Teensy 3.2 RTC crystal replacement

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Hello,

I was wondering how I can connect a high precision 32768Hz clock signal (5Vpp) to the XTAL32 or EXTAL32 inputs of the teensy board instead of using a crystal. How can I do this? Are the XTAL32/EXTAL32 pins 5V tolerant?

Best regards
 
No, the XTAL32 and EXTAL32 pins are NOT 5 volt tolerant. Only the pins on Teensy 3.2 & 3.5 with digital I/O capability are 5V tolerant.

To use an external clock signal, you must scale it to 0 to 3.3V. If you drive the pin below -0.3V or more than 0.3V above the 3.3V power supply, you will risk damage to the chip.
 
Hi Paul,

thank you for the reply. I will use a level shifter then.
But how can I connect the signal to the chip? Do I use EXTAL32 and leave XTAL32 open? Or ground it?

Best regards
 
First of all, let's be more precise about the signal names before you start connecting wires.

The chip has four signals:

EXTAL0/PTA18 - pin 32 on MK20DX256
XTAL0/PTA19 - pin 33 on MK20DX256
XTAL32 - pin 19 on MK20DX256
EXTAL32 - pin 20 on MK20DX256

EXTAL0 & XTAL0 are for the normal 16 MHz crystal.

To use an external 32768 Hz clock, you would connect to EXTAL32 (pin 20). The XTAL32 pin should be left unconnected.

This pin on Teensy 3.2 is EXTAL32:

teensy32_extal32.jpg

Of course, the ground of your signal source must also connect to Teensy's ground, as you would do for connecting any type of signal to any pin.
 
Specifically asking about the MKL26Z64 (Teensy LC processor):
Does the crystal require a high value resistor and two small value caps to ground? And is the microcontroller sensitive to crystal ESR? I have not been able to find any documentation related to these questions. I'm sure I've just been looking in the wrong place...
 
Specifically asking about the MKL26Z64 (Teensy LC processor):
Does the crystal require a high value resistor and two small value caps to ground? And is the microcontroller sensitive to crystal ESR? I have not been able to find any documentation related to these questions. I'm sure I've just been looking in the wrong place...

Nevermind, I found it in the datasheet. I can't imagine how I missed it the first time I went through it.
 
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