MicroMod Asset Tracker Carrier Board Hookup Guide
El Duderino, 
PaulZC 
Choosing a Processor Board
As you can see from the Jumpers section above, there are some things to consider when choosing a MicroMod Processor Board to go with your Asset Tracker.
The MicroMod M.2 connector pinout is very comprehensive and some of the processor chips on our MicroMod Processor Boards just don't have enough I/O pins to let us allocate one to every M.2 pin. So, on those boards we share the pins. For example, on the ESP32 Processor Board, you will see that the UART TX1 pin is shared with the G3 I/O pin and the AUD_OUT pin. We do that so that you can use that pin for TX1 or G3 or AUD_OUT. And for most users and most Carrier Boards, that's perfectly fine. However, since the SARA-R5 has so many pins that we need to connect to, the sharing of TX1 and G3 causes a small problem. We need TX1 to communicate with the SARA, and G3 would normally be used to enable power for the ICM-20948 IMU. We get round that by including a normally-open split pad jumper on G3 and making the IMU power circuit default to on. Simple!
So, you can use any SparkFun MicroMod Processor in the Asset Tracker, but if you need complete access to all of the pins then here are the restrictions:
ESP32 Processor
TX1 is shared with G3. This means you cannot close the G3 split pad to enable control of the IMU Power. The IMU power will need to be always on.
RX1 is shared with G4. This means you cannot close the G4 split pad to access the SARA Ring Indicator / Clear To Send 2 pin.
AUD_LRCLK is shared with G1. G1 controls the micro-SD power and AUD_LRCLK is connected to the digital microphone. If you want to use the microphone, you will need to open the G1 split pad jumper. The micro-SD power will default to on.
AUD_BCLK is shared with G2. G2 can be used to turn the SARA-R5 on/off and AUD_BCLK is connected to the digital microphone. If you want to use the microphone, you will need to open the G2 jumper. You won’t then be able to have the processor turn the SARA on and off. The SARA always powers on automatically when power is connected, so no worries there, and the SARA On button is there if you need it.
Artemis Processor
The Artemis Processor does not support the M.2 TX2 and RX2 pins. This means you cannot use the Artemis to talk to the SARA using any of the dual-UART variants (called variants 2, 3 and 4 in the SARA-R5 System Integration Manual). This means you cannot, for example, use UART 1 for your LTE traffic and UART 2 for your GNSS data. That's all. You can do everything over a single UART anyway.
nRF5280 Processor
The nRF52840 Processor Board supports all of the pins used by the Asset Tracker. There are no restrictions for this Processor Board.
SAMD51 Processor
The SAMD51 Processor Board does not support RTS1 and CTS1 and so you will not be able to perform hardware handshaking when using the UART. Serial buffers are so large these days that you probably won’t even notice this restriction, but it is something to be aware of.
RP2040 Processor
AUD_LRCLK is shared with CTS1. This means that if you want to use hardware handshaking for the UART communication, you won’t be able to use the digital microphone.
Likewise AUD_BCLK is shared with RTS1, again preventing you from using the microphone and hardware handshaking at the same time.
I2C_INT is shared with TX2. This means that you cannot use the ICM-20948 IMU interrupt and SARA DTR / TXD2 simultaneously. You can open the ICM_INT split pad to isolate the IMU interrupt signal.
G4, G5, G6 and G7 are shared with the SPI connections SPI_SCK, SPI_COPI, SPI_CIPO and SPI_CS. This should not be an issue for you, but it does mean you cannot close the G4-G7 jumpers to enable access to: SARA_RI, SARA_INT, SARA is On, and SARA_DSR.
STM32 Processor
The STM32 Processor Board does not support RTS1 and CTS1 and so you will not be able to perform hardware handshaking when using the UART. Serial buffers are so large these days that you probably won’t even notice this restriction, but it is something to be aware of.
The STM32 Processor Board does not support the M.2 TX2 and RX2 pins. This means you cannot use the STM32 to talk to the SARA using any of the dual-UART variants (called variants 2, 3 and 4 in the SARA-R5 System Integration Manual). You cannot, for example, use UART 1 for your LTE traffic and UART 2 for your GNSS data.
G7 is not supported on the STM32 Processor Board. This should not be an issue for you, but it does mean you cannot access the SARA is On signal.