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Registered Member #1643
Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039
I was thinking it would be kinda cool to take an AVR...And have buttons so i could step a frequency up/down by 1hz, or duty cycle up/down by 1%. I've already done this in the past...Having a PWM, variable duty, and variable frequency, limited it 1hz to 1Mhz.
But if I want to run 7-segments to display frequency and duty, how would I do this without screwing up the output frequency? To run up to 5 7-segments to display XXX frequency XX Duty, i would have to tell the AVR to go one by one. Meaning turn on segment 1, display a 1. Turn on segment 2, display a 5. etc etc, at a fast enough scan rate, that it will just read "155 50" meaning 155hz, 50% duty.
However, The time it takes for the chip to run that, will interfere with outputting 155hz wont it? Is there a way so i can have it range 1hz to 1Mhz, without slowing down how fast the refresh rate of the display is? Or will I have to get a 2nd chip, like ATTINY2313, dedicated to the segments?
Questions please ask...Hard to explain, I just remember last I tried a similar concept, when I told it to do 100hz, it made a 100hz tone, but the LCD was off during the time it was emitting that tone. It was fine at high frequency, but low frequency, it was annoying...Tips?
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
Most models have a PWM unit as part of the timer that generates a frequency with a given duty cycle, if not you will have the problem you describe.
You can only make frequencies with a period that is an integer multiple of the oscillator period. That means at high frequencies you can't control the frequency with high resolution. As an example, with a 1 MHz clock you can make a 1 MHz signal or a 500 kHz signal, nothing in between. If you want 1 Hz resolution at high frequencies you need to use an external chip or use direct digital synthesis (DDS) like this:
Registered Member #1643
Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039
Yeah, kinda figured I might have to resort to having a seperate chip so one can control the 7-segment resolution, so I dont have and flickering, and keep one chip dedicated to just perform frequency. Will see what I can do, thanks for the tips/links, Bjorn.
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
The frequency output will not be affected by the code driving the display if you use a modern AVR. The frequency output is driven by the internal timer/counter and will run in the background so you can do what you want with the rest of the ports.
Registered Member #3215
Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780
anyways, you may use the watchdog to count cycles and as it interrupts, you set your port high or low
once the watchdog has interrupted any operation will halt, you'll manipulate the output, then recalculate the cycle count for your frequency and duty cycle, put it in the watchdog counter, and after it is all done you're back in your main code which will only update the 7-segs and acquire your set point values
that's the most efficient way of generating a variable duty cycle variable frequency signal when you have a simple µC (even a 8-bit one will be accurate) but you have to be sure to have at least 20 times the desired frequency as a clock (eg. 20Mhz clock for 1Mhz output would be fine, leaving enough cycles to compute everything even at the highest output freq.)
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AVR -- Multi-function question
