Mohawk

Mohawk

Mohawk is a Raspberry Pi Zero µHAT-sized UHF/VHF radio interface built around a DRA818 radio module which comes in a UHF and a VHF variant (DRA818U / DRA818V). It brings the radio's audio, configuration UART, and push-to-talk control to a Raspberry Pi, and provides an RS-232 port for an external device. The current design is a KiCad 10 schematic and two-layer PCB. Hardware validation has not been recorded in this repository.

Linux has drivers for both the SPI UART interface and the codec, so two hardware TTYs show up and the codec shows up as a regular alsa soundcard.

Project files

The schematic and board were saved with KiCad 10. Custom footprints use ${KIPRJMOD}/lib/KiCADv6/footprints.pretty.

Hardware overview

The BoM will be finalized once a verified working prototype has been assembled and specific part variants have been nailed down and confirmed good.

The Pi's SPI0 bus is shared by the UART bridge (CE1) and codec (CE0). The codec's digital audio uses the Pi's PCM/I²S pins. The bridge's two UART channels connect separately to the DRA818 and ‎MAX3232, leaving the Pi's built-in UART pins available on the 40-pin header. The codec connects to the radio's analog input and output; its GPIO_MFP5 pin drives the radio's PTT net. The codec's right headphone output feeds the two-pin speaker connection through a coupling capacitor.

Functional Diagram

External connections

Pin numbers below come from the current schematic/netlist. Check the PCB pad numbering and your cable orientation before attaching hardware.

J3 carries 5 V power but uses the Pi's 3.3 V GPIO/I²C signals. GPIO17 Is brought out for use as an interrupt, but is just a GPIO pin, so it could be used for an idicator LED or a button, etc. Use the netlist/pin table above when wiring a UI board.

There is no USB UART or 2 × 5 IDC RS-232 connector in the current schematic. The external serial connection is the ‎MAX3232 and J4's five SMD pads.

The HAT ID EEPROM uses ID_SDA/ID_SCL on J1 pins 27/28. Those are separate from the ordinary I²C1 pins used at J3. The Device Tree overlay is present but has not been tested on hardware.

Control signals on the Pi header

The radio's PTT is connected to the codec's multifunction GPIO rather than directly to a Pi GPIO. Software must configure that codec pin before attempting to transmit. These are schematic connections, not a tested Linux setup or a promise of a particular /dev/tty* assignment.

Device Tree overlay

The overlay targets the Raspberry Pi Zero family. It enables SPI0 with the codec on CE0 and the SC16IS752 on CE1, enables I²S, and creates a simple ALSA sound card. UART channel A connects to the DRA818; channel B connects to the MAX3232. The bridge's interrupt and reset use GPIO25 and GPIO5. The codec reset uses GPIO27.

Fit R15 (10 Ω) before using the codec. The schematic marks it do-not-populate, but it is the only connection from GPIO4/GPCLK0 to the codec MCLK pin. The overlay supplies a 12.288 MHz master clock on that pin. With R15 absent, the audio clock is disconnected regardless of Device Tree configuration.

On a Raspberry Pi with the kernel Device Tree headers installed, compile and install the overlay:

cpp -nostdinc -undef -D__DTS__ -I /lib/modules/$(uname -r)/build/include -x assembler-with-cpp mohawk-overlay.dts /tmp/mohawk-overlay.dts
dtc -@ -I dts -O dtb -o mohawk.dtbo /tmp/mohawk-overlay.dts
sudo cp mohawk.dtbo /boot/firmware/overlays/

Add dtoverlay=mohawk to /boot/firmware/config.txt and reboot. On systems using /boot/overlays and /boot/config.txt, use those paths instead. The HAT ID EEPROM is not programmed by this overlay; add the dtoverlay line explicitly until it is.

The codec's MFP5 pin is initialized high so the radio's active-low PTT stays released. Its ALSA MFP5 GPIO control can later drive it low to transmit; ensure the control is set high again when transmission ends. The overlay also describes the radio receive audio on IN1_L, transmit audio on LOL, and the monitor headphone output on HPR. Audio mixer routing and levels still need to be set in ALSA and verified on a prototype.

Design and build status

The PCB is routed in the saved design, and KiCad reports no unconnected items. A KiCad 10 check of this checkout found 26 ERC violations, 59 PCB DRC violations, and two schematic/PCB footprint-name parity warnings. The DRC findings include silk overlap, copper-to-edge and hole-clearance errors, alongside warnings about copper islands, dangling copper, footprint library matches, and text thickness. Review and resolve the reports in KiCad before ordering boards. The schematic also marks R5, R10, R14, and R15 as do-not-populate.

To rerun the checks and export a BOM from the project root:

kicad-cli sch erc -o /tmp/mohawk-erc.rpt mohawk/mohawk.kicad_sch
kicad-cli pcb drc --schematic-parity -o /tmp/mohawk-drc.rpt mohawk/mohawk.kicad_pcb
kicad-cli sch export bom -o /tmp/mohawk-bom.csv mohawk/mohawk.kicad_sch

Build Notes

U1 (the I²C EEPROM) and U4 (MAX3232) are technically optional. You do not need the MAX32322 or its supporting passives if you do not need RS-232 support. The I2C EEPROM lets a DeviceTree overlay be automatically loaded at boot so linux kernel modules know what the pin assignments are, but this is just a convenience and also not strictly required.

Legal Notice

Neither this device nor the DRA818 module is, obviously, certified by the FCC. Only a suitably licensed individual may transmit using this device: you must have an amateur radio license. It is up to the operator to ensure they are following FCC requirements.