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Thread: Can teensy 3.5 board take 5V analog input?

  1. #1
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    Can teensy 3.5 board take 5V analog input?

    Hello, :)

    I was trying to find documents/info about analog input pins on the Teensy 3.5 board and I couln't find much info about analog input. Does the analog input pins take 5V input? If yes, would it show as 3.3 or 5V? Are there pins specifically used for 5V input?

  2. #2
    Senior Member PaulStoffregen's Avatar
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    Voltage from 0 to 3.3V will read 0 to 1023 using analogRead().

    Voltage from 3.3V to 5V will read as 1023, but does not damage the Teensy 3.5 pins.

    However, the analog-only pins (A10, A11, A25, A26) are not 5V tolerant. Applying more than 3.3V to those pins can damage the chip. The other analog pins, which can also be used as digital, are the ones which can withstand 5V.

    Other than VIN (to power the board), none of the pins are really meant for 5V. All the signals are meant to be 0 to 3.3V. The 5V tolerance merely means you can apply 5V safely to the digital inputs.

    Also be aware, touching 5V to the 3.3V power, even for a brief moment, will kill the whole board. Never apply 5V to the 3.3V power!

  3. #3
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    Quote Originally Posted by PaulStoffregen View Post
    Voltage from 0 to 3.3V will read 0 to 1023 using analogRead().

    Voltage from 3.3V to 5V will read as 1023, but does not damage the Teensy 3.5 pins.

    However, the analog-only pins (A10, A11, A25, A26) are not 5V tolerant. Applying more than 3.3V to those pins can damage the chip. The other analog pins, which can also be used as digital, are the ones which can withstand 5V.

    Other than VIN (to power the board), none of the pins are really meant for 5V. All the signals are meant to be 0 to 3.3V. The 5V tolerance merely means you can apply 5V safely to the digital inputs.

    Also be aware, touching 5V to the 3.3V power, even for a brief moment, will kill the whole board. Never apply 5V to the 3.3V power!
    Good morning!

    I wonder if applying a voltage above 3.3 V to an analog input has an undocumented [permanent] side effect.

    I’m using Teensy 3.5 in a couple of projects.

    One is an adaptation of Loftur Jonasson, TF3LJ / VE2LJX Power and SWR meter (https://sites.google.com/site/loftur...wr-meter---rev)

    The second is a QRP transceiver loosely based on QRP Labs QCX, with an UI ported from a project by UB7KPV (http://ub7kpv.blogspot.com/2017/01/s...2-tft-lcd.html. In Russian, sorry).

    I managed to “convince” the code of both projects to run on a T3.5. I use a 340x480-3.5 inch display with parallel interface (MCUFRIEND_kbv + Adafruit_GFX). Adapting the code to use a different display and hardware platform was not without pain; but otherwise all went fine until yesterday, when I started to observe a strange behavior in the analog readings.

    The issue appeared only in one of my prototypes, the QRP radio. All analog readings started to behave as if the analog reference voltage was 5 V, and not 3.3 V. When applying 3.3 V to any analog input, the reading was 676 instead of 1023 (10 bit resolution).

    To be sure that the issue was real and not an artifact, I ran the same test program in both prototypes, using the ADC library from Pedvide. Readings were (resolution set to 12 bit):

    For PWR+SWR Meter:
    Voltage in REF analog input (Fluke meter): 0.956 V. AnalogRead: 1183 == 0.953 V. Not bad!

    For QRP transceiver:
    Voltage in REF analog input (Fluke meter): 3.297 V. AnalogRead: 2732 == 2.201 V!!! (but 2732 would equal to 3.336 V if Reference voltage was 5V!).

    All started when -while testing the Tayloe mixer and its I / Q amplifiers- I applied more than 3.3V (and less than 5V) to analog inputs A8 and A20. From that point, all readings in the T3.5 installed in the QRP transceiver are scaled as if reference voltage was 5V.

    Did anybody else observed a similar behaviour?

  4. #4
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    Issue solved!.
    Shame on me. I managed to short 3.3V to VIN. So, the voltage applied to MK64F512 was 5V. But it was not fried, and continued working; for quite a long time, indeed. AREF voltage was 4.9V, so readings were referred [almost] to 5V. When I removed the short, readings returned to normal. And, luckily, there is apparent damage to the chip.

  5. #5
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    Quote Originally Posted by Angrodri View Post
    Issue solved!.
    Shame on me. I managed to short 3.3V to VIN. So, the voltage applied to MK64F512 was 5V. But it was not fried, and continued working; for quite a long time, indeed. AREF voltage was 4.9V, so readings were referred [almost] to 5V. When I removed the short, readings returned to normal. And, luckily, there is apparent damage to the chip.
    Should say... There is NO apparent damage to the chip!

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