best way to add 3.3v flash to a 5v uP?

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previously, I have successfully connected an SPI based flash memory to
a 5V system using resistors to drop the voltage down from 5v to 3.3 v
for the CS, CLK & SI lines and a FET controled by the flash for the
3.3v to 5v, SO connection.

I now have to directly connect a flash chip to the address and data
lines of a microcontroller. I now have to deal with something like 34
lines instead of 4 and the speed of the signals will probably be a lot
faster than the SPI Flash.

Can I still get away with using my simple resistor interface?

I have been looking at some devices called TruLevel Bus Switches from
Fairchild which can do the level translation.

EG. http://www.fairchildsemi.com/ds/FS%2FFSTD16861.pdf

and other devices  FSTD16450, FSTUD16450, FSTD3125 and FSTUD32450.

However, I don't seem to be able to find a UK supplier. Google doesn't
seem to be much help either.

Any ideas?

Thanks.


Re: best way to add 3.3v flash to a 5v uP?
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Most 3.3V flash is 5V safe, i.e. you can directly connect all the data
lines. Just needs a 3.3V supply.

What flash device is it?
--
Tim Mitchell

Re: best way to add 3.3v flash to a 5v uP?
It's Intels 28F320J3 StratFlash.

Problem is, reading intels AP-477 section 3.2.1.2 5V to 3V Interface
quotes;

"If the 5v supply ramps much faster than the 3v supply, the substrate
diode may still be forward biased temporarily, leading to reliability
problems"

Since my 3v supply is derived from the 5v supply using National
Semiconductors LP2988  then I may well have this situation.


Re: best way to add 3.3v flash to a 5v uP?

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From what I've seen with linear regulators, if the 5V has a
'normal' smooth rise, the 3.3V will track that rise fairly
closely up to it coming into regulation.

Re: best way to add 3.3v flash to a 5v uP?


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What about selecting a 5V Flash instead? Even if it's more expensive,
the board will simpler and the cost should balance out.

Alternatively, try lowering the voltage on the rest of the board to
3.6V. You should be within tolerance for the Flash.

--
Magnus McElroy
Electrical Engineer (EIT)
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Re: best way to add 3.3v flash to a 5v uP?

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I have used the Fairchild 74LVXC4245MTC device for connecting a 3V CPU
to 5V peripherals. There are a number of varients depending on whether
the control signals are on the 3V or 5V side.
You can buy these and other Fairchild devices from http://www.ebv.com .
Normally one has to buy a minimum of one tube of devices.

Regards
  Anton Erasmus



Re: best way to add 3.3v flash to a 5v uP?

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TI (Texas Instruments) makes a compatible part to the above part number.
It is SN74LVC4245A.

There is also a 16-bit versions, SN74ALVC164245.

Both are available from DigiKey.

I'm not an employee of Texas Instruments - but I was at one time.

Urb


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