AVR-Chat

Peter Gargano wrote:
> I have the reverse concern - Driving AVRs from parts designed to be 
> used with TTL. Most AVRs (eg Mega-168) have a 0.6VCC spec. on VIHmin 
> meaning 3.0 Volts when run at 5.0 Volts (well, actually worse case is 
> 0.6 x VCCmax which may give VIHmin of 3.15 Volts).
> 
> Many parts you may want to interface to an AVR have a VOHmin of 2.4 
> Volts - meaning they are not guaranteed to drive the minimum required 
> 3.0 Volts.
> 
> So, how come people can get away with it? Here's an example (and I 
> could quote many more, I use this example because it was ready to hand).
> 
>    http://www.prllc.com/productcart/pc/viewPrd.asp?idproduct=9
> 
> If you check the schematic on the User's Guide you'll see they have a 
> Samsung memory chip (with the above VOHmin of 2.4) interfaced directly 
> to the M128 running at VCC of 5.0 Volts (VIHmin of 3.0 Volts).
> 
> Peter

You are looking at worst case, typical case is that it will work well.

But this is not the problem for the OP, I would say his problem is more 
likely grounding (either 2 separate ground nets, chips missing ground) 
or he is trying to drive the avr outputs with them set to inputs (which 
would do exactly what he says, reasonable "1" but needs pull down for a "0"


-- 
Regards,

Kat.
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