How to use config tool to set endpoint max packet size over 64

[AmberLin]

Hi community,

I made LPC55S28-EVK as an USB HID generic device. When I tried to modify endpoint max packet size as 1024 byte, there is an error that the max packet size cannot over 64 byte. If this error exists, config tool cannot generate any USB related file.

Is there any method to set endpoint max packet size over 64 with config tool?

Config tool is very useful for me. Although I can modify usb_device_descriptor.c and usb_device_descriptor.h manually to fulfill my purpose, this method is inconvenient when I need to add new function by config tool next time.

By the way, I had checked that LPC55xx is able to transfer 1024 byte, and I also remember to modify report count in HID report descriptor.

Best regards,

Amber

[Alice_Yang · Accepted solution]

Hello @AmberLin 

Answer as below from Config tool team:

The limitation is implemented due to USB Specification 2.0 that states in the "5.7.3 Interrupt Transfer Packet Size Constraints" chapter:

All high-speed device default interface settings must not include any interrupt endpoints with a data payload
size (specified via wMaxPacketSize in the endpoint descriptor) greater than 64 bytes. Alternate interface
settings may specify larger data payload sizes for interrupt endpoints. If the interrupt endpoints have a large
data payload size, it is recommended that additional configurations or alternate interface settings be used to
specify a range of data payload sizes. This increases the chances that the device can be used successfully in
combination with other USB devices.

Hope it helps,

BR

Alice

[AmberLin]

Hi, Alice

Thank you for your replay!

I will add additional alternate in my code to comply with the interrupt transfer limitation.

Best regards,

Amber Lin