[SI-LIST] Re: Level Translator parts
- From: steve weir <weirsi@xxxxxxxxxx>
- To: keithK EPD <epd2001usa@xxxxxxxxxxx>
- Date: Fri, 10 Sep 2010 13:06:38 -0700
Keith a scope photograph would help, but for this kind of really long
data cycle stuff, I suspect you have your buffer direction controls
messed up. The fact that your unidirectional signals work OK, and that
when you disable the translating receiver both speak to this. You would
have to have a really, really long cable or some humungous capacitive
load to cause ringing for 100's of nsec.
Steve.
keithK EPD wrote:
> 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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--
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
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www.ipblox.com
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