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Reverse & Gearshifting

Direction and speed profile are selected through a single linear gear ladder that the driver moves one rung at a time with the shift paddles (Hori wheel) or the SE/SF bumpers (RC remote).

The shift ladder

flowchart TB
    High["High (blue LED)"] --- Med["Med (cyan)"] --- Low["Low (green)"] --- Park["Park (amber)<br/>throttle inhibited"] --- Reverse["Reverse (red)<br/>REV line held"]
Rung Direction ESC speed mode Throttle
High Forward High live
Med Forward Med live
Low Forward Low live
Park — Low inhibited (neutral)
Reverse Reverse Low live
  • Up = right paddle (wheel bit 13) or SF bumper (RC); down = left paddle (wheel bit 12) or SE bumper. One rung per press.
  • Every rung is freely selectable at any time — there is no standstill gating. (An earlier build gated Reverse on "vehicle stopped," but on stands the wheels coast for a long time and "stopped" is derived from speed pulses, so Reverse became unreachable. The ESC's own direction handling is the backstop against a hostile reverse-at-speed.)
  • Whenever the kart is not armed/driving, the ladder is forced to Park, so a fresh arm always starts neutral — no throttle is possible until the driver deliberately upshifts into a drive gear.

The pure ladder logic lives in firmware/kart-core/lib/kartcore/shift_ladder.h (host-tested).

Reverse is a held line

The ESC's REV input (Teensy pin 3 → ESC pin 8) is driven as a held ground — it is grounded for as long as the Reverse rung is selected.

Why held, not pulsed

An earlier version pulsed the REV line briefly. On the FarDriver app this only flickered the ESC into reverse for a split second and then back to first gear. Holding the line keeps the ESC in reverse for as long as Reverse is selected. Not-armed / Park resolves to forward, so a disarm always ends pointing forward.

Speed mode is a "gear-cycle button"

This ESC does not have discrete Low/Med/High select lines that you set directly. Instead its high-speed line behaves like a momentary button that cycles the gear each time it is grounded:

Low → Med → High → Low → …   (one press = one step up, wrapping)

So the firmware tracks the ESC's current mode open-loop (g_speedMode) and emits the right number of button pulses on the high-speed line to reach the mode the ladder selects:

  • an upshift (e.g. Low → Med) = 1 pulse;
  • a downshift (e.g. Med → Low) = 2 pulses (down-one equals up-twice in a 3-gear wrap).

Park ↔ Low and Park ↔ Reverse do not move the ESC speed mode (both hold it in Low), so they emit no pulse. Pulses are generated by a non-blocking pulser (ground for ~80 ms, release for ~80 ms, per pulse) so the control loop never stalls.

Open-loop model can drift

Because the ESC gives no feedback on its current gear, the firmware's model can drift from what the FarDriver app shows (e.g. after a missed pulse). Resync it with the GEAR <low|med|high> serial command, which sets the model without pulsing. The LED gear color (green/cyan/blue) is the quick visual check.

Medium is the ESC default

Medium speed mode is the ESC's default when neither the high-speed nor low-speed line is grounded. The FarDriver-app-side configuration of the speed profiles (what each mode actually does — the per-gear current and speed limits) is set in the ESC, not this repo. Those values are captured in the FarDriver ESC Settings reference (see Ratios in Gear); remaining open questions are tracked in Hardware & Open Items.