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Topic: USBASP correct schematic Very confused here (Read 2735 times) previous topic - next topic

Zapro

fewer caps ? that isn't going to work!

The caps are necessary for the circuit to work, do NOT omit them!

// Per.

john1993

#16
Mar 25, 2013, 09:05 pm Last Edit: Mar 25, 2013, 10:01 pm by john1993 Reason: 1
The first is a bad copy... It  'should' work but I'd not use it W/O the diodes.. It's very likely to be 'intermittent' when the diodes that are missing aren't doing their job.

Designing electronics equipment isn't about how many parts you can throw into a circuit. It's about understanding how it is supposed to work and how it must interface with 'other' stuff. Arbitrarily chopping parts out because you don't Think... that they are required would Indicate that you know more than the original designer... were that truly the case...
YOU WOULDN'T BE ASKING WHAT AND WHY HERE...   IMO
Doc


i dont have a doctorate (yet), only ee. but one of the first things ive learned is to design according to how things ACTUALLY work and not how they SHOULD or are SUPPOSED to work. i dont mean throwing something together and if it works ship it. but there are usually reasons for paticular components.

checking some app notes for northbridge chips i found the zeners ARE requred. not so much for protecting from chassis grounds or test equipment though but "ringing" and in this case clamping 5v signals from the mega8. since its running directly off the usb vcc there is potential to violate max Vih for the motherboard device. this can result in shutdown which i think is the reason my simplified version failed but the ebay unit works w/o diodes. the ebay mega8 is powered off a 3.3v ldo so didnt trigger an exception.

to test this i went back and altered my kluge to run off 3.3v. guess what... it worked. there are other reasons for the zeners and 68ohm involving slew rates and setup times but my circuit was close enough for usb1.0 operation.

people are of course free to add as many caps, resistors, diodes, or whatever. but personally i choose do so for a reason, not because i think they are SUPPOSED TO or SHOULD be there. ymmv.

Zapro

Of course it works when you run it at 3,3V, then the zeners is not needed. The signalling voltage on USB is indeed 3,3V and not 5V - so if you try to interface with 5V pins it MIGHT work, but shouldn't. If you lower the I/O voltage to that of USB standards, no problems.

Still, decoupling caps are needed - always!

// Per.

john1993

true. but i still cant get over how simple this circuit is. i just tried it on 4 of my computers and the 2 my wife has. write/read 100kbyte test program no problems. next im going to try junking the crystal and boosting oscal to 12mhz just for yucks.

Zapro

The xtal can indeed be omitted, but omitting decoupling caps is just asking for trouble.

// Per.

JoeO


The first is a bad copy... It  'should' work but I'd not use it W/O the diodes.. It's very likely to be 'intermittent' when the diodes that are missing aren't doing their job.

Designing electronics equipment isn't about how many parts you can throw into a circuit. It's about understanding how it is supposed to work and how it must interface with 'other' stuff. Arbitrarily chopping parts out because you don't Think... that they are required would Indicate that you know more than the original designer... were that truly the case...
YOU WOULDN'T BE ASKING WHAT AND WHY HERE...   IMO
Doc


i dont have a doctorate (yet), only ee. but one of the first things ive learned is to design according to how things ACTUALLY work and not how they SHOULD or are SUPPOSED to work. i dont mean throwing something together and if it works ship it. but there are usually reasons for paticular components.

checking some app notes for northbridge chips i found the zeners ARE requred. not so much for protecting from chassis grounds or test equipment though but "ringing" and in this case clamping 5v signals from the mega8. since its running directly off the usb vcc there is potential to violate max Vih for the motherboard device. this can result in shutdown which i think is the reason my simplified version failed but the ebay unit works w/o diodes. the ebay mega8 is powered off a 3.3v ldo so didnt trigger an exception.

to test this i went back and altered my kluge to run off 3.3v. guess what... it worked. there are other reasons for the zeners and 68ohm involving slew rates and setup times but my circuit was close enough for usb1.0 operation.

people are of course free to add as many caps, resistors, diodes, or whatever. but personally i choose do so for a reason, not because i think they are SUPPOSED TO or SHOULD be there. ymmv.


What college did you get your EE degree from?

john1993

why do you ask? any of my statements not sitting right with you? if so let me know as i enjoy nothing more than setting straight those who think they know more than they actually do. i can usually go into much more detail and happy for the chance to do so.

anyway, no personal info. the LAST thing i need is having some of these internet characters flollowing me home.

JoeO


why do you ask? any of my statements not sitting right with you? if so let me know as i enjoy nothing more than setting straight those who think they know more than they actually do. i can usually go into much more detail and happy for the chance to do so.

anyway, no personal info. the LAST thing i need is having some of these internet characters flollowing me home.

That is what I figured.  You don't have an EE degree.  It is obvious.

hiduino

The Arduino forums are all about learning and helping one another.  Let's try not to bash each other.
It's about having fun with tinkering.  Arduino is for the everyday layperson not only engineers.


john1993

true. few months ago i was not an engineer myself but now this and similar forums helped me achieve my goal. i am quite grateful for that but must admit to getting carried away once in a while. must remember not everyone has the same level of expertise but everyone can enjoy the arduino experience.

Docedison

So... JUST BECAUSE you think YOU ARE AN ENGINEER...
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

john1993


So... JUST BECAUSE you think YOU ARE AN ENGINEER...


maybe... but i certainly dont think you are a "doc". sorry.. couldnt resist. :)

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