Taking the Kinetis L FRDM board to the next level, a real product

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Taking the Kinetis L FRDM board to the next level, a real product

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steve_fae
Senior Contributor I

Ok, customer got their first product working using the KL25 FRDM board with an attached daughter card.  They are using USB in their product.

1.  The FRDM board doesn't add much in they way of projection on the usb lines , just a few Schottky diodes.  We have noticed some noise coupling into the laptop that connects to the FRDM board when power is applied to the board other then USB power.  Any suggestions on what we should add for additional USB protection?

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2.  What should the power scheme be if 3.3v would be applied equivalent to P3V3 KL25 from a different power source, but if USB is attached, will it be able to enumerate?  Sort of new to designing a USB product.  We have already cut the path for the P5V KL25Z trying to address noise for test.  If we plan on adding the debug header, are we missing anything other then decoupling caps on power?

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Paul_Tian
NXP Employee
NXP Employee

For your first question. I would like to provide a standard USB protection circuit for your reference.

USB protection.png

For your second question, if you use external powre to P3V3, it will not effect USB power because there is a diode.

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steve_fae
Senior Contributor I

So if I don't want to power the device, only communicate for data, should I not connect Vbus?  Just want to connect what is basically needed for communication.  Don't plan on using SDA for debug, will be adding debug header for reprogramming.

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Paul_Tian
NXP Employee
NXP Employee

The USB transceiver power supply comes directly from VOUT33 (voltage regulator output). Therefore, the regulator must be enabled to supply 3.3 V to the transceiver. So our Vregin need no less than 3.3V input.

Then you can use two types connection. One is VBUS connect to Vregin. The other is MCU_VDD connect to Vregin. If you want to use another power source range from 2.7V to 5.5V for Vregin, I think it is OK.

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steve_fae
Senior Contributor I

Zhe, your last message is what I need to make sure I implement correctly.  If a PC attaches to our board, it will provide

5v usb to 4179_4179.png

that should drive a 3.3v regulator that connects to what on our board so that the micro enumerates as a CDC device, but that the 5v doesn't provide power to the rest of the board?

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Paul_Tian
NXP Employee
NXP Employee

Hi, Steven

As you can find, the USB bus power(P5V_KL25Z) support two parts. One is connected to 3.3V regulator and another one is to KL25 usb regulator in pin(Pin12). So this power source can both provide power to regulator and KL25 chip usb regulator in.

Hope my reply can help you.

Best Regards

Paul

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