3.3 to 5v translator needed?

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Hello hw folks. I have a mega128 running on 5v, and it talks to several spi peripherals at 5v. The boss threw in a 3.3v atmel spi flash rom. The new hw guy just hooked it up to the spi clk and data. Does this usually work, or should they have thrown in a 3.3 to 5v translator? Will it 'clamp' the spi data and clk to 3.3? If so this blows my other 5v spi periphs out of the water. (obviously, it doesnt work, and of course, the hw guy thinks its sw, and me, the sw guy, thinks its hw.... aint life grand?)

Imagecraft compiler user

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Running that 3.3V peripheral at 5V is asking for trouble, especially in the future. It may work, for a while!

Try 74LCX125 - its 5V complient on both input and output side. It can be used going outbound. Use one section for clock and one for MOSI. Connect the enable of each section to ground and you have a non-inverting 5V to 3.3V translator.

MISO side is harder because it wants an open "collector" output that is 5V complient. But, you can use one section 74LCX125 for that also. It has tristate output and each section has its own output enable. Use pullup resistor to +5 (which should already be there). Connect the peripheral's data output to the enable input and, on that same section, connect the normal logic input to ground. You end up with a non-inverting open collector 3.3V to 5V translator! Oh, you may need a 3.3V pullup on the input because the peripheral's output should be open collector.

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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Search the Forum for "dataflash level 3.3" to uncover two recent threads that are fairly comprehensive.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Doesn't Atmel have either an appnote or a spot in the Dataflash datasheet that discusses this exact topic as well?

-Colin

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Your HW guy should be able to show you the data sheets where:

(1) The 3.3V device can generate valid 5V "high" levels.
(2) The 3.3V device can tolerate 5V at its input.

If either (1) or (2) are false or unproven, then you do need a level translator. This one hagtech.com/pdf/translator.pdf is a clever one which requires only two transistors and 4 resistors. Where the signal is flowing in one direction only, you only need half of the circuit.

Regards,
Mike

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I would use:
74ALVC164245 - 16-bit dual supply translating transceiver (3-State)
or
74LVC4245A - Octal dual supply translating transceiver

You can choose the direction and the Suply Voltages! May be there are another similar ICs.
3,3V and 5V are compatiable, but do you know what will happen when temperature changes or there any noise sources or the PCB is not designed so good...It's hazard, I think!

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You can also use the MAX3377.
I use it to interface an MMC (3.3V) to an ATMEGA.
It works fine.

JP

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Atmel has made an application note about this problem, just read this and you will know how to interface the 3.3-volt DataFlash correctly.

Interfacing the 3-volt DataFlash with a 5-volt System:
http://www.atmel.com/dyn/resourc...

This article was published in The Atmel Applications Journal, Issue 2 - Spring 2004.
You can download a pdf version here:
http://www.atmel.com/journal/spl...
Or order printed versions for free by filling out this form:
http://www.atmel.com/journal/Def...