onehorse
Well-known member
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.
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.