[SI-LIST] Re: Level Translator parts
- From: steve weir <weirsi@xxxxxxxxxx>
- To: "Muranyi, Arpad" <Arpad_Muranyi@xxxxxxxxxx>
- Date: Fri, 10 Sep 2010 13:53:00 -0700
Muranyi, the combination of his ferrite beads, the bypass network and
drivers in conflict can ring for that period of time. A cable would
have to be really, really long.
Steve
Muranyi, Arpad wrote:
> I am not sure a really long cable would cause/let things
> ring for 100's of ns. Aren't the losses going to be large
> due to the length and damp the ringing fast? I guess the
> answer may depend on the frequency of the oscillations too...
>
> Regarding the direction controls, another thing to keep in
> mind is that a disabled I/O (to act as a receiver) is also
> a high impedance "termination" allowing full reflections,
> which may only be clamped outside the supply rails, but if
> the signaling is well inside the rails, they may never get
> clipped by the clamps...
>
> Arpad
> =============================================================
>
> -----Original Message-----
> From: steve weir [mailto:weirsi@xxxxxxxxxx]
> Sent: Friday, September 10, 2010 3:07 PM
> To: keithK EPD
> Cc: tom@xxxxxxxxxxxxx; Muranyi, Arpad; si-list@xxxxxxxxxxxxx
> Subject: Re: [SI-LIST] Re: Level Translator parts
>
> 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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>>
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>
>
>
--
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
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