548 FSM :: Quicker, easier and cheaper to make your own chip!

How it works

It’s a Moore FSM with spaced change/refund pulses and a small datapath for credit/selection.

States: st_select → st_vend → (st_mc_pulse ↔ st_mc_gap)* → st_select st_select → (st_refund_pulse ↔ st_refund_gap)* → st_select

Inputs

coin (1 bit): rising-edge–counted +1 credit per pulse (capped at 7).
btn (4 bits): user command sampled on the transition "0000" → non-zero 0000 → wait (no command) 0001 → cancel (clear selection, keep credit) 0010 → refund (return all credit as spaced pulses) 0011..1111 → product codes for items 1..13 (internal idx 0..12)

Selection & Pricing

Prices depend on the selected index (sel_idx): idx 0..3 → price 1, idx 4..7 → price 2, idx 8..12 → price 3 When a valid selection exists and credit ≥ price, the FSM vends: Credit is debited by price in st_vend. If credit remains, it enters the make-change sequence (spaced pulses). Otherwise, it returns to st_select.

Refund vs. Make-change

Refund (btn="0010") returns all current credit as pulses with 1-cycle gaps. Make-change happens after a vend if debit left credit > 0; same pulse/gap pattern.

Outputs (Moore)

dispense = '1' only while in dispense_product (exactly one clock cycle).
change = '1' only while in return_change (exactly one clock cycle).
product_num (4 bits) latches the 1..13 product number on entry to st_vend, stays stable until the next coin or button press (or reset). This aligns naturally with the dispense_product strobe.

Timing/Reset reset is asynchronous, active-high: immediately sends the FSM to idle. State updates on the rising edge of clk. Inputs are sampled synchronously. The “one-cycle pulse” behavior comes from those terminal states automatically returning to idle on the next clock.

How to test

Scenario 1 —

Input: 1 coin (coin 1-cycle pulse), select btn in 0011

Expect: dispense_product='1' for 1 cycle, product_num= (10)10 = (1010)2 "until next scenario/btn press" and change='0'.

Scenario 2 —

Input : 2 coins ( 2 pulses each coin for 1-cycle pulse), select btn in 0111

Expect: dispense_product='1' for 1 cycle, product_num= (5)10 = (101)2 "until next scenario/btn press" and change='0'.

Scenario 3 —

Input : 2 coins ( 2 pulses each coin for 1-cycle pulse), select btn in 1011

Expect: dispense_product='1' for 1 cycle, (11)10 = (1011)2 "until next scenario/btn press" and change='0'.

Scenario 4 —

Input : 3 coins ( 3 pulses, each coin for 1-cycle pulse), select btn in 0011 (price product = 1 coin)

Expect: dispense_product='1' for 1 cycle, product_num = (1)10 = (0001)2 "until next scenario/btn press" and change='1' then '0' then '1' (2 pulses for 2 coins) .

Scenario 5 — Refund

Input : 3 coins ( 3 pulses, each coin for 1-cycle pulse), select btn in 0010 (to return change)

Expect: dispense_product='0', product_num = (0)10 = (0000)2 "until next scenario/btn press" and change='1' then '0' then '1' then '0' then '1' (3 pulses for 3 coins) .

Scenario 6 —

Input : 2 coins ( 2 pulses, each coin for 1-cycle pulse), select btn in 1111 (product price= 3 coins)

Expect: Nothing happens, dispense_product = product_num = change = 0

Then press btn = '0001' to cancel the transaction, and then press btn = 0100 (product price = 1)

Expect: dispense_product='1', product_num = (2)10 = (0010)2 " and change='1' then '0' (1 pulse for 1 coin) .

External hardware

Inputs:

coin → 1x pushbutton or DIP (debounced)
btn(3:0) → 4x switches to issue 0001 (cancel), 0010 (refund), and 0011..1111 (products)

Outputs:

1x LED for dispense_product (short flash on vend)
1x LED for change
4x LEDs for product_num (optional)

Clock & reset: board oscillator + synchronous active-high reset generation.

#	Input	Output
0	clk	dispense
1	reset	change
2	coin	product_num(0)
3	btn (0)	product_num(1)
4	btn (1)	product_num(2)
5	btn (2)	product_num(3)
6	btn (3)
7

548 FSM

548 : FSM