-c51a4a){kind=icon}
A Raspberry Pi Pico W based USB CDC ⇄ WiFi modem with Hayes AT-style commands and a LED indicator — now with TLS/SSL termination inside the dongle.
The host (e.g. an Oric 8-bit computer via LOCI) talks plain serial over USB; the Pico W handles the network — including the full HTTPS handshake, decryption and optional certificate verification. No extra or custom hardware is required: a bare Pico W is enough.
About this fork — adds TLS/SSL termination (mbedTLS) and the related AT commands on top of sodiumlb/PicoWiFiModemUSB, itself based on the Pico WiFi modem.
- Project lineage
- Features
- Architecture
- Installation
- First-time setup
- TLS / HTTPS
- Command reference
- Building from source
- Notes
- References
- Acknowledgements
- License
This firmware stands on the shoulders of a long line of retro WiFi modems. Direct ancestry (this fork is at the bottom):
- jsalin/esp8266_modem — Jussi Salin's original ESP8266 virtual modem.
- mecparts/RetroWiFiModem → mecparts/PicoWiFiModem — Paul Rickards / mecparts lineage, ported to the Pico.
- sodiumlb/PicoWiFiModemUSB — direct parent: native USB-CDC variant for the Pico W, designed to pair with LOCI.
- this fork (benedictemarty/PicoWiFiModemUSB) — adds TLS/SSL termination (mbedTLS) and the
ATGET https,AT$CAandAT$CVcommands.
jsalin/esp8266_modem
│
mecparts/RetroWiFiModem ──▶ mecparts/PicoWiFiModem
│
sodiumlb/PicoWiFiModemUSB (USB-CDC, Pico W, LOCI)
│
benedictemarty/PicoWiFiModemUSB ◀── this fork (+ TLS/SSL)
Other influences (ESP8266 virtual modems by Stardot and Roland Juno, WiFi232, etc.) are listed under References.
- Native USB CDC serial interface (no RS-232 hardware needed).
- Hayes AT command set (dial, answer, speed dial, Telnet, status…).
- WiFi station mode (2.4 GHz) with credentials stored in flash.
- TLS-terminated HTTPS:
ATGET https://…performs the handshake and returns the decrypted page. - Certificate verification (
AT$CV1) against a user-provided CA (AT$CA=) — refused unless a CA is present, so it never fails open. - On-board flash storage via LittleFS (settings + CA), in the area not used by the firmware.
The Pico W acts as a modem and as the TLS endpoint. The host computer never sees encrypted traffic: TLS is set up, verified and torn down inside the dongle, so even an 8-bit machine can reach HTTPS services.
Oric 8-bit LOCI Pico W modem Internet
┌────────────┐ bus ┌────────┐ USB CDC ┌────────────────┐ WiFi ┌──────────┐
│ host / │◀──────▶│ LOCI │◀───────▶│ PicoWiFiModemUSB│◀──────▶│ server │
│ software │ │ │ serial │ + mbedTLS │ TLS │ (HTTPS…) │
└────────────┘ └────────┘ └────────────────┘ └──────────┘
└─ cleartext serial ─┘ └──── TLS terminated here ────┘
Role of LOCI — LOCI is the USB host interface for Oric 8-bit computers. It carries the plain AT-command / data serial stream between the host and the modem over USB. LOCI is transparent with respect to TLS: it only ever handles already-decrypted bytes, so adding HTTPS support required no change on the LOCI side — all the crypto lives in this firmware. (Any USB-CDC capable host works the same way; LOCI is simply the Oric-oriented one.)
A pre-built firmware is attached to each release.
- Unplug the Pico W, then plug it back in while holding the BOOTSEL button.
It appears as a USB drive named
RPI-RP2. - Copy
wifi_modem-vX.Y.Z.uf2onto that drive. - The Pico reboots automatically and enumerates as a USB serial port
(
/dev/ttyACM*on Linux, a COM port on Windows).
Connect with any serial terminal (picocom -b 115200 /dev/ttyACM0,
minicom, PuTTY…). USB CDC ignores the actual baud rate, but the AT-level
AT$SB speed must match your terminal for UART-style setups.
Reflashing without the BOOTSEL button (1200-baud touch). Opening the serial port at 1200 baud reboots the Pico straight into BOOTSEL (
RPI-RP2), so you can drop a new.uf2without unplugging. E.g.stty -F /dev/ttyACM0 1200(or any terminal opened at 1200 bps), then copy the firmware to the mounted drive.
