On Tue, 10 Mar 2015 12:02:35 -0700, you wrote: >You'll have to take steps to scale the voltage down to the BBB's 3.3 V I/O >level. The best way to do this is with dedicated voltage-level translator ICs. >But depending on your application, you might be able to get away with >current-limiting resistors in series from the 74xx output to the GPIO (you'll >have to do some work to find the right value), and with the right 74xx >selection, you won't need to put anything between BBB outputs and 74xx inputs >(that will depend on what the 74xx Vhi threshold is).
It's reasonable to do that when you have a 5 volt system with a number of lines, and can't substitute the part (a 5 volt display, for instance). I would NOT use current limiting resistors, I don't thing that they solve the problem. As I remember it, and they could have changed things and this is not as much of a problem now, the way that the pin is internally constructed involves back biased diodes which connect to the power supply rails. These back biased diodes, if forward biased, make an SCR, which once turned on, stays on until the power is interrupted. The voltage causes these SCR's to become functional, then fire. The SCR then uses the internal power supply to provide enough energy to destroy the I/O structure in the chip. Current limiting resistors do not help, because it's not the current coming in that does the damage, it's the relative voltage level. This is called a "parasitic SCR" and has destroyed chips in the past. Whether or not it is a problem still, I do not know. You'd want to have chips (to connect the 5 volts to) that have 5 volt tolerant inputs. These processors do not. Better to use 3.3 volt logic everywhere possible, and level translate only when talking to a 5 volt system that cannot be redesigned. Talking, in this case, means signals going either direction. > >Or maybe you can use 3.3 V 74xx variants. This would be my preferred solution, yes. Harvey > >> On Mar 10, 2015, at 10:50 , [email protected] wrote: >> >> Hi, >> >> I come from the world of sotware engineering and I am a newcomer (nearly) to >> the world of electronic. I would like to know if it was possible to >> interface a 74LS02 to the BeagleBone Black. >> >> I plan to connect Vcc and Vss of the 74LS02 to the 5V Vdd output and Gnd of >> the Beagle and use the GPIO pins to command the logic inputs. My problem is >> that I don't know if I can plug the 74LS02 without components such as >> resistors, in order to limit the current flowing through components. I don't >> want to damage the board. >> >> The result of all the logic operations should be gathered by some GPIO pins >> configured as inputs. >> >> Is it possible to proceed that way ? >> >> Thanks in advance for your answers >> >> -- >> For more options, visit http://beagleboard.org/discuss >> --- >> You received this message because you are subscribed to the Google Groups >> "BeagleBoard" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected]. >> For more options, visit https://groups.google.com/d/optout. > > >-- >Rick Mann >[email protected] -- For more options, visit http://beagleboard.org/discuss --- You received this message because you are subscribed to the Google Groups "BeagleBoard" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
