Recommended minimal connections between nRF9160, nRF52832 and satellite modem

[okwestern]

I'm developing a system of nRF9160, nRF52832 and Iridium 9603 (satellite modem) - all on the same main board. The nRF9160 is the brain of the system and handles LTE, GPS and a bunch of on-board sensors. The nRF52832 handles a proprietary bus over BT between external units and the main board. The Iridium 9603 is handling communication where LTE is out of reach. nRF9160 <-> nRF52832 are connected over UART (only tx/rx), nRF9160<->9603 also over UART (only tx/rx).

I have a few pins to spare on both nRF, but don't want to waste them in case I need them in a later revision. Would there be any benefits with more connections between the two nRF besides tx/rx? 

I'm also trying to understand COEX and MAGPIO. I've connected COEX0 to control the power on of a GNSS LNA. Anything else that would be wise (either to nRF52 or the satellite modem)?

[Kenneth]

I have a few pins to spare on both nRF, but don't want to waste them in case I need them in a later revision. Would there be any benefits with more connections between the two nRF besides tx/rx? 

If you add HWFC (2pins: CTS and RTS) then you get better control of power consumption, since you can disable the UART interface when there is no data. 

I've connected COEX0 to control the power on of a GNSS LNA. Anything else that would be wise (either to nRF52 or the satellite modem)?

Not that I can think of. 

[okwestern]

Thanks! Would UART be the best way to communicate between the nRF91/nRF52, given it uses only 2 or 4 signals?

[Kenneth]

UART is a common serial interface, so I would say yes.

Kenneth

[okwestern]

Would UART also be enough for FOTA from nRF9160 to nRF52832? Or do I need some direct reset signal, etc? I would think the nRF91 could send a firmware update message to the nRF52, then upload the firmware in nRF52 flash, and the nRF52 resets himself into update mode...?

[Kenneth]

I suggest to add an additional pin that can be used to trigger DFU mode.

Kenneth

[okwestern]

I just discovered the nRF52832 needs its single UART for other things, so the question is, what is the best alternative? As an emergency solution, I was thinking an UART to SPI bridge chip, but if I can do it all in software that would be best (and something that is easy to implement/ready libs in Zephyr).