NAND latch

// An SR latch: two NAND gates that hold each other's answer.
//
//   Q    = NAND(set,   Qbar)
//   Qbar = NAND(reset, Q   )
//
// Both inputs are active low and sit at 5 V through their pull-ups. Flip the
// slide switch: the pole is grounded, so it pulls one input down, that gate's
// output goes high, and the other gate (seeing a high on both inputs) goes
// low and feeds that back. Let the input go and nothing moves: each gate is now
// holding the other one where it is. That is one bit of memory, and it is the
// same cell that is inside a static RAM.
//
// It is also the standard cure for switch bounce. A mechanical contact chatters
// for a few milliseconds when it closes; the latch flips on the first edge and
// then ignores every one after it, because the input it is bouncing on is no
// longer the one deciding anything.