ModemHAT

Modem HAT for the Raspberry Pi

This HAT is a modem that connects the Raspberry Pi to a plain old analog telephone line.

The modem presents itself to the Raspberry Pi on an UART and is controlled with the classic AT command set. It supports various standards (V.22bis, V.32bis, V.34, V.90, ...) at speeds of up to 56 kbps.

The design is based on a Skyworks ISOmodem chipset, specifically Si2457 data modem and Si3018 line side driver. The implementation closely follows the circuits described in application note AN93.

The Modem HAT was successfully tested on the Shadytel phone network at Toorcamp 2022.

[!CAUTION] Whatever your jurisdiction, we're pretty positive that connecting this device to the public phone system IS NOT LEGAL! Don't blame us if you get into trouble.

Raspberry Pi Configuration

[!NOTE] This information was tested on Raspberry Pi 4 and Raspberry 5, running Raspberry Pi OS 12 (Bookworm). There may be differences for other versions of the Pi and/or Pi OS.

The modem HAT requires a Raspberry Pi 4 or newer.

The modem communicates through the Pi's UART on GPIO 8-11, and requires CTS and RTS for flow control. The modem supports automatic baudrate detection at speeds up to 307.2 kps.

Before first use after power-up, the modem needs to be reset. To reset the modem, GPIO25 (pin 22) is set low for at least 500ms, then set high, followed by a delay of at least 300ms.

By default, the modem is configured for operation in the United States. For other countries the modem may have to be reconfigured after reset. See AN93 chapter 6.2 for details.

Below are the configuration steps for Raspberry Pi 4 and Raspberry Pi 5.

Raspberry Pi 4

To enable the UART, edit /boot/firmware/config.txt and add the following line at the end of the file:

dtoverlay=uart4,ctsrts

After rebooting the Raspberry Pi, the UART is mapped to /dev/ttyAMA4.

To reset the modem, run the Python script reset-pi4.py.

The modem is now ready to use. For example connect to the modem by running:

screen /dev/ttyAMA4 115200

Once connected to the modem, if you type at and press enter, the modem should respond with OK.

Raspberry Pi 5

To enable the UART, edit /boot/firmware/config.txt and add the following line at the end of the file:

dtoverlay=uart3-pi5,ctsrts

After rebooting the Raspberry Pi, the UART is mapped to /dev/ttyAMA3.

To reset the modem, run the Python script reset-pi5.py.

The modem is now ready to use. For example connect to the modem by running:

screen /dev/ttyAMA3 115200

Once connected to the modem, if you type at and press enter, the modem should respond with OK.

AT Command Set

The modem supports the basic Hayes command set.

For a complete list of supported AT commands as well as configuration registers refer to AN93 chapters 5.4 through 5.7.

Raspberry Pi GPIO Pinout

Required connections:

Optional connections:

In the standard configuration of the HAT, the modem boot config and interrupt pins are not electrically connected to the Rasbperry Pi GPIO header. Populate R29-R32 with 0-ohm resistors to use these pins.

Building Your Own HAT

General

Large parts of the HAT are based on the hardware design reference in application note AN93 chapter 4. Where applicable, identical component designators are used to simplify cross-referencing.

I used solder paste, stencil and reflow oven for assembly of the HAT. I also ordered partially assembled boards from JLCPCB, and manually soldered the parts that they didn't have in stock (modem chipset and high-voltage capacitors).

After soldering the through-hole parts (RJ11 connector, speaker), make sure to clip the leads on the bottom side of the HAT to avoid shorting with metal parts on the Raspberry Pi.

Modem

The modem chipset consists of the digital part Si2457 (U1) and analog part Si3018 (U2).

These are expensive parts, costing US $13-$16 and $6-$8 respectively when purchased from reputable distributors. While tempting, beware of sources like Ebay and AliExpress as parts may be salvaged, old or even fake. It will be much easier to diagnose problems without having to second guess whether the core components of the HAT are genuine.

The HAT uses a 32.768 kHz crystal (Y1) to clock the Si2457, mainly chosen for small size and because I already had a few in my parts bin. The downside of this choice are the slow reset sequence (800 ms) and that this clocking option is only supported by Si2457 revision D (part number SI2457-D-FT) or newer.

If you suspect that you're stuck with an old part, you could try using a 4.9152 MHz crystal instead. In that case you must remove R17 which configures the clock speed. You may also have to replace the load capacitors (C40, C41) with appropriate values. See AN93 chapter 4.1.3 for details.

On the analog side, most of the resistors, capacitors, diodes and transistors have critical voltage, current and/or power ratings. These are all called out in the schematic. See bill of material in AN93 chapter 4.3 for full details.

For bring-up and troubleshooting of the modem portion of the circuit, refer to AN93 chapter 10.1.

Audio Output / Speaker

The audio output is optional and allows monitoring of connection progress via a speaker, and who doesn't love the old-school sounds of two dial-up modems negotiating a session. It's half the reason to build this project! :)

The audio amplifier LM4819 (U4) can be substituted with the higher power, pin-compatible LM4862. Cheap, off-brand substitutes are probably fine too.

The HAT has a footprint for a 15mm speaker, like for example CUI Devices CVS-1508. Alternatively, footprint J3 provides connectivity via a JST-PH connector, or directly soldering speaker wires to the pads.

If audio is not needed, the audio amplifier (U4) and associated parts (C12-C19, R19-R24, SW1, J3) can be left unpopulated.

To enable audio output on the speaker, make sure MUTE switch is set towards the LEDs. The AT command to enable speaker output while dialing and handshaking is ATM1. Speaker volume is controlled with ATL1 through ATL4, where 4 is the loudest.

Modem Boot Config EEPROM

The modem boot config EEPROM is optional and can store AT commands that the modem automatically runs after reset. See AN93 chapter 2.7 for details.

The EEPROM can be programmed by the modem with AT commands. For hacking, the EEPROM can also be connected to the Raspberry Pi by populating R29, R30 and R31. For the latter, keep the modem in reset while interacting with the EEPROM.

If this functionality is not needed, the EEPROM (U6) and associated parts (C22, R26, R27, R28) can be left unpopulated.

Raspberry Pi HAT ID EEPROM

The HAT ID EEPROM is optional and can store information that allows the Raspberry Pi to recognize the HAT and load the required device tree overlay, i.e. automatically configure UART and other pins.

I haven't figured out the content for this EEPROM yet. Specification is here. Contributions are welcome :)

If this functionality is not needed, the EEPROM (U5) and associated parts (C21, R14, R25) can be left unpopulated.