3.3v <> 5v interfacing @ 15Mhz

rickman <gnuarm@gmail.com> wrote in message
news:1164552868.812529.247280@h54g2000cwb.googlegroups.com...
> Yuriy K. wrote:
> > rickman wrote:
> >
> > > I am starting to think the voltage issue is a red herring.
> >  > ...
> > > So maybe you don't need the voltage buffers anyway.
> >
> > dsPIC I/O port VinH requirement is 0.8*Vcc.
> > At 5V it gives us VinH >= 4.0V.
> > It's a little high for 3.3V circuit, isn't it?
>
> Haven't you been reading the thread?  We discussed that and Alison
> added transistor buffers that should have fixed the problem and instead
> made it worse.  That says to me that although driving from 3.3 may not
> be meeting the spec, it is working in her case and she has another
> issue.
>

Hi Rickman,

It's working :-)  I made up a small board of solid copper, and am using a
fixed 2k/1k divider to give 3.33v.

Have a look at this picture to save 1000 words;

http://www.logicsays.com/pub/WeST_pcb.jpg

Not something I'd want to leave on my car seat at an airport!!  Most of the
wires are now redundant.  The little upright board has a MAX232 on it.  Your
transistor circuit is over there on the lower right.  The upper right are
those awful dividers from before.

Even the two most troublesome cards I have are working (they're about
3-years old).

What does that tell us in the end?  That breadboards are seriously
problematic.

Our DATA OUT signal from the SD card is now directly connected to the SPI-IN
port on the pick.

Alison

Yuriy K. <yktech@mail.ru> wrote in message
news:ekbbrh$ect$1@news.netins.net...
> rickman wrote:
>
> > I am starting to think the voltage issue is a red herring.
>  > ...
> > So maybe you don't need the voltage buffers anyway.
>
> dsPIC I/O port VinH requirement is 0.8*Vcc.
> At 5V it gives us VinH >= 4.0V.
> It's a little high for 3.3V circuit, isn't it?
>
> --
> WBR, Yuriy.
> "Resistance is futile"

Hey Yuiry!! :-)

I'm going to order a few of those ICs you suggested, as to be honest I like
the right way of doing things.  And resistor dividers don't seem to be the
100% reliable tool for the job.

If there's an IC specifically for 3.3v/5v bidirectional comms, then I think
it's good practice to use it.

That dsPIC is now recognising 3.3v as positive '1' at 15Mhz.  It had been
spoken of that PICs would work with SD cards directly, yet the datasheets
suggest otherwise.  So it's sensible to stay within spec.

This will be a data storage device afterall, so nothing can be left to
chance.

Thanks again :-)

Alison

On 25 Nov 2006 05:07:30 -0800, "PeteS" <PeterSmith1954@googlemail.com>
wrote:

>Alison wrote:
>
>> Hi,
>>
>> Please may I ask for assistance?  I need a reliable method of interfacing an
>> SPI SD/MMC card to a dsPIC.  At the moment I'm using a 1k8/3k3 voltage
>> divider on the 5v ----> 3.3v signals, and connecting the 3.3v ----> 5v
>> signals straight to port.
>>
[snip]

For 3.3V to 5V Logic...

http://analog-innovations.com/SED/LogicLevelTranslator74HC14-RevA.pdf

                                        ...Jim Thompson
-- 
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Alison wrote:
> Hi Rickman,
>
> It's working :-)  I made up a small board of solid copper, and am using a
> fixed 2k/1k divider to give 3.33v.
>
> Have a look at this picture to save 1000 words;
>
> http://www.logicsays.com/pub/WeST_pcb.jpg
>
> Not something I'd want to leave on my car seat at an airport!!  Most of the
> wires are now redundant.  The little upright board has a MAX232 on it.  Your
> transistor circuit is over there on the lower right.  The upper right are
> those awful dividers from before.

Yes, it does look a little rough.  I don't think many people still use
those push boards just because they can create problem with the speeds
of today's circuits.  The stuff we do goes directly to PCB.  In fact
the last board I designed had *no* artwork revisions and the first rev
is being shipped to the customer now.


> Even the two most troublesome cards I have are working (they're about
> 3-years old).
>
> What does that tell us in the end?  That breadboards are seriously
> problematic.
>
> Our DATA OUT signal from the SD card is now directly connected to the SPI-IN
> port on the pick.

Like you say in your other post, this is clearly not meeting the spec
and so you should consider changing it.  However, as others have
pointed out, circuit delays can add up and be a problem at 15 MHz,
especially since the SPI bus is only using half a clock for setup (33
ns).  So when you pick a buffer, make sure it is not one of the really
slow HC or HCT buffers.  Try to pick one with < 5 ns delay which should
not be a problem at 5 volts.  At 3 volts you may see a bit longer
delay, but you should be able to keep it below 10 ns.  This should all
be spec'd in the data sheet.

Jim Thompson wrote:

> For 3.3V to 5V Logic...
> 
> http://analog-innovations.com/SED/LogicLevelTranslator74HC14-RevA.pdf

Incredible. Designing complicated circuit instead of simply using 
74HCT14 / 74HCT04 / zillion other HCT74xx and AHCTxx ICs.

