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Digital Discovery with 5V Logic


Joey Gold

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Hi,

I found this thread while looking into using my Digital Discovery w/ 5V logic... (surprise!)

To answer the question in a little more detail and ask another question.  The DIN_USR_0...23 inputs are definitely 5V tolerant by virtue of having a 51k input series resistor (in parallel with a compensation capacitor) followed by a Schottky ESD/overvoltage diode to 3.3V.  That's only 33uA current (approx) through the resistor+diode @ 5V input.  The diodes are rated at 30mA, so (ignoring any diode forward voltage drop...) you could jam an awful lot of voltage into those inputs without causing damage.

The DIO_USR_24...31 I/O channels, however, I'm not so sure.  The SN74CBT3384C 10ch FET bus switch is 5V tolerant, but between the input protection PTC, FET Ron, and 10 ohm resistor I only see 46 ohms, and then it appears the DIO_FPGA_24 signal is fed directly into the FPGA I/O pins.  Any idea on whether these 16 I/O will tolerate 5V?  I haven't explored the software well enough yet to know if they can actually be used as inputs, TBH, but the name says DIO, so I'm assuming it's possible.

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