Pin-strapping

[sethp]

What is it? Getting certain pins in a particular configuration to be latched in at during startup, configuring a particular device. Afterwards,

How does it work? Resistors, mostly, pulling things up and down it seems.

[sethp]

For our GSW141 prototype board, @dougli1sqrd came up with a clever solution using a SPST (single pole, single throw) switch network attached to the strapped pin like so:

schematic screenshot tbp

A possibly overly complete model (including internal "weak" pulls inside the chip): https://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWcMBMcUHYMGZIA4UA2ATmIxAUgpABZsKBTAWjDACgAnEQmqsDQt14gUCQVTTxOQqjxngaNKCPiQ2AZXlyacuXxAAzAIYAbAM4NlKNgCUQGNCGKCHVGnmVuq2aPSr-oBDYzbjFwQipXcP9wQ1MLNgB3eTBFFIioJPTZYX5xLMIwsAyePgy1AHN5UUFSkFwlfzZsQg8UYTw+YhQQTs82ABl61toPFo9i5Qhjc0sfPzYAWREOiVywHolfdi4dJXdhjwO+OGk93u8RvqpKKWW6uQxIJRZxEW3mkYelOtf+zWweCUNXqQPqCE2yhmFistlocHBPUUYwhnnh9V8aJgQSq40R9Ra+KaAKulwm5Vi0MsEmkgKUxW8YJBXikyXaVD6eOutNRDJEeHJMTAqlUbCqKAF0X5E26-QAHipiCJsErsDQle0kSsQAAiAD2AAcGAA7AA6ZgAJhwjABLY06tgAIwuICYcgQxHoKqQamdYGI9MwFDkgJ9bAVlH2jg9ghoKHowKUFQNNr15pTxvD8J6KFlCAQEFzSETIANAFcTCZzWWDQB6C16xKZhUkchoHp4bD0NDF7VmAAu1oN5v7eoqFRMDDYAGN7I4GnPGY1wMwPIRoLwCIUMKQyEDJjA4DtF04XI4+XxmMQ2EA

Two things worth noting:

• The external resistors are likely to be stronger than the internal ones, effectively disabling the internal pull up/down mechanism (except as a small source of heat)
• The GSW part doesn't list any of its GPIO's impedances, so a lot of that's guesswork based on https://electronics.stackexchange.com/questions/270833/considerations-when-using-internal-pull-up-down-resistors