Ferry Chain Count

A chain ferry crosses the river at Aldermere by pulling itself along a 140 m chain. The new dock controller needs to know where the ferry is and which way it is moving. The chain turns a pulley with a 2-bit position sensor called a quadrature encoder, which sends signals on channels a and b. Each step of the chain flips exactly 1 of the 2 bits. The order of the flips tells you the direction.

The commissioning log shows two problems. First, at the docks, waves from passing boats rock the ferry, and the pulley jitters: 1 step forward, 1 step back. The old decoder reported a direction change on every jitter step. Each report re-arms the boarding barrier, so the barrier lights blinked all afternoon. Second, a greasy sensor sometimes misses one change, and then the next sample shows both bits changed at once. After such a jump, the sample cannot show whether the chain moved 2 steps forward or back, so the position is unknown and the count must not guess.

Your task: build the decoder. It keeps a wrapping position count on pos and a direction output dir that dock jitter cannot flip. It also raises a sticky fault flag fault (it stays set until reset) for the both-bits jumps.

Interface

Behavior

• The pair (a, b) is sampled at every rising edge and compared with the previous sample
• No change: pos, dir and fault all hold
• Exactly one bit changed: a step. Forward order 00, 01, 11, 10, 00 adds 1 to pos, and the reverse order subtracts 1. pos wraps mod 256 in both directions
• dir updates only when a step goes the same way as the step before it
• A single step against the current run (the dock jitter) moves pos but leaves dir alone
• Both bits changed at once: no count change, no direction change; fault goes high and stays high until reset
• After a double change, decoding re-anchors on the new pair; later valid steps keep counting while the flag stays up
• rst = 1 at a rising edge: pos = 0, dir = 1, fault = 0, and the previous-step memory is set to forward
• Reset also latches the current line pair as the new baseline, so an unmoving chain produces no phantom step on release

What the bench checks

• The testbench checks pos, dir and fault on every cycle
• Forward and reverse runs cover steady parking and position wrap in both directions
• Direction must change only after two consecutive contrary steps, while alternating jitter must leave it unchanged
• Both illegal double transitions must set the sticky fault without changing position, and valid counting must continue afterward
• Reset on nonzero encoder lines must clear every output and re-anchor there without a phantom step

Constraints

The encoder lines are already clean and synchronous. There is nothing to debounce (the lines never glitch). Do not add ports.