Default serial configuration: 9600 bps, 8 data bits, no parity, 1 stop bit.
AT$SSID=your WiFi network name set the network to join at power-up
AT$PASS=your WiFi network password set the password (case sensitive)
ATC1 connect now
AT&W save settings to NVRAM
Optional:
AT$SB=speed default serial speed AT$SU=dps data bits / parity / stop bits
ATNETn Telnet protocol mode AT&Kn RTS/CTS flow control
AT&Dn DTR handling
After AT&W, the modem auto-connects on power-up and is ready to dial out with
ATDT or fetch pages with ATGET.
HTTPS is terminated in the dongle: the Pico W performs the TLS 1.2 handshake with mbedTLS, then streams the decrypted response over the serial link.
Fetch a page over TLS:
ATGEThttps://example.com
By default verification is off (AT$CV0, insecure: any server certificate is
accepted). To verify the server certificate against a trusted root:
AT$CA= then paste a CA certificate in PEM form,
-----BEGIN CERTIFICATE-----
… finish with a line containing only a single '.'
-----END CERTIFICATE-----
.
AT$CV1 enable verification (returns ERROR if no CA is stored)
AT&W persist
With a CA loaded and AT$CV1 enabled, a connection whose certificate does not
chain to that CA (expired, self-signed, unknown root, wrong hostname) is
refused. AT$CA? reports the stored CA size; AT$CA- deletes it.
Note: there is no on-board clock, so certificate expiry dates are not checked; trust is established via the CA chain and hostname (SNI).
Multiple AT commands can be typed on a single line. Spaces between commands are
allowed, but not within commands (e.g. ATS0=1 X1 Q0 is fine; ATS 0= 1 is
not). Commands taking a string argument (e.g. AT$SSID=, AT$TTY=) consume
everything that follows, so no command may follow them.
| Command | Details |
|---|---|
| +++ | Online escape code: one second pause, three +, another one second pause — returns to local command mode. |
| A/ | Repeats the last command entered. Do not type AT or press Enter. |
| AT | The attention prefix preceding all commands except A/ and +++. |
| AT? | Displays a help cheatsheet. |
| ATA | Force the modem to answer an incoming connection. |
| ATC? ATCn |
Query/change WiFi connection status. 0 = not connected, 1 = connected. ATC0 disconnects, ATC1 connects. |
| ATDSn | Calls the host in speed dial slot n (0-9). |
| ATDT[+=-]host[:port] | Establish a TCP connection to host/IP (default port 23, Telnet). Speed-dial by alias or 7 identical digits. A leading +/=/- overrides the ATNET setting for the call (+ fake Telnet, = real Telnet, - no Telnet). Pressing a key before connect aborts. |
| ATE? ATEn |
Command-mode echo. E0 off, E1 on. |
| ATGEThttp://host[/page] ATGEThttps://host[/page] |
Fetch and display a web page, then close. https is TLS-terminated by the modem (see TLS / HTTPS). |
| ATH | Hang up the current connection. |
| ATI | Display network status: build date, WiFi/call state, SSID, RSSI, IP, bytes transferred. |
| ATNET? ATNETn |
Query/change Telnet mode. 0 = off, 1 = real Telnet, 2 = fake Telnet. See note on real vs fake below. |
| ATO | Return online (use with +++ to toggle command/online modes). |
| ATQ? ATQn |
Result codes. Q0 display, Q1 suppress (quiet). |
| ATRD ATRT |
Display current UTC date/time from NIST (YY-MM-DD HH:MM:SS). Requires WiFi and no active call. |
| ATS0? ATS0=n |
Rings before auto-answer. S0=0 = don't answer. |
| ATS2? ATS2=n |
ASCII code of the online escape character (default 43 = +). 128-255 disables the escape. |
| ATV? ATVn |
Result codes as words (V1) or numbers (V0). |
| ATX? ATXn |
Result code verbosity. X0 basic (CONNECT, NO CARRIER), X1 extended (with speed / connect time). |
| ATZ | Reset the modem. |
| AT&D? AT&Dn |
DTR-inactive handling: &D0 ignore, &D1 command mode, &D2 end call, &D3 reset. |
| AT&F | Reset NVRAM and current settings to defaults (SSID, password, speed dials included). |
| AT&K? AT&Kn |
