On Wed, 11 Mar 2015 12:41:40 +0100, you wrote: >Hi, > >Thank you for you response. > >I have a lot of 74xx components which may be dead. As my first project with >the Beagle, I would like to simply test the logic behind these chips >(monitor the logic to detect wrong values). > >I first planed to wire the 74LS02 like this : > > >But you mentioned that an output high ranges from 0 to 5v which could >damage Beagle GPIOs (not 5v tolerant). > >If someone could give me more advises please ...
If you're going to build an IC checker, then you need to think of several things. 1) power and ground are in different places on several chip types (7490 vs 7476). There are typically 14, 16, 18, 20, and so on logic chips, look into ZIF sockets. 2) if you don't know the chip type, you can connect an output to a chip's output. In that case, you'd like to have a passive (resistor) pullup to the supply voltage, and an open collector output driving that, and perhaps with a series resistor to limit current, since you may be trying to pull an active output (that is high) low. Inputs won't matter. 3) your pin driver also wants to be able to read the pin as well, so the BBB needs the level translation back to the chip, the 8T245 would work well there. 4) each driver needs to be individually controllable for both level and direction. You need to be able to turn it off, set it high (10 k pullup is ok, or set it low. In any of the above cases, you need to be able to read the voltage at the pin. An analog multiplexer driving an op amp running from 1.8 volts (or resistive divider in this case to limit the a/d inputs to 1.8 volts max) would do well. That way you can also identify a good or bad output, or just plain read the voltages at the pins. more than you asked for, and more complicated, but the level shifting is something to think about. it would be a nice project. Harvey > >Thanks > >2015-03-11 11:01 GMT+01:00 Eric Fort <[email protected]>: > >> first off, to properly answer your question we need to start by >> referencing the datasheet for the part you wish to use found here: >> http://www.ti.com/lit/ds/symlink/sn74s02.pdf >> >> something missed by many is that 3.3v OUTPUTS on a device such as the >> beagle will drive standard 5v TTL logic just fine! just make sure to limit >> your fan out (number of logic loads) appropriately. The reason for this >> becomes clear if one actually reads the datasheet referenced above for the >> part you wish to use. If you look at the 74LS02 VIHmin (the minimum >> voltage which the input sees as high) it's listed as 2v. since 3v3 > 2v0 >> it works fine. the problem you have is that the output of the 74LS02 >> swings from 0-5v so when the output of the 74LS02 goes high it will measure >> 5v. since 5v > 3v3 it will cook a 3v3 INPUT. So depending what you want >> to do you may or may not need voltage translation andin most cases (i2c >> buses being the major place where you find an exception and do need >> bidirectional) they need only be unidirectional and one may use simpler >> circuitry such as a transistor or resistance ladder/voltage divider. So >> I'll end by posing a question. What doyou want to accomplish with this and >> can you post a basic schematic for review for us tolookat and >> constructively comment on? >> >> Eric >> >> On Tue, Mar 10, 2015 at 10:50 AM, <[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. >>> >> >> -- >> For more options, visit http://beagleboard.org/discuss >> --- >> You received this message because you are subscribed to a topic in the >> Google Groups "BeagleBoard" group. >> To unsubscribe from this topic, visit >> https://groups.google.com/d/topic/beagleboard/hTzOjUNwFZE/unsubscribe. >> To unsubscribe from this group and all its topics, send an email to >> [email protected]. >> For more options, visit https://groups.google.com/d/optout. >> -- 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.
