Hi, I have a working design where I am using 74lvc1g123 monostable multivibrators. I am using the outputs of a 3.3V microcontroller to drive this chip’s inputs with it using a 5V supply. I designed it reading the Nexperia/NXP datasheet, but I recently looked at the TI datasheet, and it has completely different input high voltage thresholds, with the Nexperia IC easily accepting 3.3V logic and the TI clearly not. Are they really different or is one data sheet wrong? Any help appreciated.
Hello erend,
Welcome to the DigiKey TechForum.
The data sheets you posted don’t appear to be for the same part. Similar but not same.
The Nexperia one shows to be a 74LVC1G123 while the IT one is SN74LVC1G32.
We have several different version of the 74LVC series from 3 different manufacturers as in the link https://www.digikey.com/en/products/filter/multivibrators/711?s=N4IgTCBcDaIOwBYAyA1AwiAugXyA and all of these voltages show supply of 1.65V to 5.5V.
@Irfan_Koric, Looks like the voltage thresholds are the same whether it’s the SN74LVC1G32 or the SN74LVC1G123.
Based on the differences between the input threshold voltages of Nexperia’s 74LVC1G123 vs. TI’s SN74LVC1G123, specifically when powered by 5V, it appears that Nexperia designed their part to be compatible with 5V TTL-level logic, which requires input voltages of 2V to be recognized as a logic High, whereas TI’s part only recognizes CMOS-level logic inputs, which requires a logic High to be at least 0.7 x Vcc, or 3.5V at a Vcc of 5V. It’s unfortunate that they both use the “LVC” series designation when they do not have identical behavior.
Is part of the reason you are choosing to power this part with 5V to level-shift the output to 5V CMOS logic?
Hi, So sorry I mistakenly posted 32, the parts I’m using are LVC1G123 one-shots. Yes I chose the part because my 9S08 uC is 3.3V, and I’m sending it’s SPI output to 2 of these one shots. Everything downstream is 5V, so I thought I was getting both one-shot functionality and up level shifting. 3.5V Vhi really shouldn’t work, though it is doing so I guess because that is worst case. I still wonder whether one of them has the wrong numbers, given most companies making the same technology logic usually are compatible. Thank you for the response!
Well, doing a random sampling of parts from Nexperia and TI, all of the TI parts I checked in the LVC series appear to not have TTL-compatible inputs (meaning Vin-hi must be >= Vcc x 0.7). For the Nexperia parts, most that I viewed also did not have TTL-compatible inputs, but a few did, including the 74LVC1G123, the 74LVC1G14, and the 74LVC1G17. This seems odd to me, and I will look into this a bit more.
In the mean time, maybe I’m ignoring something obvious, but for what reason would you want to use a one-shot on the output of a SPI bus rather than just level shifters?
Hi David, Wow, you found the Nexperia 123 actually does have TTL inputs, amazing! I guess both respective data sheets are correct. I also noticed that the TI part does not show Schmitt-Trigger on its CLR input whille Nexperia shows it on all inputs. Well in the future I will use Nexperia in this location. I am driving a chain of WS2128B smart LEDs which require a certain pulse widths for a 1 or 0 in it’s definition of serial data. I clock out the SPI and enable one of the two one-shots depending on the bit being 1 or 0. Thanks for your help!