The USB module can work at 1.8V, but the problem is that the data lines at this voltage level are out of the USB electrical specification, so the USB host may not be able to communicate properly.
The electrical specification of USB is described in Table 7-7. DC Electrical Characteristics of the USB 2.0 specification and it can be downloaded from USB.org . The important parameters impacting this are:
Output Levels for Full-speed
High (driven) – VOH - 2.8V-3.6V
Input levels for Full-speed
High (driven) – VIH – 2.0V
This means that the USB receiver could operate at 2V minimum but the output high level should be between 2.8-3.6V to meet the specification.
Unfortunately, there is no way to separate the USB I/O voltages from the rest of the I/O pins without the use of an external level translator.
Any note or comment on choosing the "external level translator"?
I have tried to use TI's TXS0202 (refer to http://www.ti.com/product/TXS0202/technicaldocuments) but failed.
Sample program : USB Ramdisk demo program in QN9080DK-SDK
Evaluation board : QN-9080DK and TI's TXS0202 evaluation module
TXS0202 port B : USB output (D+, D-, GND) from QN9080DK USB and the external power to QN9080DK (2.4V)
TXS0202 port A : USB from PC with 3V3 LDO from Vbus to provide TXS0202 Port A power, D+, D- from PC
It may not work but it works when the QN9080DK is driven by external 2.85V and directly connected to PC.
Attached please find the measured D+ (yellow) and D- (blue) waveform (to PC) : the 1st one (S__908000132) is working (QN9080DK with external power of 2.85V, direct connected with PC, no TXS0202 module involved); the 2nd figure (S__908000133) is NG (QN9080 with external power of 2.4V, TXS202 module is involved)
You can check the different ones on the DK schematics, but as the one you are using it's not from NXP, I can't really know how it works or which can be your problem with it.
Thanks your reply. It clears my concerns on USB I/F since there is no separate power source to USB interface. Therefore, we have to take special care on design when QN908x drive at low volatge (say, 1.8V).
Ted Y. Liu