Some confusions please help: LUFA code vs. atmel spec sheets

[liudr]

It's been a while since I wanted to dive into LUFA code to understand how USB controllers work on 32u4 so I've finally got enough time to understand the code, which is really well written. I mean coming from Arduino USB code and Atmel's own demo code, I can see with the docs that come with LUFA you can actually understand what it's doing.

I'm writing my own USB device stack because of a special use case of 32u4. So my goal is to understand how it works. Been hacking the arduino USB code for some years but the way it's written is just a mess.

References I have:

• 32u4 full spec sheet with 2 chapters on USB controller. I also compared with at90USB1286, seems very similar albeit at90 is OTG capable.
• LUFA code
• Arduino USBCore.cpp etc.

Tools:

• Beagle USB protocol analyzer
• My own USB Host shield based protocol injector so it would do any requests I want.

I found some inconsistencies between the code I read and the spec sheet I read. I am hoping the LUFA folks will help me get over the difficulties because strictly following the spec sheets got my project derailed LOL

Confusions (two exactly):

1. The process of set_address. In LUFA, it is inside DeviceStandardReq.c, the function USB_Device_SetAddress() does the following (my summary):

• Read new address
• Call USB_Device_SetDeviceAddress(), which simultaneously sets ADDEN and the address. Isn't this NOT ALLOWED in spec sheet?
• Call Endpoint_ClearSETUP () to clear RXSTPI
• Call Endpoint_ClearStatusStage(), which does status stage depending on request direction determined by USB_ControlRequest global struct
• Call USB_Device_EnableDeviceAddress() to set ADDEN bit
• Set USBDeviceState to Device_STATE_Addressed if the address isn't 0 else to DEVICE_STATE_Default

lufa/LUFA/Drivers/USB/Core/DeviceStandardReq.c

Line 125 in 8a78554

static void USB_Device_SetAddress(void)

Except for the last bullet point, everything else was hardware related. According to 32u4 spec sheet, one shall NOT set ADDEN and address at the same time but LUFA did it anyway. I've seen atmel and arduino code do the same. So isn't this bad idea? The status stage (ZLP IN) should carry default address of 0 instead of new address.

2. A related confusion was inside Endpoint_ClearStatusStage(), which is in Endpoint_AVR8.c.
   The last step of status stage is either release ZLP IN or receive and clear ZLP OUT. This function does this depending on the direction of the request. BUT, the Endpoint_ClearIN() and Endpoint_ClearOUT() both clear FIFOCON besides TXINI/RXOUTI. The spec sheet of 32u4 specifically says not to mess with FIFOCON for control endpoints. So why messing with it?

lufa/LUFA/Drivers/USB/Core/AVR8/Endpoint_AVR8.c

Line 129 in 8a78554

void Endpoint_ClearStatusStage(void)

Related inline declarations:

lufa/LUFA/Drivers/USB/Core/AVR8/Endpoint_AVR8.h

Line 453 in 8a78554

static inline void Endpoint_ClearIN(void)

Unless you define CONTROL_ONLY_DEVICE, which is considered "rare" in LUFA beginner's guide, you're clearing both FIFOCON and respective interrupt flag.

[abcminiuser]

Sorry for the very late response!

Given it's been well over a decade since I wrote the code, I can't honestly say for certain why it is the way it is; it's possible that I just used the same optimizations from the Atmel demo code when debugging my own, misread the datasheet, or just did empirical testing and found those shortcuts to be "stable".

As an older, more seasoned and jaded firmware engineer, I'd strongly suggest following the datasheet's recommendations, as they are almost always important for reasons such as rare timing issues within the peripheral if certain constraints aren't respected.

[liudr]

@abcminiuser
Thanks for responding! I totally get it. I can't remember what I did a month ago on an ongoing project. It's a struggle to work on larger projects and remember details if you put it down even for a few weeks. I will definitely follow the spec sheets to see if it would work or not, again. I wrote my own code "according to spec sheet" from ground up, just a few functions, but that didn't work. I'll start with working code from arduino USB and apply spec sheet to it this time. Currently working on something else so it will take me a while to do some tests and post results, one being the loading USB address, prepping a ZLP IN, then setting ADDEN, and the other being clearing TXINI/RXOUTI without clearing FIFOCON for control endpoints.

[NicoHood]

Maybe just try out your suggestions and open a PR for public testing?