Lpc2000

>From: "Bill Knight" <BillK@...>
>Reply-To: lpc2000@yahoogroups.com
>To: "lpc2000@yahoogroups.com" <lpc2000@yahoogroups.com>
>Subject: Re: [lpc2000] Re: Olimex ARM JTAG Signals levels
>Date: Wed, 20 Oct 2004 10:03:21 -0500
>
>
>On Wed, 20 Oct 2004 14:38:48 -0000, tsvetanusunov wrote:
>
> > Hi Olimex/Others
> >
> > I have an Olimex ARM JTAG. It works ok on my homemade board if Vcc
>is
> > 3.3 V. But if Vcc is 3.1 V is does not work.
> >
> > I have look at the JTAG.
> > The board contain a 74AC244 (5V driver - power from PC)
> > The board contain a 74LCX244 (3.xV driver - power from target)
> >
> > Signals from Target through 74LCX244 OK, but from 74LCX244 to
>74AC244,
> > do not work 3.x volt to 5 volt system.
>
>Hello John,
>
>3.1V is near the down limit of the specification
>if you have problems just increase the power to 3.3V while debugging
>then if you product needs exactly 3V power use this power supply in
>production and use ISP utility to load your code
>
> > I think the 74AC244 should be replaced by a 74ACT244 ???
> >
> > What is your opinion ?
>
>bad idea as LPT signals often are not 5V but down to 4.0-4.3V (at
>least this is what we measured on two notebooks in office
>ACT needs >4.5V to operate, so the option to rise your ARM power
>supply to 3.2V while debugging seems less hassle, than to replace the
>computer you use for debugging
>
>Best regards
>Tsvetan
>===================================================================
>I'm not sure it is a bad idea.  The 74LCX244 connects to the ARM and
>gets its power (~3.3v) from the target board. The 74AC244 connects to the 
>PC
>and gets its 5v from there.  The problem might be that the signal(s)
>FROM the 74LCX244 TO the 74AC244 are not above the 74AC244's input 
>threshold
>level.  Changing the 74AC244 to a 74ACT244 would lower the input threshold
>requirement for the signal(s) coming from the 74LCX244 while still 
>maintaining
>compatablility with the PC signals.
>
>Of course making sure the target supply voltage was 3.3v would help too.
>
>BTW: I checked my Macraigor WIGGLER and it also uses the 74AC244 and 
>74LCX244
>parts.
>

I just use a 74HC244 on my own JTAG interface (it's spec'd down to 2.0 V 
supply). I haven't checked it at anything other than a nominal 3.3V, though.

Leon