For signals from the flash IC to the Arduino just connect directly (or add a series 1k resistor to protect the flash chip should you accidentally configure that Arduino pin as an output).
If you take slightly larger values for the v.div, namely 2.4K and 4.7K, and a largish 40pFestimate for gate capacitance, then the time-constant is R*C= 1.6K * 40pF = 64 nsec.That's still pretty fast. It would probably work, except at very fast SPI clock frequencies.
However, re the series resistor? Whilst this'll limit current, obviously, how does this protect the 3.3v device from a 5v over voltage? 5v is still going to flow through the 1K?
QuoteHowever, re the series resistor? Whilst this'll limit current, obviously, how does this protect the 3.3v device from a 5v over voltage? 5v is still going to flow through the 1K?It works (and only works) on chips whose input pins utilize built in 'clamping' diodes that conduct if the signal voltage is above the chip's Vcc or below it's Vdd (or ground) connections. So the clamping diode conduct to limit the voltage the input pin actually is subjected to and the external series resistor limits the current flow to the max or less of the rated current capacity of the clamping diodes. So one needs to know what the current capacity rating of the specific clamping diodes for properly sizing the series resistors correctly.
Ah, I see - that makes perfect sense.It also leads me on to another question which I was going to as elsewhere in a new thread about clamping in general; if say, in this example, 5v if fed into a pin instead of 3.3v, when the clamp diodes begin to conduct, is it just the excess voltage that is conducted away to ground, so the pin would still see 3.3v, or does ALL the voltage get conducted away?
Yes probably - but SD and microSD card interfaces are fussy as their I/O is designed for 50MHz signallingin native mode rather than slow SPI from a microcontroller. A 16MHz Arduino can drive 8MHz SPI, ie clockhigh and low times are 62.5ns, so rise and fall times of 20ns or better would be advisable for that I think. Most devices would be more like 10pF than 40pF I hope.
Also, where does the 1.6k come from in the calculation? 4k7 - 2k4 = 2k3... Not from there.
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