Building a UPDI flasher

The split-flap modules use an ATtiny1616, which is programmed over UPDI — a single-wire interface. You do not need a dedicated programmer. A cheap CH340 USB-to-TTL adapter with one Schottky diode soldered across two of its pins does the whole job, and the only tool you need is a soldering iron. This page shows the build and explains why the diode has to point one particular way.

Source: this is the flasher the firmware project itself documents in FLASHER.md. Once you have one, Flashing Modules covers putting the firmware on.

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Parts

Part	Notes
HiLetgo CH340 USB-to-TTL adapter	Sold as a 5-pack. Board marking HW-597, six-pin header 5V VCC 3V3 TXD RXD GND. Any CH340, CP2102 or FTDI adapter with the same pins works the same way.
1N5817 Schottky diode	Any small Schottky diode does the job (1N5817, 1N5819, BAT85).
3 jumper wires	Female-to-male. The female end plugs onto the adapter's header pins; the male end pushes into the holes of the module's UPDI header. They carry 3V3, GND and the UPDI line.
Soldering iron	For the one solder joint pair below.

The full chain

Computer → CH340 adapter in a USB port → three female-to-male jumper wires (female end on the adapter, male end pushed into the module) → the module's UPDI Interface header, which is labelled +3V3, G and UPDI. The adapter powers the module's controller through the 3V3 wire while it is being flashed, so on the bench the module needs no other supply — no 12 V, no rail.

The adapter

Component side	Solder side

Left, component side. Nothing here is modified. The jumper wires on the header run to the module.

Right, solder side. The one modification: a diode soldered between the TXD and RXD pads, with its white band at the TXD end. That band is the whole point of this page.

Why one wire needs a diode

A serial adapter has two data pins: TXD talks, RXD listens. UPDI has one wire, and both the programmer and the chip talk on it, taking turns. So the adapter's two pins have to be joined into one line.

The ATtiny drives the UPDI line the polite way: it only ever pulls the line low, and when it lets go an internal pull-up floats the line back high. The adapter's TXD pin is not polite. It actively drives the line high whenever it is idle, which is most of the time. Tie TXD straight to the UPDI pin and the moment the chip tries to answer by pulling the line low, it is fighting the adapter's output. The answer is swamped and nothing works.

The diode turns TXD into a polite driver. With the diode's cathode on TXD, current can only flow into TXD: when TXD goes low it pulls the shared line down through the diode, but when TXD goes high the diode blocks and TXD is effectively disconnected. The chip's pull-up holds the line high, and the chip can pull it low with nothing fighting back. RXD sits on the shared line the whole time, so the adapter hears everything.

  CH340 adapter                                    module UPDI header

  TXD ───────|<───────┐
        band on the   │
        TXD end       ├──────────────────────────  UPDI   (pull-up inside the chip;
  RXD ────────────────┘                                    the chip only pulls low)

  3V3 ─────────────────────────────────────────── +3V3
  GND ─────────────────────────────────────────── G

Only RXD goes to the module's UPDI pin. Nothing connects to TXD except the diode.

Older guides (including an earlier version of this guide) described a resistor between TXD and RXD instead. A resistor works with some adapters and not others, because it only weakens the fight rather than ending it; the diode is the reliable build, and it is what the firmware's platformio.ini is set up for.

Diodes are directional

A resistor does not care which way round it goes. A diode does; that is its entire job. It has two ends, an anode and a cathode, and it lets current flow from anode to cathode while blocking the other way. On the schematic symbol the arrow points from anode to cathode, and the bar at the arrow's tip is the cathode.

On the physical part the cathode is marked by a band printed on one end of the body. On the 1N5817 it is a white band on a black body. That band is the only thing telling you which way the diode faces, so before you solder, find it.

Solder the diode so the band is at the TXD pad.

Cathode (the banded end) to TXD. Anode (the plain end) to RXD.

If you get it backwards the adapter can push the line high but can never pull it low, so the chip never hears a single bit and avrdude stops with UPDI link initialization failed. Nothing is damaged. Desolder the diode, turn it around, and solder it again.

Soldering it

1. Flip the adapter over. On the HW-597 the pads are labelled on the solder side, so you can read TXD and RXD directly.
2. Find the band on the diode. Bend the leads so the diode lies across the two pads with the band end over TXD.
3. Tin both pads. Solder the band lead to TXD, then the plain lead to RXD. Trim the leads short so the diode lies flat.
4. Check that no solder bridges to the neighbouring 3V3 or GND pads.

Wiring it to a module

Adapter pin	Module header pin	Purpose
3V3	+3V3	Powers the module's controller while flashing.
GND	G	Common ground.
RXD	UPDI	The shared data line.
TXD	nothing	Only the diode connects here.

If it says UPDI link initialization failed

• Check all three wires are seated at both ends. Without the 3V3 wire the module has no power and cannot answer.
• Give it a moment and try again. Some modules answer only a few seconds after being connected.
• Diode direction. The band must be at the TXD pad. If it is at RXD, turn it around.
• Try a slower speed — 28800 baud instead of 57600 (upload_speed in platformio.ini, or -b 28800 for avrdude).

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Next: Flashing Modules → — the two commands per module, with or without a compiler.