topic Re: LPC546xx USB_VBUS in LPC Microcontrollers

LPC546xx USB_VBUS

padraig_fogarty — Wed, 27 May 2020 18:42:49 GMT

Figure 48 from the LPC546xx datasheet shows the pullup on USB_DP being switched from Vdd. However my understanding is that the USB specification requires this to be connected to USB_VBUS.

Given the problems I have seen with regard to USB device enumeration I suspect that the figure is in fact incorrect and the device does switch to USB_VBUS, but I would appreciate it if someone could confirm this detail. Also, if the minimum working voltage for the USB_VBUS could be provided that would also be very helpful.

Regards,

Padraig

Re: LPC546xx USB_VBUS

xiangjun_rong — Wed, 03 Jun 2020 03:50:52 GMT

Hi, Padraig,

I have confirmed with AE team, the "Fig 48. USB interface on a self-powered device where USB_VBUS = 5 V" is correct, the USB_DP is pulled up to VDD instead of USB_VBUS with the DCON bit is set in the DEVCMDSTAT register.

Regarding the minimum voltage of the USB_VBUS pin, it is VDD*0.7, if the VDD is 3.3V, it is 0.7*3.3V=2.3V.

Hope it can help you

XiangJun Rong

Re: LPC546xx USB_VBUS

padraig_fogarty — Wed, 03 Jun 2020 11:54:04 GMT

Thank you for the response.

However, I find the details concerning since I my understanding is that the USB standard requires that USB_DP be pulled up to VBUS and not VDD; or at least that the pull-up should not be attached if VBUS is not present. Can you confirm how this works on the LPCxxx?

Also, from my own testing I can say that USB device enumeration will fail if the voltage on the USB_VBUS pin is below 2.9V. It certainly does not work anywhere close to 2.3V!

Re: LPC546xx USB_VBUS

xiangjun_rong — Fri, 05 Jun 2020 03:39:25 GMT

Hi, Padraig,

This is the USB_VBUS pin voltage specs in data sheet of LPC546xx, pls refer to it.

Hope it can help you

XiangJun Rong

Re: LPC546xx USB_VBUS

padraig_fogarty — Fri, 05 Jun 2020 08:55:03 GMT

I had been looking at the datasheet revision 2.6, and it seems this table was added back in Feburary 2020 to revision 2.7; not noticing that was perhaps an oversight on my behalf. In addition to this table it would very useful if the 'Function' listed in the table could be described. I have searched the document and can find no explaination of 'VBUS_valid'. I might guess but I would prefer to have the NXP explain the term as it is intended.

However, and perhaps it is even more important that I note the revision of the datasheet has also changed the title of section 13.7 from "Suggested USB interface solutions" to "Suggested USB0 Full-speed interface solutions". And the table you show above is in a new section "13.8 USB1 High-speed VBUS threshold levels".

So, is it the case that the voltages for USB1 (and given in Table 59) are now different from those previously specified in the datasheet for both USB interfaces?

These new figures do not seem to correspond in any way with the previous description, and I would consider this a very significant alteration of the specification!

Regards,

Padraig