2026-05-07

FSM simulation — Rotary-dial pulse counter

This is the interactive simulator of the rotary-dial pulse-counting state machine that runs inside the USB HID Phone firmware. It animates the FSM in sync with a live timing diagram showing in_nsa, in_nsi, the active state and pulseCount, and is meant for teaching, documentation and screencasts.

The ISR body in the simulator is a direct port of InterruptHandlerHigh() from user/user_mouse.c — same four states, same transitions, same gating of the Leerlaufimpulse via in_nsa.

One simulated tick = one real 10 ms Timer0 interrupt.
Pulse-low / pulse-high durations in the scenarios (6 ticks / 4 ticks) are representative of a real rotary dial returning at ~10 pulses per second.
All scenarios include the two trailing Leerlaufimpulse, so you can watch them get correctly ignored: e.g. seven in_nsi pulses go by for digit “5”, but pulseCount finalizes at 5.

If the embedded view is too cramped: open the simulator in a new tab.

Scenarios

Value	Description
`dial1`	Dial “1” — single pulse
`dial2`	Dial “2” — two pulses
`dial3`	Dial “3” — three pulses (default)
`dial4`	Dial “4” — four pulses
`dial5`	Dial “5” — five pulses
`dial6`	Dial “6” — six pulses
`dial7`	Dial “7” — seven pulses
`dial8`	Dial “8” — eight pulses
`dial9`	Dial “9” — nine pulses
`dial0`	Dial “0” — ten pulses (rotary-dial encoding for zero)
`dial21`	Dial “2” then “1” back-to-back, showing successive digits
`manual`	Drive `in_nsa` / `in_nsi` yourself via the pills below the controls

Keyboard shortcuts

Key	Action
`Space`	Play / Pause
`S` or `→`	Step one tick
`←`	Step back
`R`	Reset

URL parameters

The simulator can be deep-linked, which is handy for documentation or scripted recordings:

fsm_demo.html?scenario=dial3&speed=0.1&autoplay=1

Parameter	Values	Effect
`scenario`	`dial1` … `dial9`, `dial0`, `dial21`, `manual`	Preload a scenario
`speed`	`0.01` … `1.0`	Real-time multiplier
`autoplay`	`1`	Start playing immediately
`tick`	integer	Seek to a specific tick on load (stills)

What you should see

On ↑in_nsa the FSM jumps from STATE_INIT to STATE_DIAL and resets pulseCount to 0.
Each ↓in_nsi while in STATE_DIAL flips us to STATE_PULSE_LOW and increments pulseCount.
Each ↑in_nsi flips us back to STATE_DIAL.
When ↓in_nsa arrives and pulseCount > 0, we go to STATE_DONE.
The (simulated) processIO() then computes pulseCount + HID_KP_BASE, lights up the green digit N pill in the header, flashes the STATE_DONE → STATE_INIT edge, and we are armed for the next digit.
Any extra in_nsi activity after ↓in_nsa (the Leerlaufimpulse) is ignored.

Back to the USB HID Phone project page.