Bulk and Interrupt IN Transaction Sequence

[ryan_g]

I'm developing on the nrf52840PDK and I was running the cdc_acm exmaple in the SDK to observe the token exchanges between the host and the device.

In the Nordic52840 Product Specs v1.0 under section: 6.35.10.1 Bulk and Interrupt IN transaction it says "The host issues IN tokens to receive bulk/interrupt data."

When I run WireShark on the example, the order of token exchange is as follows

1. Host->Device: Out Token with character data

2. Device->Host: Out Token Ack Response

3. Device->Host: IN Token with character data

4. Host->Device: IN Token Ack response

The IN transaction was started by the Device and not the Host. I'm not sure if I'm misinterpreting the documentation on this or if WireShark USB sniffing isn't picking up the initial IN transaction between the Host and the Device.

Thank you.

[Håkon Alseth]

Hi,

 

I haven't used wireshark for USB sniffing, so I cannot comment much on what it shows and how "deep" it goes given that it needs to wrap around the OS in order to sniff the packets.

I use a dedicated usb sniffer (ellisys usb explorer).

 

If using "interrupt" transfer, your host will poll the usb device based on the bInterval set in the descriptor (see this site for details, search for bInterval).

If using BULK type transfer (which the data transfer for CDC is), then your device is polled based on the USB Hosts availability.

Here's a typical IN transfer (IN, DATA0 returned, ACK) where the device sends data to the USB Host:

This is also covered in the packet types of USB: https://www.beyondlogic.org/usbnutshell/usb3.shtml#USBPacketTypes 

Kind regards,

Håkon

[ryan_g]

Thank you Håkon, this was helpful and clears up my confusion surrounding the documentation.