USB interface while running micro at 2.5V

Question

I'm doing a layout for someone who has asked for a USB interface on a micro that is running at 2.5V. As far as I can tell after researching this, you can't run USB down to 2.5V. So, I did a bit of digging aorund for a level shifter and found this:

http://www.ti.com/lit/ds/symlink/txs0202.pdf

On first glance it looks like just what I need. I can run the high voltage side from a 3V3 reg off the USB 5V and the processor side can run off 2V5.

However, the data sheet says:

The TXS0202 is a 2-bit voltage level translator optimised for use in Interchip USB (IC-USB) applications.

It has a diagram of it interfacing between a micro and an HD SIM card.

So, my questions are:

1. Is it right the USB will not work correcty running the micro at 2.5V?
2. Will the TXS0202 work if it's connected to a cabled interface on the high voltage side?
3. If point 2 is no, can I actually do what I need to? Are there simple devices that will level shift for cabled interfaces?

I would have throught there must be devices to do this, there must be scores of examples of micros running at less then 3V3 that have a USB interface. I cannot find anything other than three devices that all say they are for "inter chip" interfaces. I appreciate inter chip doesn't specifically say on the same board, but the typcial application diagrams seem to imply that - or at least that's what I'm inferring.

Answer 1

1. Is it right the USB will not work correcty running the micro at 2.5V?

Per STM32F datasheet, the lowest core VDD to work with USB is 2.7 V, although it is a stretch. The MCU might receive USB packets well, but protocol responses will be too weak for a standard USB host to understand. So, running the MCU at 2.5 V would need some level translation, and not just translation, but it better be something that resembles USB FS PHY drivers.

2. Will the TXS0202 work if it's connected to a cabled interface on the high voltage side?

The TXS0202 is designed for INTER_CHIP (logic-level) communication (There is some obscure addition to USB for FS mode; there is also HSIC - high-speed inter-chip protocol). The TXS0202 has some integrated pull-ups and pull-downs, so I would be hesitant to use it to drive open USB standard, these pulls might violate standard signal levels/states and confuse standard USB host.

3. If point 2 is no, can I actually do what I need to? Are there simple devices that will level shift for cabled interfaces?

I would consider some general-purpose level shifters, uni-directional, like 74LVC2T45, although no guarantee, and good testing/validation will be required. Maybe some older simple "Q-switches" will work, PI4ULS3V302 from Pericom/Diodes. MAX3372-3379 might be considered.

Solution to the typical discrepancy between the trend to use low-voltage cores and USB protocol (3.3-V signaling and 5-V tolerant) is not to use the embedded USB PHY, but use an external USB PHY connected to MCU via UTMI+ or ULPI bus. The ULPI PHYs will take care of the level translation, but you might need to use a bigger variant of STM32 processors, with ULPI interface.