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Basic Electronics » Data input voltage

January 03, 2011
by Hexorg
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Hello NerdKits. I was wondering what's the recommended voltage range for the input to be detected at 1? My MISO in SPI is getting 3.3V in, and I suspect that i'm getting a lot of noise because of that. What is to good way to step-up that 3.3 into 5v?

January 03, 2011
by Rick_S
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You could use a buffer chip with 5V tolerant inputs such as the 74LCX125 used on the SD adapter sold by the teensy folks HERE. He has a schematic of his adapter there you could maybe use for a reference in your circuit.

Rick

January 03, 2011
by Hexorg
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Rick, that's pretty much exactly what I'm doing (using SD card). except instead of buffer chip with 5V tolerant inputs such as the 74LCX125, i just have voltage dividers to give 3.3V for the card. My question is, can uC handle 3.3V as an INPUT and count that as 1?

January 03, 2011
by Rick_S
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The cut-offs are in the datasheet under Electrical Characteristics. In my datasheet they are on page 313.

The datasheet shows

V-IL (Input Low Voltage, except Xtal1 and Reset) is Maxed at .3V*VCC so if your runing 5V VCC then anything 1.5V or less is guaranteed to be considered Low.

V-IH (Input High Voltage, except Xtal1 and Reset) is Min at .6V*VCC so if your runing 5V VCC then anything 3V or greater is guaranteed to be considered High.

So if the SD card is sending out 3.3V signals to the micro, they should be seen as high, however if the pulses were to drop lower than 3V, they would be in the gray area and may get missed.

An option would be to lower the voltage to the mcu so it is closer to the sd card or run it at 3.3 if the micro you are using supports it. The 328P's will run as low as 1.8v I believe. If you ran it all at 3.3, you would eliminate all need for any level conversion... Just a thought :)

Rick

January 03, 2011
by Hexorg
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Aaah! Rick, thank you so much, as soon as I lowered the mcu's VCC to 3.3V, it started to work like a charm! XD

January 03, 2011
by Rick_S
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No problem, glad it helped out.

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