Today I want look at analysing what exactly is happening on the USB bus when we send and receive messages, to make sure that what I think is happening is actually happening. On a Windows or Linux machine this can be done with Wireshark. I think it may also be possible on macOS since the High Sierra release. However, we're working with a USB host controller chip, so Wireshark is a no-go.

There are professional-grade USB analysers from Total Phase that start at around $475 (for USB 2.0 full-speed 12MBps) all the way up to $6000 (for USB 3.0 5Gbps). While we should be able to use the 12Mbps analyser, I wanted to see if it's possible to use a little $10 logic analyser to analyse USB traffic:

That was almost ten months go, and since I'm still struggling to figure out why I'm not getting any results back from the meter, I decided to give it a try. This 8-channel 24MHz logic analyser was only £13.44 (including P&P). It comes with a set of test hook clips, which I was able to connect to the USB port surprisingly easy:


The pins at the back of the USB connector has just enough space so that you can attach the hook clips. The USB connector pinout is +5V, D-, D+ and GND, so I had another look at the schematic on the Sparkfun website. It showed that D+ and D- are connected to resistors, while +5V is connected to an overcurrent protection device. Based on this I could identify which side was +5V and connected the hook clips. I connected D- to channel 0 on the logic analyser and D+ to channel 1, and GND to ground. I also connected D2 to CH2 to use as a trigger:


Now it was time to install the sigrok PulseView software. I chose "fx2lafw" as the driver for my analyser, and when it successfully connected, defined the three channels for D-, D+ and the trigger. I selected the maximum available frequency (24MHz) and 1G samples, which should allow for 42 seconds of sample time. I then added the USB signaling decoder with USB packet and USB request decoding, and specified the appropriate signals and full-speed signalling:


I also added digitalWrite(D2, 1) in my code where I wanted the trigger to occur. I started up the Espruino code and clicked on Run:


I was successfully sniffing USB packets between the TUSB3410 chip in the cable and my USB host controller chip! After verifying that most of the packets I was sending were actually appearing on the bus, I noticed that my bulk out transfers were missing:


Above you can hopefully make out the setup packet 40 05 00 00 03 00 0A 00, which should be followed by some data. Instead it is followed by the "close" packet 40 07 00 00 03 00 00 00. So maybe that is where I should start looking why things aren't working!