I want to have both nano-SIM socket and eSIM (MFF2) to a nRF9160. Since they both will be parallel, I was thinking to break eSIM power (VCC pin 8 of MFF2), when a physical SIM is inserted using the CARD_DETECT on the SIM socket. It will never be hot-swapped (and rarely changed). Would this be a fail-safe method (given the socket doesn't malfunction). What is best practice?
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\$\begingroup\$ Doesn't nRF9160 support a firmware-only eSIM? If not, perhaps they have a newer/different chip that does? There shouldn't be a need to use hardware for it. Although in practice it may be simpler to implement to throw an eSIM chip there as you plan to. Just a thought. \$\endgroup\$– Kuba hasn't forgotten MonicaCommented Aug 29, 2023 at 19:43
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You shouldn't do that.
For any IC, the absolute ratings for pins are given w.r.t. the supply rails ground and positive rail such as VSS-0.5V to VDD+0.5V.
So, applying voltage to the pins of an unpowered (i.e. positive supply rail is grounded or left floating, doesn't matter) IC may damage the IC.
Another possibility is that the IC may try to get powered from the pins if there are clamping diodes:
simulate this circuit – Schematic created using CircuitLab
As you can see, when the CLK pin sees a positive (3.3V) pulse the VPP rail will see 2.8V even if the IC is unpowered. So the IC's internals may try to draw current through the clamping diodes. And this will cause some trouble since the SIMs are physically in parallel. NOTE: I don't know if there are clamping diodes in the SIM's control chip, but if there are then there's a risk.
answered Aug 29, 2023 at 11:42