Can you point me to where you see the CD pin? I just looked over both of the pin diagrams, but I didn’t see it. Maybe a fourth cup of coffee will help.
The firmware supports 4 hardware select pins for SPI, but it is really easy to use any other GPIO to select additional devices. If you’re clever, you can use logic gates to expand the number of target devices to a very high number. Additionally, you can use a SPI based logic extender to control even more devices.
SOUT is indeed MOSI and SIN is indeed MISO. I prefer the full nomenclature, but decided to match what’s in the datasheet for the chip.
All pins on the chip are +3.3V logic level. The pins are not rated for RS232 voltage levels. You would need additional circuitry to step up the voltage. The flow control is on many microcontrollers, but on dev boards, often aren’t exposed to the user.
One of the UARTs is tied up with communicating with the system IC, which talks to the modem. The UART on the chip is connected to two sets of pins. It’s possible to switch it from one set of pins and move it to another. We currently don’t support that feature, but it’s possible. The same is true for I2C0, except that is only used when the system is in bootloader mode. We’re waiting on certification to release the schematics, but then you’ll be able to see everything for yourself.