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Author Topic: How fast and stable can arduno trigger digital IOs?  (Read 1366 times)
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Hallo,
I am very new to Arduino and to electronics in general, so before I start with the wrong path, I thought I ask here first smiley

I have connect my Arduino Mega to my oscilloscope and upload a simple Blink LOOP with the delayMicroseconds() command. It seems that maximal
speed Arduino can do is 62us at an inaccuracy of +/- 300ns    (on 1 digital port, port 12 in this case)

For my project I will need stable//accurate signals in a range of 60ns - 40ms on 11ports (10 for a LED array and 1 to control another device)... is that even possible with arduinos programming environment or do I need assembler for that?

Thanks!!

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Given the fact that Arduino runs at 16 Mhz, it is easy to see that 60ns signals is at the edge/beyond its capabilities.

Imho : for 60 ns signals you need a processor at least in the 100Mhz range, a realistic minimum is 200Mhz I guess.
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Rob Tillaart

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@robtillaart
Thanks for the answer!
I can't find the post anymore, but I am sure I have read somewhere that he has a cycle of 2ns.. but I may be wrong on that since I don't understand much of the actual internal work of a MC.
Anyway... I would be happy with a stable 40ms signal.

Example: 2ms ON, 38ms OFF, LOOP...     even at that speed, he is jigling too much for my taste.
Is the delay() command the usualy way to go for that, or is there any better command for that?

The more I google, the more I get the impression that I need assembler for accurate timings for thos signals.
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If you're looking for 2ms pulse width you may find delayMicroseconds more to your liking (http://arduino.cc/en/Reference/DelayMicroseconds) or
you may want to avoid using delay and just loop checking micros() to see if the pin needs pulsing. The other thing to be aware of is that digitalWrite() takes quite a while, if you want a faster response you may need to manipulate the port directly.
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@sixeyes
I got to try DelayMicroseconds tomorow!
"you may need to manipulate the port directly"  what you mean by that?
How do I manipulate the port directly?
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Using DDRx, to set direction and PORTx to write to the pin you want.
If the 40ms is fixed using a timer is the best way to have 0 jitter, you see jitter because the millis values are returned from an interrrupt routine and the port access's are not atomic, and they can be interrupted in the middle of a write.
To learn about direct port manipulation see this:
http://hekilledmywire.wordpress.com/2011/02/23/direct-port-manipulation-using-the-digital-ports-tutorial-part-3/
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@Senso
Thanks for the link!
I will play around with it on monday... here I have no oscilloscope to test it :/

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The jitter on an arduino output signal is caused by the interrupts running in the rest of the system doing things like the real time clock and the handling of the serial ports.
To remove this you must disable the interrupts, then delay() will not work and you have to write your own delay loops.
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I just tried direct port manipulation and it goes down to ns range completely stable smiley-grin

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I don't think you connected the grounds, Dave.
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Quote
can't find the post anymore, but I am sure I have read somewhere that he has a cycle of 2ns
Memory can play funny tricks on you, can't it?

Quote
I just tried direct port manipulation and it goes down to ns range completely stable
Tens of nanoseconds, I could believe. Getting towards the hundreds.
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Its 60ns on my signal and 1ms on the loop. Should work just fine.
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Its 60ns on my signal
In my language, that's tens of nanos.
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If it all works please share your code how you got it to work.

Thanks,
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Rob Tillaart

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In theory the jitter could be sub nanosecond, its the granularity that's 66ns.
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The LEDs and a Camera are controlled by the sync signal of the Arduino. I use the array to measure how long the Camera needs from the trigger signal to the first picture recorded on different frame-rates. I will post the code once its done... its a very slow progressing project since every time I need something, i need to google and play around till it works smiley    ..but its a good way to learn this stuff.
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