Hey guys, I have a 3.3v based microcontroller that I am wanting to use to communicate with SDI-12 sensors. For reference, SDI-12 sensors are 3-wires (12VDC for power, ground, and then a data line that is 5V). 5V is logic 0. 0V is logic 1.
Here is an example from the SDI-12 specifications:
http://www.sdi-12.org/current_specification/SDI-12_version-1_4-Jan-30-2021.pdf


Am I on the right track with using a circuit like this? This uses a p-channel mosfet, and an n-channel mosfet.


I'm guessing it might be better to use 2x mosfets on the top portion so my logic doesn't get inverted? What about for the lower portion?

Thanks and any help or advice is greatly appreciated!

 

Maybe consider this: sn74lv1t04.pdf Available to ship at Mouser: 9745933

 

I have just been through this... I was running SPI from 3v to 5v to a max7291.. BUT!! the mosfet style logic level convertor was not fast enough and the 5v was never being reached... So (as Yaakov just said) I bought a couple of 74hc244's... Perfik!!!

Edit*** The mosfet convertors were running common gate..

 

Maybe consider this: sn74lv1t04.pdf Available to ship at Mouser: 9745933

I have just been through this... I was running SPI from 3v to 5v to a max7291.. BUT!! the mosfet style logic level convertor was not fast enough and the 5v was never being reached... So (as Yaakov just said) I bought a couple of 74hc244's... Perfik!!!

Edit*** The mosfet convertors were running common gate..

How would I use these components in this situation? I basically have one GPIO that I am using to both receive and transmit SDI data.

 

Would a circuit like this be more preferable? This is more of a standard 3.3V to 5.0V level shifting circuit, I just didn't know how well it would play with the 500 ohm, 200K pulldown, etc.? p.s. that is a N-channel mosfet:


It seems like that 10K pullup resistor to 5.0V kind of messes this up... because when the GPIO is set to input to read, and the SDI-12 needs to drive the data now, ideally that 10K would not be there for this part.

 

So I came across this in the SDI-12 specifications for an example circuit:

I updated my level shifter like this. Will that 10K resistor matter? I also removed the last 510 resistor because it was causing issues with the mosfet:

 

The actual mosfet circuit is this....


But I didn't realize the speed was an issue.. I was running at a pulse of 1us ...

 

The actual mosfet circuit is this....
View attachment 251415

But I didn't realize the speed was an issue.. I was running at a pulse of 1us ...

This is for SDI-12 though - baud rate of 1200.

 

That should be fine.. I was running full SPI.. I used a 74hc244 and it levels up perfectly.

 

So I came across this in the SDI-12 specifications for an example circuit:
View attachment 251392
I updated my level shifter like this. Will that 10K resistor matter? I also removed the last 510 resistor because it was causing issues with the mosfet:
View attachment 251391

Did any of this work? I’m trying to do the same thing having a issues figuring out how to handle 3.6 coming back from teros 12

 

Maybe consider this: sn74lv1t04.pdf Available to ship at Mouser: 9745933

Hi,

Doesnt the data line have to be bidirectional?
Maybe not for all apps though.

 

Hi,

Doesnt the data line have to be bidirectional?
Maybe not for all apps though.

Not on SPI you have MISO and MOSI as a data ring on the SPI bus..

 

Not on SPI you have MISO and MOSI as a data ring on the SPI bus..

Looking at this drawing i see what looks like a bidirectional buss on the left.

 

Whoooooaaa!!! Like everyone else I read SPI.... SDI is 1 wire.... I am sooo sorry MrAl ( I did wonder why you asked that question..)

Shutting up now...

 

This application note provides a simple bi-directional level shifter based on an N-Channel MOSFET.

 

Whoooooaaa!!! Like everyone else I read SPI.... SDI is 1 wire.... I am sooo sorry MrAl ( I did wonder why you asked that question..)

Shutting up now...

Don't feel too bad, I made the same mistake in the other SDI thread where the title actually said "SPI - 12" until one of the mods corrected it. the diagram from the spec clearly shows how to use a tristate buffer and a receiver (could be a tristate buffer with the output always enabled) to make a bidirectional data line.

 

Don't feel too bad, I made the same mistake in the other SDI thread where the title actually said "SPI - 12" until one of the mods corrected it. the diagram from the spec clearly shows how to use a tristate buffer and a receiver (could be a tristate buffer with the output always enabled) to make a bidirectional data line.

It doesn't help that the SPI system names the pin SDI ... That is my excuse LOL..

 

Whoooooaaa!!! Like everyone else I read SPI.... SDI is 1 wire.... I am sooo sorry MrAl ( I did wonder why you asked that question..)

Shutting up now...

Hi,

Hey no problem my friend and it's always nice to talk about this stuff anyway as it brings other ideas out too so thanks just the same

 

This application note provides a simple bi-directional level shifter based on an N-Channel MOSFET.

Hi,

Oh you just reminded me of some boards i purchased long time back now when i was more heavily into microcontrollers (still love them though).

The board are sold just for the purpose of converting voltage levels usually 3.3v to 5v or vice versa. They have the mosfets right on them and some resistors.
The only problem i saw with any of these solutions is you have to have a reliable clamp voltage so that the circuit has a way to limit the 3.3v signals.
I'll see what i can dig up. The nice thing is everything is on that one tiny board, and often has several channels worth so you can do several pins with just one board. They are cheap too. I should have some notes around if no one else looks them up first.

As to the clamp voltage (usually 3.3v or something) you have to make sure that any and all currents that can possibly come through the clamp can never raise the voltage of the lower voltage (like 3.3v) or it can overpower the chip running under 3.3v power. If the clamp voltage current is too high it will raise the Vcc level of the 3.3v circuit and thus could blow out any chips being used on 3.3v, including the microprocessor/controller (such as the more expensive type like the ARM).

Here is just one such board:
BOB-12009 SparkFun | Mouser

They are available on Amazon also. There may be single channel, dual channel, etc., boards.
In the drawing you can see there are several channels with mosfets and resistors.

 

Yes, I have some of these. They work well.