I/O lines standards]

Hello the list: I need to use the digital I/O from my 2400 daughterboard
to drive a sequencer for a t/r system. I wanted to convert the USRP
to 5 V safely.
I tried 5 Fairchild type H11L1 optoisolators, current production,
available from Digi-Key, in a test setup. Put a 1k ohm resistor in
with the input diode. All 5 drew less than 2 ma current at 3.3 volts,
switched state from low to high at .8 to .9 ma (about 2.7 volts).
According to the spec less than 5 ma drain per line, these will be safe
use as output buffering and transition to 5v logic. Note that they will
high (5 v) with the inputs at either 3-state or low. To use 5v logic
the USRP lines as inputs, use the Fairchild driving a voltage divider to
yield about 2.5 volts to the input pin. Note the Altera device is
operating as LVCMOS so it will not draw enough current to unbalance the
divider. The H11L1 is an open collector output, so a simple divider with
1k resistors or even larger should work fine. With the H11L1 connected
a 5v supply, no condition will exceed the 5 ma limit.
Note that you can use the USRP lines directly with any of the PIC
processors that run on 3.3 volts. I still suggest optoisolation just
because there are two different power supplies involved…
Naturally, I’m not assuming any responsibility if your USRP gets goofed

---------------------------- Original Message

Subject: Re: [Discuss-gnuradio] I/O lines standards
From: “Matt E.” [email protected]
Date: Fri, March 20, 2009 15:36
To: “Don L.” [email protected]

5 each

Don L. wrote:

to the Cyclone FPGA. If that’s the case, are they set to LVCMOS or

Dr. Don L. AJ7LL
Six Mile Systems LLP
17850 Six Mile Road
POB 134
Huson, MT, 59846
VOX 406-626-4304

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