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" <m...@ettus.com> Date: Fri, March 20, 2009 15:36 To: "Don Latham" <d...@montana.com> -------------------------------------------------------------------------- 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 Huson, MT, 59846 VOX 406-626-4304 www.lightningforensics.com www.sixmilesystems.com _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
