On Wed, 11 Mar 2015 17:21:09 +0100, you wrote: >Hi, > >The circuit should look like something like this : >
Which is OK if you want to run the input of a 5 volt TTL chip directly from the BBB. I'd advise against it simply because of paranoid engineering. > >? >You also mentioned that I need to monitor voltages : > >*"An analog multiplexer driving an op amp running from 1.8 volts" * > >You mean that I need to use the onboard ADC ? But I have a problem, I don't >know how to use such a system (mux + opamp). Could you explain me a little >bit more please ? (If this isn't BBB related (as in a project), then someone say so please.... Ok, look at testing an IC, any IC, any pin can be the following: 1) input 2) output 3) bidirectional 4) ground 5) supply. things not to do: 1) connect any pin on the processor to a source of voltage greater than the processor's VCC 2) connect as output two pins (processor to chip) together (if in opposite states, with active pullups, excessive current will flow, letalone pulling up a pin on the processor too high. 3) try to supply the IC through a processor pin. Also trying to ground the chip through the processor pin. Kinds of faults you may see in the IC under test (assuming vcc and gnd are connected elsewhere). 1) pin is open 2) pin is stuck at ground 3) pin is stuck at VCC 4) pin as output does not go to logic 1 (bad level) 5) pin as output does not go to logic 0 (bad level). Try the following: Running the processor to an open collector inverter allows the pin to be grounded without harm, all that you are doing is grounding the pullup resistor. Using the level converter allows you to see a logic 1 or a logic 0 at the chip pin. However, sections 4 and 5 do not get covered. What you'd like to do is measure the voltage at any pin with the chip's A/D converter. The following scheme is used a lot to do just that, with the operational amplifier protecting the far more expensive processor. The first thing is to realize that you want to measure more than one pin. You could have a circuit for each pin, but that would limit you to the 8 A/D channels. You can use an analog multiplexer, it works like a digital multiplexer (8 inputs selectable to 1 output), except that analog voltages go through, rather than digital levels. Google "analog multiplexer". This will run at the voltage levels of the IC (chip you're testing). So for a single chip, you'd have 3 lines (binary selection) picking which input gets selected. This voltage appears at the output of the multiplexer. It's still (at the worst) 5 volts or 0 volts. Too much for the processor. You need a non-inverting opamp circuit (again, look it up) with a gain of 1. This gives you a bit of margin. 5.0 volts at the input of the opamp will give you 5 volts at the output. Run this through a resistive divider with the ratio of 1.5/5.0. This gives you some safety margin. In this case, it's ok to use a resistive divider since you're not dealing with digital levels and it's one way only. This goes to the A/D input. (chip is safe). Run the opamp from the 5 volt supply going to the chip. (the opamp works as a buffer and minimizes the load on the circuit) Draw a block diagram, look at the voltages, then see what goes inside each block. You'll want one multiplexer input for each pin on the chip. This is *if* you want to check the voltage levels on the chip. If not, then what you've written (with an output driver I'd put in) would be the reasonable thing to do. Checking the voltage levels on the chip can help identify the chip, and can also determine if you've got bad pins on the chip (that still read 1 or 0, but not always a good 1 or 0). More complicated than you were asking, but this is a direction the design can go. To simply take the design to the point where the chip can be directly connected to the processor, use 3.3 volt capable logic. (back to the simplest design). Harvey > >Thanks > >2015-03-11 14:24 GMT+01:00 Harvey White <[email protected]>: > >> 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 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. 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