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PICList Thread
'[PIC]SD Card interfacing'
2006\05\11@025157 by Per Linne

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Which strategies are common when it comes to interfacing
3.3V SD Cards to a 5V PIC? I need to interface both 5V
devices and an SD card over SPI. Are there any level shifters that
are meant for this purpose? The 16-bit M-chip seminar suggested
that there will be an application note on the subject. Has anyone
seen that app note yet?

Regards,
PerL

2006\05\11@043921 by Mike Harrison

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On Thu, 11 May 2006 08:51:15 +0200, you wrote:

>Which strategies are common when it comes to interfacing
>3.3V SD Cards to a 5V PIC? I need to interface both 5V
>devices and an SD card over SPI. Are there any level shifters that
>are meant for this purpose? The 16-bit M-chip seminar suggested
>that there will be an application note on the subject. Has anyone
>seen that app note yet?
>
>Regards,
>PerL

While searchiing for other 3v3 translators recently I notice that TI listed one aimed at SD card
applications - check their site - can't remember if it was under logic or interface sections.


2006\05\11@082924 by Maarten Hofman

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www.k9spud.com/sdcard/ thinks you can use 74VHC08 chips, though it
seems they didn't try it yet.

Greetings,
Maarten Hofman.

2006\05\11@111500 by Bob Blick

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> Which strategies are common when it comes to interfacing
> 3.3V SD Cards to a 5V PIC? I need to interface both 5V
> devices and an SD card over SPI. Are there any level shifters that
> are meant for this purpose? The 16-bit M-chip seminar suggested
> that there will be an application note on the subject. Has anyone
> seen that app note yet?

I haven't seen a Microchip app note, but the way a lot of people do it for
just a few lines is with a mosfet for each line. Page 43 of this Philips
document gives an example:

http://www.semiconductors.philips.com/acrobat/literature/9398/39340011.pdf

Cheerful regards,

Bob


2006\05\11@115219 by Harold Hallikainen

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>> Which strategies are common when it comes to interfacing
>> 3.3V SD Cards to a 5V PIC? I need to interface both 5V
>> devices and an SD card over SPI. Are there any level shifters that
>> are meant for this purpose? The 16-bit M-chip seminar suggested
>> that there will be an application note on the subject. Has anyone
>> seen that app note yet?
>
> I haven't seen a Microchip app note, but the way a lot of people do it for
> just a few lines is with a mosfet for each line. Page 43 of this Philips
> document gives an example:
>
> http://www.semiconductors.philips.com/acrobat/literature/9398/39340011.pdf
>
> Cheerful regards,
>
> Bob

The mosfet approach is used mostly where you have a bidirectional data
line, but SPI lines are all unidirectional. While the mosfet approach will
definitely work, it has quite a few parts when compared with a single
level translator chip from Maxim or others. Maxim also has bidirectional
level translators that use the mosfet trick plus a speed up monostable
trick.

Harold
(the ideal design has zero parts)

--
FCC Rules Updated Daily at http://www.hallikainen.com - Advertise on
hallikainen.com - $100/year!

2006\05\11@123208 by Bob Blick

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> The mosfet approach is used mostly where you have a bidirectional data
> line, but SPI lines are all unidirectional. While the mosfet approach will
> definitely work, it has quite a few parts when compared with a single
> level translator chip from Maxim or others. Maxim also has bidirectional
> level translators that use the mosfet trick plus a speed up monostable
> trick.


Ahh, for some reason I read I2C. For unidirectional interfacing in 5 to 3
direction, use an NPN transistor as common-base amplifier - use the
emitter as input, collector as output(with pullup to 3.3 supply) and base
through a resistor to 3.3 supply. In the 3 to 5 direction, on a normal TTL
level pin, just hook it directly. However the SPI interface on the PIC has
Schmitt trigger inputs so that is marginal(at best). You can use the
common base method slightly modified, but at that point your three(or
four) pins have gobbled up a dozen little parts. Cheap parts, but what a
hassle. CD4049 for 5-to-3 + CD40109 for 3-to-5, perhaps?

Cheerful regards,

Bob

2006\05\11@151251 by Robert Rolf

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Bob Blick wrote:
>>The mosfet approach is used mostly where you have a bidirectional data
>>line, but SPI lines are all unidirectional. While the mosfet approach will
>>definitely work, it has quite a few parts when compared with a single
>>level translator chip from Maxim or others. Maxim also has bidirectional
>>level translators that use the mosfet trick plus a speed up monostable
>>trick.
>
>
>
> Ahh, for some reason I read I2C. For unidirectional interfacing in 5 to 3
> direction, use an NPN transistor as common-base amplifier - use the
> emitter as input, collector as output(with pullup to 3.3 supply) and base
> through a resistor to 3.3 supply.


Isn't it simpler to use a diode? You have a diode drop from the
transistor emitter anyway.

     p
     u
     l
     l
     u
     p
3.3V-+--|>|--5V


> In the 3 to 5 direction, on a normal TTL
> level pin, just hook it directly. However the SPI interface on the PIC has
> Schmitt trigger inputs so that is marginal(at best). You can use the

So use a series resistor from the 3V with a pullup on the 5V side
to move the 3.3V range up over the s/t threshold.

> common base method slightly modified, but at that point your three(or
> four) pins have gobbled up a dozen little parts. Cheap parts, but what a
> hassle. CD4049 for 5-to-3 + CD40109 for 3-to-5, perhaps?

Robert

2006\05\11@160422 by Bob Blick

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>>For unidirectional interfacing in 5 to
>> 3
>> direction, use an NPN transistor as common-base amplifier - use the
>> emitter as input, collector as output(with pullup to 3.3 supply) and
>> base
>> through a resistor to 3.3 supply.
>
>
> Isn't it simpler to use a diode? You have a diode drop from the
> transistor emitter anyway.

No, common base does not work like that. Collector-emitter voltage is the
saturation value of the transistor, you can get it down to 50 millivolts
pretty easily.

>> In the 3 to 5 direction, on a normal TTL
>> level pin, just hook it directly. However the SPI interface on the PIC
>> has
>> Schmitt trigger inputs so that is marginal(at best). You can use the
>
> So use a series resistor from the 3V with a pullup on the 5V side
> to move the 3.3V range up over the s/t threshold.

Then you miss the bottom threshold, and your total voltage swing is
reduced. You need more voltage swing, not less.

Cheerful regards,

Bob


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