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TM4C123 doesn't start

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Hi guys,


I have a simple problem (I hope) : I have designed a custom board with a TM4C123, and I implemented a simple USB port with a linear regulation on 5V to generate 3.3V for the MCU. As there is another source of power on this board for 3.3V, I added a schottky diode to prevent short circuits between supplies. 


My problem is when I power my board with USB, It doesn't start, but with the main supply it's okay. I checked voltages : from main = ~3,3V and from USB = ~3V  So I guess that 3V is not enought.


What can I do against that ? I tried unsuccessfully to disable BOR but I'm not sure at all of what I tried (not a lot of information on the web). My IDE is Energia but as it's not a problem directly link to it, I posted here.



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Chapter 24.3 of the datasheet specifies 3.15 to 3.63 V for recommended operating conditions.



Also see chapter 24.6.2:

The POK (Power-OK) monitor is used to keep the digital circuitry in reset until the VDD power supply is at an acceptable operational level. The digital Power-On Reset (Digital POR) is only released when the Power-On Reset has deasserted and all of the Power-OK monitors for each of the supplies indicate that power levels are in operational ranges. The BOR0 (Brown-out reset) and the BOR1 monitors are used to generate a reset to the device or assert an interrupt if the VDD supply drops below its operational range. The BOR1 monitor's threshold is in between the BOR0 and POK thresholds.

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Thanks for your answer.


I think you're right, even if table 24-10 seems to show that I'm ok versus power-ok. I cannot add any other component as my pcb was design like this.


I will try without diode for tests and if it's that issue I will try with another model of diode

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  • 3 weeks later...

Ok so I tried without diode and it's okay. I did another test with a PTC fuse instead of the diode, it does not work too. So for now I directly connected the 3V3 from USB to the main 3V3 power but now I know that I should choice an ajustable regulator to compensate the voltage drop ;)

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You can try to compensate the diode voltage drop by putting another (series) diode (same type as the protection one) between the GND pin of the 3.3 V LDO and the GND of your circuit board.

In this way you will raise the LDO's reference with the same amount you loose on the protection diode.

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