3.3V to 2.8V TTL Logic Conversion

Hi there,

I'm powering an ESP-12 module with 2.8V but I want to program it with a CP2104 board that uses 3.3V logic. How can I make sure it's safe? I'm assuming that the "TX" pin on the ESP-12 doesn't need protection, but the "RX" on the ESP does (since it will be receiving 3.3V logic). I am aware of logic level shifters, but I'd like to keep this as simple as possible.

Taking a look at Adafruit's ESP8266 breakout board schematic (here) all they use is a single 1N4148 signal diode pointing away from the RX pin on the ESP-12 module. How does that work exactly? Is RX on the ESP8266 normally HIGH, then when the FTDI pulls it to LOW, the diode conducts and RX on the ESP also becomes low? Will this also work for 2.8V protecting against 3.3V logic?

Thanks!

This very inexpensive level shifter works for all popular voltage combinations.

jremington:
This very inexpensive level shifter works for all popular voltage combinations.

Inexpensive? $2.95 for that is highway robbery. On ebay you can get ten-packs for under $3 shipped (of comparable 4 channel level shifters)

:stuck_out_tongue:

But yeah - that's definitely the thing to use - though you could probably also get away with just a resistor in series (relying on the protection diodes to clamp the voltage on the low side).

androidfanboy:
I'm powering an ESP-12 module with 2.8V...

Any reason why.
The datasheet states a minimum VDD of 3volt (3.0-3.6volt).
Leo..

You don't need a diode,C1 and C2 and that regulator there but you will need to connect 2.8V where Vout of regulator is (pin 5).I think you might get away without R3 and R1 but I'm not sure.

@Wawa: The ESP8266 spec sheet actually says it can operate from 1.7V - 3.6V or something like that. The ESP was tested by various people and instability occurs below about 2.2V. Anything above 2.5V is deemed to be the safe zone. Anyway, please just accept that I'm using 2.8V (it draws a lot less in deep sleep mode).

So I got a NodeMCU board and measured the RX pin on it, and it's normally high. Does this mean that I can just use a single 1N4148 diode like the Adafruit HUZZAH breakout board does? (I liked the schematic in my question post). I am aware of that Sparekfun logic level converter, but it has more channels than I need, plus you have to hook up VCC for both low and high voltages. I want to know if i can just use a single signal diode pointing away from the RX pin of the ESP. Thanks!

Diodes are unstable under load and I wouldn't reccomend them.Schematics that I gave you is good and you can make as many shifters as you need,or you can make just one for RX of ESP,that will be enough.If DIY is not your thing you can always buy pre-made one exactly the same for like $3 for 10 of them

What kind of "load" are you referring to? Isn't it just logic high/low with maybe a teeny tiny bit of current? (Which is why most circuits for CP2104 include current-limiting resistors on the TX/RX pins to make sure it's safe for the MCU). If I just stick a 1N4148 wouldn't it just not conduct nomrally (since RX seems to be pulled high on the ESP by default), then when the CP2104 TX line goes low, the RX on the ESP also is pulled low because now the 1N4148 conducts?

It might work,but my expiriences with diodes are not that great.

Check this out.

But I would avoid voltage deviders with resistors.

@Nikola19992: That last example is exactly what I was talking about, using just a 1N4148 pointing away from the RX of the ESP. How exactly does it work though? The explanation says it "lowers the voltage of the signal". By how muhc? They're using it from 5V to 3.3V, but what about from 3.3V to 2.8V?

As a side note, it says the ESP has built-in RX protection. What's the extent of that?

"Lowering voltage" depends on load.But it should be in datasheet

Here's the 1N4148. I'm assuming it lowers the voltage by the forward voltage, which is about 1V. This means if I am using it for 3.3V to 2.8V it will chop the 3.3V down to 2.3V, which still should be good enough, right?

Yes,at 10mA of current "voltage drop" will be MAX 1V

Page 2, Fig. 2 - Forward Current vs. Forward Voltage for more details.
Yes 2.3V will be good for 2.8V device.But what if your load will be less than 10mA?

Well from that graph it looks like the minimum Vf is 0.5V (when it draws basically no current), so that's actually perfect since 3.3V - 0.5V = 2.8V

Yes that should be perfect for you.Good luck with your project

Thanks!

If you need any other help you can PM me

Awesome, thanks!