[Fwd: Re: [Discuss-gnuradio] I/O lines standards]

[Don Latham]

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 lines
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 series
with the input diode. All 5 drew less than 2 ma current at 3.3 volts, and
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 to
use as output buffering and transition to 5v logic. Note that they will be
high (5 v) with the inputs at either 3-state or low. To use 5v logic with
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 2
1k resistors or even larger should work fine. With the H11L1 connected to
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 :-)
Don


---------------------------- Original Message ----------------------------
Subject: Re: [Discuss-gnuradio] I/O lines standards
From:    "Matt Ettus" <address@hidden>
Date:    Fri, March 20, 2009 15:36
To:      "Don Latham" <address@hidden>
--------------------------------------------------------------------------


5 each

Don Latham wrote:
> Sorry, Matt, one more Q? 5 ma/line or 5 ma total?
> Thanks v much for your time!
> Don
>
> Matt Ettus
>> Don Latham wrote:
>>> I'm just checking so's I don't destroy my USRP mboard. The I/O lines
>>> such
>>> as io_tx_13 etc. that are available on the flex 2400's seem to go
>>> straight
>>> to the Cyclone FPGA. If that's the case, are they set to LVCMOS or
>>> LVTTL?
>>> And, is the voltage used 3.3 V? I think I read the answer on the
>>> schematics, but I need to check!
>>> Thanks, All.
>>>
>>>
>>
>> They go straight to the FPGA.  They are 3.3V CMOS.  Don't pull more than
>> about 5mA.
>>
>> Matt
>>
>
>




-- 
Dr. Don Latham AJ7LL
Six Mile Systems LLP
17850 Six Mile Road
POB 134
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VOX 406-626-4304
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