Up/down counter

// An up/down counter: a 74HC193, steered by a 74HC02.
//
// The four LEDs are the count in binary, Q3 on the left and Q0 on the right,
// stepping once a second.
//
// Click the slide switch to change direction. The 193 has two clock inputs,
// CPU (count up) and CPD (count down), and the one not in use must be held
// high. The 74HC02's four NOR gates make two OR gates:
//
//   CPU = clock OR UP_N       CPD = clock OR DOWN_N
//
// The switch pulls either UP_N or DOWN_N low. The low one lets the clock
// through to its input; the high one holds the other input high. So the
// switch picks the direction without ever touching a clock wire.
//
// Count up past 15 and it rolls over to 0; count down past 0 and it rolls
// over to 15. TCU and TCD, the carry and borrow, go low for half a clock at
// those points; wire them to a second 193's CPU and CPD for an 8-bit counter.