Lpc2000

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.

Regards
-Bill Knight
R O SoftWare








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