[SI-LIST] Re: Level Translator parts
- From: keithK EPD <epd2001usa@xxxxxxxxxxx>
- To: <tom@xxxxxxxxxxxxx>, <arpad_muranyi@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Fri, 10 Sep 2010 15:58:40 -0400
At first blush the answers you have given sound correct , but I left off a few
details
1) I can not change the HCT245 part (it's on a driver board, and fully
qualified).
I have full control design over a "display" PCB to drive the LCD data
and control pins
as I said before the un-dir pins (or control pins) work fine.
2) the display doesn't have 5 volt compatible inputs, requiring the 3.x volt
conversion
3) the series ferrites, placed right after the 245 driver, also can not be
changed
(un-fortunately, it would have been nice to change the driver part to a
HC245 and run off of 3.3v...no can do)
4) I'm not sure about how fast the edge rates on the ST2378 / but I can turn
off the enable pin (low) and the
in-coming signal(s) from the 245 clean up nice / it appears that the
ST2378 acts like an low impedance device (?)
it seems like the reflected wave on the signals is trying to find a
home but kinda oscilates due to the inductor
in the path at the source.
5) I will be trying to put series resistors on the Display PCB, to dampin (or
match) the ST2378 Z.
and
6) then replace the ST2378 by a HC245 and use the control pins to drive the
display ...
thanks for your responses - I still haven't read them all, but I will ......
Keith Kowal
Electronic Product Design - MA
781-593-0199 (h)
www.product-designs.com
> From: tom@xxxxxxxxxxxxx
> To: Arpad_Muranyi@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Level Translator parts
> Date: Fri, 10 Sep 2010 12:21:11 -0700
>
> 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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