Electronics > Projects, Designs, and Technical Stuff

74LVX245 -- what the heck?!

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SiliconWizard:

--- Quote from: Buriedcode on May 12, 2020, 03:47:03 pm ---Datasheets have indeed been vague about this, with different manufacturers leaving out the caveat "5v - tolerant inputs are /OE and DIR" - that are the only true inputs of the device.

--- End quote ---

Yeah - I understand the why, but I still think the datasheet is confusing, and I personally consider this design to be a fucked-up design. Very limited use if only /OE and DIR are really 5V-tolerant.

Pinkus:

--- Quote from: ebastler on May 12, 2020, 10:42:14 am ---
--- Quote from: Pinkus on May 12, 2020, 08:24:36 am ---I recently used a 74AC16245 (16 channel, there should be also a 8 channel version) for bidirectional 3V <> 5V conversion. No problems.

--- End quote ---

Hmm, I wasn't aware that the AC series could be used for voltage translation, and the datasheet seems to state that inputs shouldn't be driven above Vcc?

Do you mean the SN74LVCH16T245 from TI by any chance? They have dual supply voltages, and can hence do "proper" level translation -- including the up-conversion from 3.3V outputs to 5V. I was wondering whether I should use those for level-shifting my outputs: At the moment I am using the FPGA's 3.3V outputs directly, which is fine when interfacing with old TTL logic, but marginal when interfacing with HC and other families with a 2.5V threshold.

--- End quote ---
I checked the PCB: you are right, it is a 74ALVC164245DGG (I shall update this in the schematics) and yes, it has dual power supply. One note: do not let the direction pins float. You need a definate direction which you do not have until the mikrocontroller was programmed. Use pull up or pull down resistors for them, otherwise the level shifter might be destroyed soon after the first power up.

 

ebastler:

--- Quote from: Pinkus on May 12, 2020, 04:40:10 pm ---One note: do not let the direction pins float. You need a definate direction which you do not have until the mikrocontroller was programmed. Use pull up or pull down resistors for them, otherwise the level shifter might be destroyed soon after the first power up.

--- End quote ---

Good point, thank you. In my case I'm driving the bus drivers from an FPGA, which I have set up to have pullups on its I/Os already while its configuration is being loaded from flash. I hope that will take care of things.

Buriedcode:

--- Quote from: SiliconWizard on May 12, 2020, 04:08:40 pm ---
--- Quote from: Buriedcode on May 12, 2020, 03:47:03 pm ---Datasheets have indeed been vague about this, with different manufacturers leaving out the caveat "5v - tolerant inputs are /OE and DIR" - that are the only true inputs of the device.

--- End quote ---

Yeah - I understand the why, but I still think the datasheet is confusing, and I personally consider this design to be a fucked-up design. Very limited use if only /OE and DIR are really 5V-tolerant.

--- End quote ---

Well, as far as I am aware, the 245 is the only bidirecitonal device in the VHC (or LVX) family, so as the input/output design (5v tolerant on input) is the same for all members of the family, I don't know why they would redesign it just for one device.  Over voltage tolerance isn't the only reason to use the 245 in the LVX family, so it's not like it's useless.

The problem is the datasheets are pretty much the same for all members of the logic family, so I assume they just used the same specs for the family, but added the caveat for this one device - and some manufacturers appear to have left this out.  I spotted it in quite a few datasheets many years ago when designing mixed level systems (fairchild, TI, ON semi) but in recent years, the exception seems to have either been left out or... implicit in the "Recommended Operating Conditions" Input voltage - simply leaving out "Vin" leaving only the T/R and /OE.    The OP is correct, in that it is indeed a gotcha.

Note I only spotted this on the VHC and LVX families, not the LVC.  I haven't seen any indications there is a problem in the AHC family either, but I don't know if thats because by design, or simply poor documentation.

ebastler:

--- Quote from: Buriedcode on May 12, 2020, 07:22:42 pm ---The OP is correct, in that it is indeed a gotcha.

--- End quote ---
It certainly got me.  ;)


--- Quote ---Note I only spotted this on the VHC and LVX families, not the LVC.  I haven't seen any indications there is a problem in the AHC family either, but I don't know if thats because by design, or simply poor documentation.

--- End quote ---
The LVC documentation seems very explicit in confirming that also the bus I/Os of the '245 will be 5V tolerant. I'm banking on that, have parts on order...

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