Open-source USB interface: Audio Widget

Hi Unicycle,

Replacing the MCU is quite relevant, yes! But it's a huge job to port all the IO functions and switch to a newer FreeRTOS.

The code base started out as an Atmel code example many moons ago. It has been rewritten many times after that, but there remain some old copyright notices that bind it to AVR32. This leads the control freak in the direction of Microchip parts. While not fluent in their legalese, I believe they will have less issue with ancient Atmel code going to a younger family member than to a competing vendor.

There's a VBUS_EN signal from the MCU to control the PSUs. And the same PSUs have to deliver power to the MCU...

The USB Specs have a rule that the data lines cannot be energized before the +5V VBUS. As a result, many processors that support USB have some way to detect and/or control the USB VBUS separately from other power supplies. I have not looked at the specific schematics that you're referring to, but I suspect that the VBUS_EN only controls some of the power supplies.

That said, I've seen many designs where power supply voltages could leak from one supply rail to another, with the result being that the device could be powered up with the power switch in the off position, provided that USB cables were attached at the same time. i.e. Not everyone gets this right, so you're right to be suspicious and pay close attention to the details.
Hi Nessor, which version of the schematic are you referring to?

The VBUS_EN signal, if I remember correctly, is mainly to conduct VBUS to various power loads in the system. The MCU's regulator defaults to ON.

Rsdio, the MCU has a VBUS detect pin that is connected to VBUS on the USB plug. I trust the MCU to consider this signal when it turns the USB data pins on and off.