LPC55S69 USB questions

[manuela_mendez]

Greetings!

For a medical device project, we are using LPC55S69, and I was unable to find info about the questions below from the user manual or hardware design manual, so I would appreciate your help on this:

1. Do USB0 and USB1 PHY already include series termination resistor (90 ohm requirement) as well as pull-up/pull-down resistors for electrical speed signaling or detection?

2. If a external crystal of 24 Mhz is used as clock source for the MCU, then will USB0 and USB1 use it to derive their clocks as well? IOW, crystalless solution is no longer relevant, correct?

3. If USB1 is being used as a host controller, is there a way to force it to operate at FS/LS controller only; IOW, the host controller should reject any HS device connecting to it

Thanks in advance!
Manuela

[Pavel_Hernandez · Accepted solution]

Hello, I have more details for your questions.

The first question is related to the USB specification for having 90ohms differential impedance. We use 33ohm resistors in series because the impedance includes the impedance of the USB transceiver. The impedance of the USB transceiver is lower than the requirement of the USB specification, so the 33 Ω resistors are required to bring the device impedance within the required specification limits. Please check the schematic of the EVK as a reference. 

2. Crystal is required for host operation on full speed USB and host/device operation on the high-speed USB. Proper clock configuration is needed to ensure you are selecting the correct clock source and the derivatives. MCUXpresso Clock Config tool can be used for this to make it easier.

3. For host operation, due to IP specs constraints, there's no settings to limit the USB1 to full speed when both ends are HS capable. I'd recommend using USB0 if applicable. Other possible solutions may include adding a full-speed USB hub IC to USB1, effectively limiting the speed to full-speed. For device operation on USB1, there is a FORCE FS field to operate as a full-speed device. 

Best regards,
Pavel

[manuela_mendez]

Hello,

Per your guidance, I checked the schematics of S69-EVK, and 33 ohm series resistors are included only for USB0 but not for USB1. Could you clarify why?

Thank you!

[manuela_mendez]

Hi Pavel, Thanks for the prompt response. Have a great weekend!

[Pavel_Hernandez]

Hello, I reviewed your question, and I did a check on the reference manual and the datasheet, I could not find enough information, I will ask to the internal engineer.

Best regards,
Pavel

[Pavel_Hernandez]

Hello, my name is Pavel and I will be supporting your case, let me get more information about USB, when I have more details I will contact you.

Best regards,
Pavel