Seeed Studio has asked that I put a link to their site on this blog. I have used Seeed Studio Fusion service and I was/am satisfied with them. So I have added a link in the right hand column.
I decided to research potential processors for the level converter circuit.
Wikipedia has a list of common microcontrollers. This is a good resource when looking for possibilities. I will only look at a few processors from companies I am familiar with. If this co-processor chip is going to act as a level shifter, It needs to be able to drive and accept 5 Volts.
ST Microelectronics makes STM8 and STM32 lines of processors. I will look at the STM32 line next week when I research 32 bit controllers.
The STM8 line has many processors that have a max operating voltage of 5.5 Volts. Of the processors that can operate at 5.5 Volts, the lowest operating voltage is 2.95 volts. This might be high for some ARM systems I would want to support. ST Microelectronics supports their devices very well on Mac and Windows. There is a good open source compiler available for Linux called Small Device C Compiler(SDCC). The STM8 line of chips doesn’t have any built in USB capabilities. The STM8 is a possibility.