t,int16_t or uint32_t.

Your real error is using _delay_ms() with a variable parameter. This is not allowed, as it is a macro involving floating point calculations - at least in avr gcc. You are supposed to use something like this:

void DelayMs(int delay) {
  while(delay--) _delay_ms(1);
}

Best Answer

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