custom Teensy build

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onehorse

onehorse

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I have a couple of questions about my custom Teensy 3.X design I hope someone who has designed a custom Teensy can help with:

The board design will use the USB for programming and, since there are LiPo-battery-driven motors, I am using a switching regulator to provide 3V3 to the Teensy (MK20DX256LH7 pins 3, 7, 30, 48) and other circuitry. I have separate 0.35 V Schottky diodes between VUSB and regulator VIN as well as VBAT and regulator VIN so I can use either or both for powering the circuit for programming/testing. In use, only the VBat will power the Teensy, other circuitry, and motors, etc. I am using a p FET for polarity protection on the VBat to VMotor connection. My question concerns the Teensy 3.1 schematic <http://www.pjrc.com/teensy/schematic.html> , where these two diodes (or equivalent) seem to be connected at both the input and the output. This would seem to defeat the isolation purpose. I would have though they would be connected only at the output (at VREGIN).Do I misunderstand the schematic here?

Also, I notice the crystals do not seem to have any load balancing capacitors in the schematic. I am using a 4 pF 32 kHz crystal so this might be OK but the Epson 16 MHz 8 pF crystal I intend to use is balanced with 12 pF caps when used in the nRF51822 circuit. Shouldn’t I have these caps in my custom Teensy 3.1 design?

Thanks in advance for any feedback.
 
So many things about this question make no sense to me. But let's start with the simplest. You talk of "Vbat". Maybe you're talking about power from your Lipo battery, or maybe the VBAT input that's located on Teensy 3.2 next to pin 12?
 
onehorse said:
My question concerns the Teensy 3.1 schematic <http://www.pjrc.com/teensy/schematic.html> , where these two diodes (or equivalent) seem to be connected at both the input and the output. This would seem to defeat the isolation purpose. I would have though they would be connected only at the output (at VREGIN).Do I misunderstand the schematic here?
Click to expand...

Those are reverse polarity diodes. Only one is needed but I suspect that since they came two in a package, Paul paralleled them together.

Now it does look like a better place for the shorting jumper is in between the two anodes of the diode. That would have provided true isolation between Vin and Vusb (when cut) and give you the option of powering the Teensy from either. I am sure Paul had his reasons for doing it the way he did.
 
Now it does look like a better place for the shorting jumper is in between the two anodes of the diode. That would have provided true isolation between Vin and Vusb (when cut) and give you the option of powering the Teensy from either.
Click to expand...

Yes, this is what I thought and what I plan to do, that is have separate diodes with no connection between anodes.

Maybe you're talking about power from your Lipo battery, or maybe the VBAT input that's located on Teensy 3.2 next to pin 12?
Click to expand...

Sorry to be unclear. Yes, I meant LiPo battery voltage, which I want to use to power the Teensy, associated circuitry, as well as the motors. However, during program debugging, I want to use the USB to power the Teensy and associated circuitry.

It seems like having separate Schottky diodes on each line from a power source to the voltage regulator input makes the most sense here.

Any help on the crystal load caps?
 
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