LV Usbasp.

[yafch]

Hi.

After reading about this multi usb AVR programmer on many forums i,ve decided to build one for myself.fischl.de - USBasp - USB programmer for Atmel AVR controllers
i just want to ask few simple questions.
The project page also provides some modified designs by users, Now some of the designs are missing two 3.6V zener diodes at usb connector and also used 18Pf capacitors in place of 22pf capacitor from crystal to ground, like one made by J.A. de Groot. Are these diodes not necessary and what about the exact value of above mantioned capacitors.

There is a version of same projects called LV Usbasp made by Pawel Szramowski.Which one you suggest me to build myself,As i know that the LV version is for programming low voltage devises.All i need to know is That Will it work for the devices supported by original programmer as well?
Thank you.

 

[Sceadwian]

The capacitance on the crystal will 'pull' the clock frequency slightly, if you need a really precise clock you use trimmer caps and adjust them until the crystal outputs the exact frequency you need. I'd use whichever you have available, if in doubt purchase two sets of capacitors, and test them both ways, the exact value you use is actually up to what the crystal maker suggests. It shouldn't be that critical though. The zeners look like a safety measure for both the USB line and the AVR itself, to protect the chip and bus from transients they're not absolutely required, but not a bad idea.

The LV version probably runs on a lower voltage, and may or may not work with normal voltage devices. I've found the voltage an AVR runs on once the oscilator is started is actually far outside it's listed specs. The LV versions are just garunteed to start and operate normally under a lower voltage range. All LV parts should be programmable using a normal voltage programmer in general, but I'm not sure what specific chips you're talking about so that piece of advice should be taken with a grain of salt.

 

[yafch]

Sceadwian sir, thank you very much for your help.
As for as the capacitors are concerned, the trimmer capacitors may not be easy to adjust perfectly for me so i think i might be able to use 3-pin oscillator said to have built in matching capacitors, is made by "MURATA".

And about the LV version, i Don,t need it at present time. I was just curious to know its advantages/dis-advantages over the original one.
The 3.6v zener diodes are yet to be purchased but i have 3.9V at present. If they are for protection purpose then i thinK 3.9V can also be used. Am i right ?

 

[Sceadwian]

If this is for USB I would definitely NOT use a ceramic oscillator, use a crystal and just use the capacitors that are recommend, around 20pf is standard don't get too picky. Even if you use the wrong capacitance as long as it starts up the crystal will be more regular than the 3 pin oscillator you're talking about. Using a crystal is typically more about stability rather than absolute precision, and the ceramic oscilator is going to be worse in both departments.
3.9V zeners should work fine.

 

[yafch]

yep that,s right crystals are more reliable than ceramic resonators. I am going to act upon your advice.
I got 18PF,20PF both npo types And 22PF not an npo type.The crystal i have right now says "12MHZ 18PF(20PF)"So both values are recommended. Although a 18PF may not be exactly 18PF depending on thier temperature stabilities and particular frequencies of the circuits they are used in.They are subject to drift.
As you said its about stability rather than absolute percision.Got my mind clear. Problem well discussed. Thank you very very much.

 

[mneary]

22PF not an npo type

FYI, npo is really np0 where the zero stands for very low temperature coefficient.

 

[yafch]

Mneary ,Thanks for correction.