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High level USB traffic causes increased emission

Question asked by Phuc Le Dinh on Nov 16, 2015
Latest reply on Nov 16, 2015 by Phuc Le Dinh

Hi all,

 

I'm using a K21FN1M0 device for a USB cdc project. I've got the USB CDC PE component from MCUoneclipse source forge page: FSL_USB_STACK.

 

When first connected to a PC, the USB full-speed packets appear every one millisecond.

TEK00004.PNG

 

But as soon as the COM port is open, the rate of traffic increases massively. Packets are sent as frequently as every 3us and with varying length. However, the K21 sits in idle and only transmit a string every 5 seconds.

TEK00005.PNG

 

My questions are:

1 - what are this traffic? Are they standard USB traffic?

2 - If this traffic is not necessary, is there any software change to get reduce (or get rid of) this massive amount of traffic as it causes EMC test to fail in 7-10MHz region?

 

Any help is appreciate.

Kevin

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