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ausserirdischegesund
12-18-2012, 08:33 PM
I want to connect my Teensy 3 to the serial port of a TP-Link MR-3020 router. This serial port gives me 3.3 V of power. Would that be enough for the teensy 3 if I do not need USB while the Teensy is connected to the router? Or should I try to tap 5V somewhere on the router (which is also possible, but mechaniclally more difficult.

TIA
Ralph

Jp3141
12-18-2012, 10:16 PM
I want to connect my Teensy 3 to the serial port of a TP-Link MR-3020 router. This serial port gives me 3.3 V of power. Would that be enough for the teensy 3 if I do not need USB while the Teensy is connected to the router? Or should I try to tap 5V somewhere on the router (which is also possible, but mechaniclally more difficult.

TIA
Ralph

The mk20dx128vlh5 MCU can be run from 1.7 to 3.6 V -- http://cache.freescale.com/files/32bit/doc/data_sheet/K20P64M50SF0.pdf?fpsp=1 p11. You can actually run it from 3.3 V on VIN (marked 3.7 to 5.5 V on the pinout). The schottky (0.3 V) diodes mean that the actual V the MCU sees could be about 0.3 V lower than this, so since the undervoltage reset is ~ 1.7 V, you can run down to about 2 V on the VIN pin -- depending on temperature and and additional loads you drive. Note that the LED brightness will change as VIN is reduced.

Be sure you have enough current available (~ 100 mA if running at full speed)

PaulStoffregen
12-19-2012, 12:17 AM
Yes, you can apply 3.3 volts directly to the +3.3 volt pin, to power Teensy 3.0.

Be careful to get the polarity correct, and make sure it really is 3.3 volts. If you accidentally connect the power backwards, or apply more than 3.8 volts, the board will be destroyed. A quick double check on the voltage and polarity is a good idea before connecting the wires.

ausserirdischegesund
12-20-2012, 08:16 PM
Thanks, Paul&JP3141! The Teensy 3 is truely wonderful in its versatility.

Derek
02-04-2015, 02:16 AM
Yes, you can apply 3.3 volts directly to the +3.3 volt pin, to power Teensy 3.0.

Be careful to get the polarity correct, and make sure it really is 3.3 volts. If you accidentally connect the power backwards, or apply more than 3.8 volts, the board will be destroyed. A quick double check on the voltage and polarity is a good idea before connecting the wires.

Paul,

The K20 Sub-Family Reference Manual (K20P64M72SF1RM Rev. 1.1, Dec 2012) states "The USB Voltage Regulator module is a LDO linear voltage regulator to provide 3.3V power from an input power supply varying from 2.7 V to 5.5 V."

Why do you mention 3.8v? Would a LiPoly battery rated 3.7V work? It provides 4.2V when fully charged...

It seems pictographer implies this is OK in the third post on this page: https://forum.pjrc.com/threads/25783-Teensy-3-1-Power-supply-clarity

Derek

Jp3141
02-04-2015, 03:13 AM
Paul is referring to the voltage you would connect to the VDD ('3.3 V') line in place of using the internal regulator. It's not particularly unambiguous in the Freescale document, but I believe it intends to say that the absolute maximum you can apply to the '3.3 V' line is 3.8 V (non operational - section "4.4 Voltage and current operating ratings"), while the maximum you can apply and operate the part is 3.6 V ("5.2.1 Voltage and current operating requirements"). The internal regulator regulates the USB supply down to 3.3 V.

A LiPo would be better connected to the USB regulator's input which will regulate it down to 3.3 V (or lower) as the battery discharges.