About the Teensy 4.0 3.3V limit.

zacgarby

New member
The card which came with my teensy says "Teensy 4.0 signal pins are not 5V tolerant. Do not apply more than 3.3 volts to any pin, except VIN or VUSB".

I was wondering how strict this rule is, and what would happen if I were to apply 5V to one of the digital pins, for example. Would it cause irreversible damage?
 
The card which came with my teensy says "Teensy 4.0 signal pins are not 5V tolerant. Do not apply more than 3.3 volts to any pin, except VIN or VUSB".

I was wondering how strict this rule is, and what would happen if I were to apply 5V to one of the digital pins, for example. Would it cause irreversible damage?

I've damaged previous Teensys that weren't 5v tolerant (Teensy 3.0, Teensy 3.6) with accidental 5v inputs. Usually the pin becomes unusable, but the rest of the Teensy limps along. I don't remember if I fried the teensy so it didn't work any more.
 
I've damaged previous Teensys that weren't 5v tolerant (Teensy 3.0, Teensy 3.6) with accidental 5v inputs. Usually the pin becomes unusable, but the rest of the Teensy limps along. I don't remember if I fried the teensy so it didn't work any more.

Ah, well that kind of sucks. Thanks though
 
That's fair enough, I suppose they don't need to have voltage regulators in the circuit or whatever which is bound to save space.

If it's easy to explain, do you know of any fairly straightforward methods to "shift" the 5V signal down to 3.3V?
 
At 600 MHz the T 4 MCU runs at 1.25 volts and 3.3 times the 180 MHz spec freq of the T_3.6 in a smaller package with a bigger feature set inside. So indeed guarding/supporting I/O pins at 5.5V would not be as Teensy.

There are Forum notes on appropriate level shifters for the task as needed where some aren't fast enough for the required bus interface if not simple I/O. Not used any but those Forum notes and SparkFun or Adafruit offerings should get to a starting point.
 
That's fair enough, I suppose they don't need to have voltage regulators in the circuit or whatever which is bound to save space.

If it's easy to explain, do you know of any fairly straightforward methods to "shift" the 5V signal down to 3.3V?

It depends on how fast you need the conversion to. There are a bunch of level shifters that are made for i2c/spi/uart type conversions that will do the transfers in either direction. But they have limitations in terms of speed, etc. My favorite that I used back in the day when I used 5v i2c displays with the Teensy is made by pololu:

Adafruit and Sparkfun also make some variants:

Opto-couplers is one way to electrically isolate the Teensy from the sensors (an opto-coupler has a sealed package that has a LED and a light sensor in the package, so there is separation between the two events).
 
I've damaged previous Teensys that weren't 5v tolerant (Teensy 3.0, Teensy 3.6) with accidental 5v inputs. Usually the pin becomes unusable, but the rest of the Teensy limps along.

Teensy LC, 3.2, 3.5, 3.6 have special ESD protection circuits that do not look like a diode between the pin and 3.3V. This is sort of a mix blessing. It lets you more easily drive the pin higher than ~3.9V, but if you do, there isn't a conductive path to the 3.3V power line, so just 1 pin takes to take the abuse.

Teensy 4.0, and also Teensy 3.0, do have diodes as their ESD protection. Perhaps that protection is more robust? Difficult to say, since Freescale (now NXP) doesn't disclose much info about the ESD protection circuits. It's probably a closely guarded trade secret.

The main consequence of a diode-based ESD protection is driving the pin very hard (low impedance) can put a lot of current through the diode. Most modern chips don't have latch up problems, but that could happen. You could also pour in enough current to possibly power the entire chip, raising the 3.3V power up higher.

The other side effect of the diodes is the possibility of a phantom power effect if you drive the pin while the power is turned off. Defragster saw quite a lot of this during the beta test. It's not an issue we can solve in software. It's a matter that the pins do have diodes.

So the behavior of Teensy 4.0 when abused will be somewhat different than most of us are used to with Teensy 3.1 to 3.6 and LC. We haven't had a 3.3V Teensy with diode-based ESD protection since the original Teensy 3.0.
 
How about three parallel 10k pullups to +4.096 V?
That implies less than 0.25 ma into each of two I2C pins (probably much less due to the diode drop).

This is a Teensy 4.0, and I need to power two MPC4725 DACs and an ADS1115 ADC from a 4.096 V low-noise reference.
These are little AdaFruit boards, and each has a 10k pullup to VDD on the I2C signals.

It worked on my breadboard (I had mistakenly thought the Teensy pins were 5V tolerant).
We'll build a handful of these boxes, but we want them to last for years.

I suppose I could un-solder those pullup resistors from the ADC and DAC boards and install 10k to +3.3 V.
Or I could leave the pullups and install 12k to ground, making the pullup voltage be +3.2 V.
 
For reading an analog signal, I see analogPins are not 5.0v tolerant, but what really is the limit? 3.3000000000 or could 3.5 volts be applied to a pin for ~100ms?
 
I was wondering how strict this rule is, and what would happen if I were to apply 5V to one of the digital pins, for example. Would it cause irreversible damage?
Typically instant death of the pin and likely CMOS latchup to fry the whole chip. Those pins are definitely not 5V tolerant in any way.
 
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