pico-serial Line — phone-line modem stick (draft 0.1)¶

A USB modem for a real telephone line, in the shape of Apple's USB Modem: a stick about as long as a disposable lighter, with an RJ11 socket at one end and a USB-C receptacle at the other (Apple's had a captive USB-A cable).

It is a softmodem: the stick is the telephone-line interface (a DAA and its codec) with an RP2040 between it and USB. The modem itself, every modulation from V.21 to V.90, fax and voice, is D-Modem's open DSP running on whatever the stick is plugged into:

• Linux: dmodemd drives it through libusb, as it drives Smart Link's SmartUSB56 today (§6). The modem appears as a pty.
• Windows: dmodemd drives it through libusb on WinUSB, which Windows binds by itself (Microsoft OS 2.0 descriptors: no INF).
• Android: the D-Modem app gets a real telephone line beside SIP (§6.2).

So the stick has no modem firmware of its own to go out of date, and everything that makes the connection is open, from the line interface up.

Everything in SPEC.md about the RP2040 core (power, USB-C, flash, crystal, buttons, UF2 updates) still applies. There is no RS-232 side.

1. Requirements¶

2. Block diagram¶

 USB-C ── ESD ── RP2040 ──────── MCLK 48 MHz (GPOUT0) ──►  Si3056  ═╪═  Si3018/19 ── bridge ── fuse ── RJ11
  (UFP)          │   PIO ◄─ SCLK, FSYNC, SDO ─┤                (system   ║   (line      hook
  LDO 3.3 V      │   PIO ── SDI ─────────────►│                 side)  C1,C2  side)     switch,
  flash, xtal    │   GPIO ── OFHK, RESET, FC ►│                        Y2 caps          ring, CID
                 │   GPIO ◄─ RGDT, INT ───────┤                        ║
                 └── LEDs (OH, RI)                                     isolation barrier

3. The line interface: Si3056 + Si3018/19¶

Skyworks' (formerly Silicon Labs') Si3056 "Global Serial Interface DAA" is the part made for softmodems: a codec with a serial port for a modem DSP, at any sample rate from 7.2 to 16 kHz (Fs = FPLL2 / 5120). Its line-side partner (Si3018 or Si3019) holds the hook switch, the DC and AC termination, the ring detector and the caller-ID path, programmable for every country's line (impedance, current limit, ring thresholds). The two talk across two high-voltage capacitors: that is the isolation barrier.

Why not the alternatives: - Si3050 (Skyworks' voice DAA, same line side): its PCM highway runs at 8 kHz only. D-Modem's V.90 needs 9600 Hz (at 8000 Hz on a clock of its own it loses the PCM signal every few seconds). - Si2494/Si3018 ISOmodem: a complete modem chip; it would make the stick plug-and-play but closed, with no path for D-Modem. - A transformer DAA + codec: more parts, a relay or opto hook switch, and every country's termination done by hand.

3.1 Circuit¶

The datasheet's typical application (Si3056 datasheet rev 1.03, Figure 17, and its bill of materials), unchanged in topology:

The part numbers are in the netlist script (LINE_PARTS in hardware/netlist/build_netlist.py; --variant line) and its BOM (bom-jlc-line.csv), each checked against JLCPCB's stock on 2026-10-22. The line side is the Si3018-F-FS (active; the Si3019 has the same pins and a few more line-level options, at $2.60 more). Neither Skyworks chip is stocked at LCSC: they come from DigiKey, and are fitted by hand or sent to JLC as consigned parts, as is the through-hole RJ11.

3.2 Connections to the RP2040¶

The RP2040 is the serial port's slave: the Si3056 makes SCLK and FSYNC from MCLK, so the sample clock is the board's crystal through the USB PLL, and the USB side follows it (§5).

3.3 Layout rules (Skyworks AN67)¶

• No ground plane under the line side; the line-side parts close together, inside the barrier.
• The barrier: C1, C2, C8, C9 bridge it; nothing else does. Creepage and clearance across it at least 2.5 mm on every layer (UL60950 basic insulation for a telecom network at 250 V), wider where the board allows.
• RV1, the fuse and the ferrites right at the RJ11; TIP and RING short and wide.