-- 
WBR, Yuriy.
"Resistance is futile"

rickman wrote:

> Haven't you been reading the thread?  We discussed that and Alison
> added transistor buffers that should have fixed the problem and instead
> made it worse.  

This is exactly what expected from *transistor* buffers at this speed. Duh.

-- 
WBR, Yuriy.
"Resistance is futile"

  "Alison" <alison@logicsaysNOSPAM.com> wrote:

  > Have a look at this picture to save 1000 words;

  > http://www.logicsays.com/pub/WeST_pcb.jpg

  > Not something I'd want to leave on my car seat at an airport!!

  Alison,

  I hate to tell you this, but that solderless breadboard is about the
  worst thing you could be using. With edge rates of 10ns  and periods
  of 33  ns, you are talking serious ringing and  ground  bounce. This
  kills data  integrity  regardless of how  good  the  paper schematic
  looks. The  breadboard  will also  develop  intermittent connections
  after a  while, so you end up spending all your time  chasing random
  problems.

  Walt Kester, Analog Devices states:

    "Matrix board,  Vectorboard,   wire-wrap,  and  plug-in breadboard
    systems are, without exception, unsuitable for high-performance or
    high-frequency analog prototyping. The resistance, inductance, and
    capacitance of these breadboards are too high."

  He shows  some examples of high-frequency prototyping here,  but the
  photos are too small:

    http://www.lpkfusa.com/articles/prototyping/edn_2_96.pdf (198KB)

  Here are  some  examples of "deadbug"  and  "solder-mount" prototype
  circuits:

    http://tinyurl.com/y9ljsq (598KB)

  Also look  for  "Manhattan" (and  the  incorrectly  spelled version,
  "Manhatten") breadboards.

  Terry Ritter has an excellent example at

    http://www.ciphersbyritter.com/RADELECT/BREADBD/BREADBD.HTM

  I hope this helps. If you need any more information, please ask.

  Regards,

  Mike Monett

  Antiviral, Antibacterial Silver Solution:
   http://silversol.freewebpage.org/index.htm
  SPICE Analysis of Crystal Oscillators:
   http://silversol.freewebpage.org/spice/xtal/clapp.htm
  Noise-Rejecting Wideband Sampler:
   http://www3.sympatico.ca/add.automation/sampler/intro.htm

Jim Thompson wrote:

>>>Please may I ask for assistance?  I need a reliable method of interfacing an
>>>SPI SD/MMC card to a dsPIC.  At the moment I'm using a 1k8/3k3 voltage
>>>divider on the 5v ----> 3.3v signals, and connecting the 3.3v ----> 5v
>>>signals straight to port.
>>>
> 
> [snip]
> 
> For 3.3V to 5V Logic...
> 
> http://analog-innovations.com/SED/LogicLevelTranslator74HC14-RevA.pdf

Wow! This is the case when the extra knowledge only harms.
Why didn't you use a simple gate of a standard LS or HCT series?


Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

Mike Monett <No@email.adr> wrote in message
news:Xns988770837D9B4Noemailadr@208.49.80.251...
>   "Alison" <alison@logicsaysNOSPAM.com> wrote:
>
>   > Have a look at this picture to save 1000 words;
>
>   I hate to tell you this, but that solderless breadboard is about the
>   worst thing you could be using. With edge rates of 10ns  and periods
>
>   I hope this helps. If you need any more information, please ask.
>
>   Regards,
>
>   Mike Monett
>
>   Antiviral, Antibacterial Silver Solution:
>    http://silversol.freewebpage.org/index.htm
>   SPICE Analysis of Crystal Oscillators:
>    http://silversol.freewebpage.org/spice/xtal/clapp.htm
>   Noise-Rejecting Wideband Sampler:
>    http://www3.sympatico.ca/add.automation/sampler/intro.htm


Hi Mike,

Thanks for the info.  Reading through now.

I'm going to try and get the circuit onto PCB in the next week or so.  The
final design will also switchover to TQFP devices.  At the moment it just
about serves the purpose of providing a base from which I can develop the
firmware for the chip.

To be honest, I hate breadboard!!  But am not in a position to create PCBs.
The closest I can do is 0.1" pitch, printing out the circuit, drilling the
holes through, dalo-ing out the bits I want to keep, and then ferric-ing off
the rest.  That's just toooooo messy.

Prototype PCBs here in the UK are still astromonically expensive compared to
the US.

Thanks again,

Alison

Vladimir Vassilevsky wrote:

>> For 3.3V to 5V Logic...
>>
>> http://analog-innovations.com/SED/LogicLevelTranslator74HC14-RevA.pdf
> 
> Wow! This is the case when the extra knowledge only harms.
> Why didn't you use a simple gate of a standard LS or HCT series?

74LS: VoutH >= 2.4V   It is actually worse, than 3.3V CMOS.
-- 
WBR, Yuriy.
"Resistance is futile"