[SI-LIST] Re: Level Translator parts

Arpad

Since Keith was having issues getting a good low I suspect his new driver
has a much faster edge than his old 5V driver.  Faster edges can cause all
kinds of SI issues.  And many drivers are asymmetric in their edge rates
where the falling edge is much faster than the rising.

I'm also curious if Keith even needed to go through this exercise.  Many
modern 3.3V receivers are 5V tolerant...  And if the new driver has a "TTL"
type output the output level will not drive all the way to 5V.

Just a thought.

Tom Dagostino
Teraspeed Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
tom@xxxxxxxxxxxxx 
www.teraspeed.com 

Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
401-284-1827
www.teraspeed.com  


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Muranyi, Arpad
Sent: Friday, September 10, 2010 12:02 PM
To: Muranyi, Arpad; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Level Translator parts

To add to my previous posting, now that I think of it, the
parts didn't even have to be Bipolar.  Chip designers started
to make output stages with CMOS pullup transistors with their
substrate tied to 5 V while the channel was tied to 3.3 V.
This would active the parasitic diode only above 5 V, in
other words the clamping action only started to kick in above
5 V.  The result was the same, the T-line looked unterminated
to signals above 3.3 V and they rang like crazy.

The same thing could also happen with an N-channel pullup,
which acts like the Bipolar emitter follower I talked about
before...

Arpad
================================================================

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Muranyi, Arpad
Sent: Friday, September 10, 2010 1:47 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Level Translator parts

Keith,

I am not familiar with the  ST2378  part, and glancing at the data
sheet didn't reveal to me whether it has a CMOS or Bipolar output
stage.  Do you know what technology this part is made with?

The reason I am asking this question is because I have seen nice
ringing signals with Bipolar devices in the days when the signals
were going down to 3.3 V from 5 V and people started using "5.0 V
safe 3.3 V" chips.  The pullup transistor being an emitter follower
would shut down when the signal would go above 3.3 V, which means
that the line looks like as if it was completely unterminated at
those higher voltages.  This high impedance, open ended line rang
like a bell with a nice overshooting signal going above 3.3 V.  I
suspect you may be running into a similar situation if this part
behaves similarly...

On the other hand, if you are observing the ringing at its input
side, the question is what voltage rail does the input's clamping
clamp to?  If they are clamping to your 5 V supply rail, once again,
your input is high impedance all the way to 5 V, allowing the lower
voltage signal to ring way above its signaling levels.

Just a couple of clues...

Arpad
=================================================================== 



-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of keithK EPD
Sent: Friday, September 10, 2010 12:38 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Level Translator parts

I just experienced an un-expected signal response.    It involved a 5v
driver  (HCT245), series ferrite, ribbon cable,
and a 5v to 3 volt part ST2378 driving a 3volt data bus req'd for the
new LCD - this whole thing was due to a display going
obsolete and a new one to replace the old one.   (bus cycle is around
1usec)
 
 So, you think that this would be a simple data bus tranlation part to
accomplish the job .... the LCD data bus is bi-directional (r/w, of
course) ....   On the bi-directional data bus,  I got a wonderful
ringing on the bus while a write bus cycle was occuring, that didn't
allow the bus to find a good low level.   Sounds like the signals
experienced an open circuit....
 
the 5 control uni-directional signals to the display behaved as
expected.
 
comments / solutions welcome.



Keith Kowal
781-593-0199 (h)

www.product-designs.com
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