[SI-LIST] Re: Level Translator parts
- From: "Muranyi, Arpad" <Arpad_Muranyi@xxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Fri, 10 Sep 2010 11:47:24 -0700
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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