Lpc2000

----- Original Message ----- 
From: "Bertrik Sikken" <bertrik@...>
To: <lpc2000@yahoogroups.com>
Sent: Thursday, December 08, 2005 8:23 PM
Subject: Re: [lpc2000] Re: JTAG Programmer Confusion - Noob Question


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> Leon Heller wrote:
>> ----- Original Message -----
>> From: "sixtyfivebit" <sixtyfivebit@...>
>>> 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.

Leon 

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