AVR Freaks

Interfacing between devices with different logic levels

[ka7ehk] Recently, there was a discussion in the General Electronics forum about interfacing between an SDI-12 bus (required 5V logic) and a processor, typically running at 3.3V. The discussion was here, for anyone interested:
https://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=83056
Glitch finally came up with a solution, and I thought it would be useful to frame it more generally because this is a constant issue that comes up.

First, you should know about the 74AHCTxxx family. It is designed to run from 5V. But, the minimum input logic high level is 2.0V. This means that you can drive it from any logic, including AVR processors, with a supply voltage above about 2.2V, or so. The output will be full-swing 5V. This family is available from TI and several other sources. This appears to also be true for 74HCTxxx (or any of the 74-series logic that ends with "T" in the middle group of letters). What is important about this logic family is that it gives you a way to "step up" an output from a lower logic voltage to 5V

Secondly, you should know about the 74LVCxxx family. It is designed to work from a supply voltage in the range of 1.65V to 5.5V. When powered from the same supply as your processor, its inputs and outputs are directly compatible with the processor. What is really important, however, is that the inputs tolerate +5V input even if the chip is powered by a lower voltage, say 3.3V. This gives you a way of accepting 5V input signals and stepping them down to 3.3V or less.

As a general "philosophical" comment, I would like to point out that many of the logic functions of these two families are available in small-gatecount packages. Different manufacturers call them "1 Gate" or "TinyLogic" or some variation on these names. Most of them are available in SOT-23 packages which are relatively easy to hand-solder due to the wider pin spacing. Many of the basic functions are available for about $0.40US from DigiKey in single quantity. Many will ask why a single gate when you can get 6 or 8 or more in a single package with traditional packages. However, when you do circuit board layout, if logic is needed in widely scattered parts of the circuit, it can be a major challenge just to route the traces to the central chip. Then, it can be a real help to put the logic precisely where it is used.

Many thanks to Glitch who spent some time helping to sort this out.

[leon_heller] MOSFETs are useful for conversion, especially if there are only one or two signals.

[ka7ehk] Agree. You need, however, to watch in low power applications for an MOSFET output that has to idle in the low state. That means it is drawing power though its pull-up resistor. In this case, complementary inverter can save power.

[glitch] I'll just clarify a bit... The 'T' stands for TTL compatible (inputs). Thus the 'AHCT, and 'HCT parts offer CMOS outputs, with TTL compatible inputs. Which is where that 2V threshold comes from.

[DocJC] Illustrations of using the 74HCT series, as well as some examples of High Side Drivers and Load Side Drivers, and simple LED and switch interfaces, which have also come up recently, can be found in a ZBasic Application Note by Don K., AN-213 External Device Interfacing http://www.zbasic.net/appNotes/AN213.pdf
This builds further on what Jim very nicely describes above.
The ZBasic series is based upon AVRs.

[alwech]TI has just released a guide on level translation and has lots of usefull help. http://focus.ti.com/paramsearch/docs/parametricsearch.tsp?familyId=705&family=analog&uiTemplateId=NODE_STRY_PGE_T&CMP=KNC-GoogleTI&247SEM

[dl8dtl] Just for completeness (and though I know Maxim's got a reputation of
suffering from long lead times): Maxim has a number of level shifters
that can translate between a wide range of supply voltages, about 1.2
... 1.8 V minimum on the low-side, and 2.5 ... 5.5 V on the high side.
They are available in various combinations of high-to-low
vs. low-to-high channels. There are also bidirectional translators.
The low-side voltage can be anything between the respective minimum
and the Vcc of the high-side (so you can translate between 1.8 V and
3.3 V, 1.8 V and 5.0 V, or 1.8 V and 2.5 V, for example).

It's the MAX3372...3379 and MAX3390...3393 series. They are, for
example, also used in the JTAG ICE mkII.

[fmaimon] This thread: https://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=63957&start=0&postdays=0&postorder=asc&highlight=
In this thread, this post from AndersAnd has a very good info about mosfet level translators:
https://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&p=569068#569068
and a link to this AN from NXP:
http://www.standardics.nxp.com/support/documents/i2c/pdf/an97055.pdf

[crwper] It's probably overkill, but is there any reason not to use an honest-to-goodness bidirectional level translator like the ADG3304?
http://www.analog.com/static/imported-files/Data_Sheets/ADG3304.pdf
I figure this hasn't been suggested yet for two reasons: (1) cost; and (2) the OP has no need for bidirectional translation.

I'm using this part for a project which requires 1.8V to 3.3V bidirectional translation (I2C), and was surprised to see no mention of its ilk here. Just wanted to make sure I'm not completely out to lunch, as it's my first time using a level translator IC.

Discrete solution: https://www.avrfreaks.net/index.p...