Flow control: &K0 off, &K1 hardware (CTS/RTS). |
| AT&R? AT&R=server pwd |
Query/change the password required for incoming connections (3 tries in 60 s). |
| AT&Vn | Display current (&V0) or stored (&V1) settings. |
| AT&W | Save current settings to NVRAM. |
| AT&Zn? AT&Zn=host[:port],alias |
Store up to 10 speed-dial entries. Example: AT&Z2=particlesbbs.dyndns.org:6400,particles → dial with ATDS2, ATDTparticles or ATDT2222222. |
| AT$AE? AT$AE=startup AT cmd |
Query/change the command line executed at startup. |
| AT$AYT | Send a Telnet "Are You There?" if connected to a Telnet remote. |
| AT$BM? AT$BM=server busy msg |
Query/change the message returned to an incoming connection while busy. |
| AT$CA? AT$CA= AT$CA- |
TLS CA management. ? reports the stored CA size (bytes), = uploads a CA in PEM form (end with a line containing only .), - deletes it. See TLS / HTTPS. |
| AT$CV? AT$CVn |
TLS certificate verification. 0 = off (insecure, accept any cert), 1 = on. AT$CV1 returns ERROR unless a CA is stored. |
| AT$MDNS? AT$MDNS=mDNS name |
Query/change the mDNS network name. With a non-zero TCP port set, reach it via telnet mdnsname.local port. |
| AT$PASS? AT$PASS=WiFi pwd |
Query/change the WiFi password (case sensitive, max 64 chars). Set empty to clear. |
| AT$SB? AT$SB=n |
Query/change baud rate: 110, 300, 450, 600, 710, 1200, 2400, 4800, 9600, 19200, 38400, 57600, 76800, 115200. Must match your terminal. |
| AT$SCAN | Scan for nearby 2.4 GHz WiFi networks. Lists one access point per line as <index> <ssid> (1-based, de-duplicated by SSID, hidden SSIDs skipped), then OK. The index is meant to drive a numbered menu whose choice is fed back via AT$SSID=. See oric/wificonf.bas for an Oric configurator that uses it. |
| AT$SP? AT$SP=n |
TCP server port to listen on (0 = disabled). |
| AT$SSID? AT$SSID=ssid |
Query/change the SSID (case sensitive, max 32 chars). Set empty to clear. |
| AT$SU? AT$SU=dps |
Data bits (5-8), parity (N/O/E), stop bits (1-2). Default 8N1. |
| AT$TTL? AT$TTL=telnet location |
Telnet SEND-LOCATION value (default "Computer Room"). |
| AT$TTS? AT$TTS=WxH |
Telnet NAWS window size (default 80x24). |
| AT$TTY? AT$TTY=terminal type |
Telnet TERMINAL-TYPE value (default "ansi"). |
| AT$W? AT$W=n |
Startup wait. $W=0 no wait, $W=1 wait for Enter at startup. |
Real vs fake Telnet — with real Telnet a CR sent by the modem is followed by a NUL; some BBS implementations don't strip that NUL (use fake Telnet for those, e.g. Particles!). Inbound, real Telnet strips a NUL following a CR; fake Telnet passes it through.
The Pico SDK and dependencies are vendored under src/. With the ARM toolchain
installed:
cmake -S src -B src/build
cmake --build src/build -j
The firmware is produced at src/build/wifi_modem.uf2. A Wokwi test variant
(UART console instead of USB CDC) can be built with -DWOKWI=1.
See CHANGELOG.md for version history.
If you sz a binary file from a Linux box over a Telnet connection and the link
drops at a reproducible point, it is the Linux telnet daemon, not the modem:
it chokes on blocks containing many 0xFF bytes. Xmodem and Ymodem (128-byte
blocks) work; Zmodem does not. Modem-to-modem transfers are unaffected.
- Retro WiFi Modem
- WiFi232 — An Internet Hayes Modem for your Retro Computer
- WiFi232's Evil Clone
- Jussi Salin's virtual modem for ESP8266
- Stardot's ESP8266 based virtual modem
- Roland Juno's ESP8266 based virtual modem
- Jussi Salin for releasing the original ESP8266 virtual modem code.
- Paul Rickards for the hardware inspiration.
- The Stardot contributors.
- sodiumlb for the USB-CDC Pico W port and LOCI.
- And Dennis C. Hayes, for a command set simple and elegant enough to outlive the modems it was made for.
GNU General Public License v3.0 — see the source headers. This is a derivative work of the projects listed above, distributed under the same terms.