Level Translator TTL Compatible? - Page 1
Forum section: electronics / projects

author: hal9001
author_rank: Regular Contributor
timestamp: June 09, 2021, 11:25:11 am
content: Does any one know if 74LVC1T45 level translator is TTL compatible?I would want to know if it works for CMOS and TTL logic but the data sheet doesnt say CMOS or TTL specifically. Is there a specific key word to look for in level translators to know this?Cheers

author: Kleinstein
author_rank: Super Contributor
timestamp: June 09, 2021, 11:35:13 am
content: The chip has the typical logic levels for CMOS logic. That is 0.3-0.7 * Vcc as the undefined range to avoid and output levels close to the supply.TLL is different in that the logic high level is lower. So they accept already some 2.5 V has high and the output may not reach 3 V for a high signal.So to work with a TTL level signal it would need a lower supply (e.g. some 3 V) for the input side. It looks like the chips is OK with a input signal higher than the supply.TTL inputs are OK to accepts CMOS output signal from some 2.7 to 5 V supply.

author: hal9001
author_rank: Regular Contributor
timestamp: June 10, 2021, 05:00:43 am
content: Quote from: Kleinstein on June 09, 2021, 11:35:13 amThanks @Kleinstein. Is it correct to say its fine to run this level translator at 3V for inputandoutput to a TTL device since TTL accepts 2.5V as high?

author: jonpaul
author_rank: Super Contributor
timestamp: June 10, 2021, 07:30:02 am
content: This discussionis about voltage, but TTL is a current-sinking logic, a current must be sunk from the inputs to bring them to a logic ZERO.The driving stage sinks up to 1.6 mA from TTL inputs with V < 0.4 volts.TTL inputs often left floating for logical ONE , (not recommended)Thus the ZERO = current sink < 1.6 mA , V<0.4V ONE = floating, no current sink, voltage will be ~ Vcc.https://en.wikipedia.org/wiki/Transistor–transistor_logicEnjoy,Jon

author: Kleinstein
author_rank: Super Contributor
timestamp: June 10, 2021, 10:10:52 am
content: True TTL logic is not use very much. There are also CMOS chips that to accept the TLL logic levels. To get the current flow or not one still needs the correct votlage. Inputs can be wired in parallel, because the voltage matters. Usually the current / load is relatively low (excpet maybe for 74S...). The current may be limiting for multiple inputs in parallel, more for the low level. As low needs < some 0.7 V , while for high level the limit is >2 V. For practical use one should exceed these levels somewhat, to get fast and reliable operation. So getting < 0.4 V us a good idea.The slightly tricky direction is usually from TTL level towards a CMOS chip, as TTL ouputs are at some 3-3.5 V and 5 V CMOS may need at least 3.5 V for a reliable high signal.Often is works with TTL to 5 V CMOS, but not always and not reliable.

author: David Hess
author_rank: Super Contributor
timestamp: June 11, 2021, 02:58:17 am
content: TTL low is below 0.8 volts, and TTL high is above 2.0 volts. The high level is what matters in this case and the 74LVC1T45 meets the 2.0 volt requirement with a supply voltage of 3 to 3.6 volts. It would not work with a supply voltage of 4.5 to 5.5 volts unless the TTL output used a pull-up, which is also a completely valid solution.

author: hal9001
author_rank: Regular Contributor
timestamp: June 11, 2021, 07:57:25 am
content: Quote from: David Hess on June 11, 2021, 02:58:17 amThats very helpful.Thanks!

author: bson
author_rank: Unknown
timestamp: June 12, 2021, 06:06:15 am
content: LVC in general is 5V input tolerant. However, 5V TTL may not be fast enough; most LVC logic has amaximumtransition time of 2.5ns. There are some exceptions however, mainly in buffers and latches since they often sit at the end of noisy buses. (LVC245 for example has a max transition time of 10ns.) If the max is exceeded you risk metastability and poor reliability. Even 10ns is pretty fast for vintage LS logic, which if memory serves is around 50ns. It might still work though, especially since it's the transition to 1.65V or so that matters, not the full 5V TTL swing. So, worth a try with buffers, but probably not really with regular discrete logic.

author: tooki
author_rank: Super Contributor
timestamp: June 12, 2021, 12:41:01 pm
content: Page 4 of TI’sLogic Guidecovers the input voltages and family compatibility nicely.And TI also has an entireguide on level translationthat covers the topic in great detail.