Lpc2000

--- In lpc2000@yahoogroups.com, "Leon Heller" <leon.heller@b...> 
wrote:
>
> 
> ----- Original Message ----- 
> From: "Bertrik Sikken" <bertrik@z...>
> To: <lpc2000@yahoogroups.com>
> Sent: Thursday, December 08, 2005 8:23 PM
> Subject: Re: [lpc2000] Re: JTAG Programmer Confusion - Noob Question
> 
> 
> > -----BEGIN PGP SIGNED MESSAGE-----
> > Hash: SHA1
> >
> > Leon Heller wrote:
> >> ----- Original Message -----
> >> From: "sixtyfivebit" <sixtyfivebit@y...>
> >>> I'm glad this thread is helping other people, and after some 
research
> >>> things are a bit more clear...
> >>>
> >>> Since I really don't want to buy a wiggler and would rather 
build one
> >>> my self (for experience, learning, etc.), is there any 
particular
> >>> schematic I should go with?
> >>>
> >>> Leon_Heller, nice site and I saw your prototype page earlier
> >>> (bookmarked it), but I couldn't find your jtag.zip. I went 
through
> >>> most of the pages on your site but didn't see it... did I miss 
> >>> something?
> >>
> >> It's in this groups Files area.
> >
> > Hi Leon,
> >
> > Sorry if I'm asking a stupid question (I'm not a hardware 
designer),
> > but is your circuit safe for the parallel port?
> >
> > As far as I can see the 74HC244 takes its VCC from the 3.3V from
> > the target board while the 5V parallel port signals are directly
> > connected to the inputs of the HC244. In the HC244 datasheet I
> > notice that there are protection diodes between the inputs and
> > Vcc / ground.
> >
> > It looks like there is a current path from the 5V parallel ports 
pins
> > through the HC244 protection diodes into the target 3.3V without 
any
> > obvious current limit. Doesn't this risk blowing out the parallel 
port
> > drivers? Also, wouldn't this lift the 3.3V on the target to a 
possibly
> > dangerous voltage?
> 
> It would be better to use a 74AC244, but I don't have any problems 
with it 
> as it is.

I should also have said that parallel port outputs don't generate 5V 
signals (I think they are TTL), and they are protected.

Leon