Camera Trap Flash

The wildlife unit's newest camera traps in the Herrick oak woods carry a xenon flash, which fires from a stored electric charge. The first firmware treated it like an LED that can fire any time. One busy night, a badger family filed past the lens, and the trigger fired 9 times in a minute. Recharging that fast dragged the power supply down until the whole camera lost power and rebooted. The night's 9 triggers gave 1 usable frame and a dead trap until morning.

The hardware fix is already on the board: a bank that stores up to 4 flash charges, topped up at 1 charge every 8 cycles.

Your task: build the block between the motion trigger and the flash tube. Spend 1 banked charge per photo, and ignore triggers while the bank is empty. A missed photo is fine; a crashed camera is not.

Interface

Behavior

• After reset the bank is empty: charges_o = 0, fire_o = 0, and the delivery cadence restarts
• A charge is delivered on every eighth rising edge after reset (the 8th, 16th, 24th, ...), regardless of traffic
• A delivery with the bank already at 4 is discarded
• trig_i = 1 at a rising edge with at least one charge available: fire_o pulses high for the following cycle and the bank drops by one
• trig_i = 1 with the bank empty is ignored: no pulse, no debt, no queue
• On an edge where a delivery and a trigger coincide, the delivery lands first and the trigger draws second
• On a coincident edge, an empty bank still yields a flash, and a full bank ends the edge at 3 charges
• trig_i held high simply requests a photo on every edge: a full bank gives 4 back-to-back pulses
• When trig_i stays high after that 4-pulse burst, fire_o follows the delivery cadence exactly
• charges_o always shows the post-edge bank level; fire_o and charges_o are registers
• rst_i = 1 at any rising edge empties the bank, clears fire_o, and restarts the cadence

What the bench checks

• The testbench mirrors these rules with a reference model and checks fire_o and charges_o on every cycle
• The testbench drives the fill cadence from empty, the cap at 4, the 4-flash burst, and a long stretch with trig_i held high across many deliveries
• It also drives coincident delivery-and-trigger edges on both an empty and a full bank, a random storm, and a mid-run reset

Constraints

TIMING: everything is synchronous to clk_i; both outputs are registers.

SCOPE: this models the charge bookkeeping between the motion trigger and the flash tube. The charger electronics, the xenon tube, and the camera's power supply are out of scope.

CADENCE: the delivery cadence is fixed by the charger and counts from reset, not from the last delivery or the last flash.

Do not add ports.