4. Rest of the board¶

The RP2040 core of SPEC.md §4.2 (the chip, W25Q flash, 12 MHz crystal, AP2112K-3.3, USBLC6 ESD, the USB-C with its CC resistors, a polyfuse), plus: - two LEDs: OH (off hook) and RI (ringing); - the BOOTSEL and RESET buttons (inside; the stick also reboots to its UF2 loader on a vendor request); - test pads for SWD, 3.3 V and the serial port.

The RS-232 chips, the DB25 and their GPIOs are not fitted: the netlist script's line variant leaves them out.

5. USB interface (firmware)¶

One vendor-class interface (class 0xFF), USB 2.0 Full Speed, VID 0x1209 (pid.codes) with a test PID until a real one is assigned. It is shaped like the SmartUSB56's, so D-Modem's driver for that is the model for this one:

Vendor control requests (EP0): the stick's info and version; the sample rate; the hook (off, on, a timed flash or pulse); the line monitor (on hook, for caller ID); a Si3056/Si3018 register read and write (the host sets the country's line from D-Modem's table); reset; reboot to UF2. The exact numbers go in firmware/PROTOCOL.md with the firmware.

Samples run both ways from the moment the host starts them, on hook too, so ring and caller-ID detection can run on the samples as they do on ALSA codecs.

6. Host software¶

6.1 D-Modem (Linux)¶

A second device kind beside the SmartUSB56 in frontend/dmodemd/usb_line.cpp: dmodemd --usb=auto finds either. The differences from the ST7554: - samples at 9600 Hz by default (V.90); - the hook and the line monitor by vendor request, not GPIO registers; - caller ID (the ST7554's codec hears nothing on hook; this one does); - the country's line settings written to the line side's registers.

6.2 The D-Modem app (Android)¶

The app gets the stick through Android's USB host API (a UsbDeviceConnection's descriptor handed to libusb), so the same line code runs on the phone: the modem on a real telephone line, beside SIP and the line to another phone.

7. Mechanical¶

• Length about 82 mm, the length of a disposable lighter (a BIC classic is 81 mm), with a cross-section set by the RJ11 socket.
• RJ11 at one end, its latch toward the top; USB-C at the other.
• A 3D-printed shell in two halves (as the W hood: MJF PA12), the board held between them. Light pipes or thin windows for the two LEDs.
• hardware/enclosure/stick.py makes the shell, the board outline and the placement file, and checks the fit.

8. Compliance (only if you sell it)¶

• Connecting equipment to the telephone network is regulated: in the US, FCC Part 68 / TIA-968 registration (the ACTA database) and a ringer equivalence number; in Europe the radio and low-voltage rules with EN 62368-1 and the telecom network interface standards; elsewhere each country's own. The Si3056 is designed to meet them all ("globally compliant"), but the product still needs testing.
• Safety: UL/EN 62368-1 (formerly 60950) for the barrier and the overvoltage tests; the datasheet's appendix lists what passes.
• None of this applies to building your own, but use it on your own line and at your own risk.

9. Open questions¶

1. The Si3056 is "not for new designs" at Skyworks (October 2026: DigiKey about 1,400 Si3056-D-FS and 2,600 on reel; Si3056-FS obsolete). Fine for building these; for a long run, buy ahead. The fallback is the Si3050 (active, same line side), at 8 kHz only: V.34 and below, no V.90.
2. The RJ11 (Ckmtw R-RJ11R06P-A800, LCSC C189750) is 13 mm tall, so the stick is about 18-19 mm thick, against a lighter's 15. A mid-mount jack would save 3-4 mm.
3. Whether to run the samples isochronous (as the SmartUSB56) or bulk: isochronous keeps D-Modem's driver as it is; bulk is simpler on Android without libusb.

10. Next steps¶

1. Netlist and schematic (--variant line).
2. The shell (stick.py), then the PCB.
3. Firmware: the Si3056's serial port in PIO, the USB interface.
4. D-Modem's driver, against a fake device first (as fake_st7554).