Part Number: AM6548
Other Parts Discussed in Thread: TMDX654IDKEVM
Hi everyone.
My name is Enrique and I am quite new hardware engineer. For the company I'm working now, we are designing a Carrier card for a AM6548 based SoM, and during the design I've had some questions (I've checked some schematics from dev. kits to learn about the design). This questions are specially related to the USB_DRVVBUS and USB_VBUS signals.
Anyway, I will try to explain what I understood from reading the datasheet and the TRM of the AM6548, so you can correct me whenever I made a mistake. Please, be patient with me, I might have misunderstood more than one thing. I'll start by saying that I want to base my design in the TMDX654IDKEVM Dev. Kit. I'll upload the schematic.
I'll start by saying that our card will be the host for USB Type-A memory sticks. This means that the ID signal in the AM6548 will be pulled to the ground. From what I read, the differential pair on the USB device will have pull-up resistors in D+ or D-, depending on which transfer speed they are able to allow. This would be also the way of telling the host that a new device has been inserted. I hope at least until here I got it right.
I like the idea of using a switch for powering up the USB system. But here is where I had more trouble understanding how it works, because the TRM doesn't explain a lot. As far as I understand, when the USB is inserted and detected by the host, the USB_DRVVBUS should be asserted right? This would make the switch to close and allow 5V in the output that would feed the whole system. This tension is then brought up and downscaled to 1V through a resistor divider (as shown in the image taken from the TRM) and fed to the USB_VBUS input of the AM6548. From what I also understood, while the USB_VBUS detects the correct tension in his input, he will keep the USB_DRVVBUS asserted. Until here am I right?
If I have it more or less until here, then some questions arise to me:
- What are the conditions to deassert the USB_DRVVBUS signal? When I remove the stick the host will detect it and pull down the signal by software? Or does it work in any other way?
- I've read that some people had problems with the USB_DRVVBUS signal. They state that short after they insert the USB device the USB_DRVVBUS signal gets pulled down, thus, removing the power from the USB, mostly because of overcurrent events (but I don't know in which part of the circuitry exactly). The question is, what considerations should I take in order to make a clean design and avoid this kind of events?
Lastly, the TRM says something about third-party USB controllers. I would like to understand under what circumstances I would need to get this third-party documentation. I mean, what would be an example of one case that I could be interested in getting these.
Thank you very much for your attention!
EDIT: what's the FL2 symbol displayed in the schematic, used in the power switch. As it has faradays as units there, it seems like a 3 pin capacitor, but might be also some kind of inductor to avoid current overshoots?
