When I was in high school studying electronics on my own as a hobby (it was not taught in the curriculum. No, physics people culturally hates electronics, they consider it a chore.), I followed the logic states of the bistable (two NAND gates) meticulously. However, it was tedious and hard to remember correctly.<\/p>\n
There’s a fast way to reconstruct the explanation from scratch. You’ll need these invariants:<\/p>\n
By leaving external inputs (S and R) at ‘1’, we are letting the state pins decide, behaving like the two chasing NOT gates.<\/p>\n
The only way to disturb the state is to create a ‘0’ (clear) action<\/strong>. The circuit is symmetric, so ‘S’ and ‘R’ is arbitrary as long as you are willing to switch the roles of Q and Q’.<\/p>\n There are no other valid actions<\/strong> in this configuration.<\/p>\n Side note: persisting the clear action will lead to 0 & 0 = 0 at the applied input and 1 & 1 = 1 at opposite NAND gate, which the achieved state remains. Normally we want to return the external inputs back to 1 to receive future commands (actions) correctly, both external inputs asserting low is invalid.<\/p>\n It’s more natural to have S and R being active high in transistor’s implementation. NAND’s ‘S’ and ‘R’ are active low (so technically, I should use S’ and R’ instead, but I’m following the more common nomenclature for the moment for the NAND gate implementation).<\/p>\n\n
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