ElPiHat

Introduction

The HB9UF EchoLink Hat is a hat for the Raspberry Pi 3 and 4. It is designed to connect a transceiver to a Raspberry Pi to implement a link transceiver, but can also be used for APRS and any other application where a radio needs to be connected to a Pi. The board features a sound card, a DC/DC converter and some I/O circuitry to interface with the transceiver. A hardware timout-timer is included to clear the PTT line if the Raspberry Pi asserts PTT indefinitely.

Sound Card

The sound card is based on a Cirrus Logic WM8731SEDS Audio CODEC. The line input and line output (both stereo) are routed to JST XH connectors. The I2S bus is routed to the Raspberry Pi expansion header after series-termination with 33 Ω. The microphone input and headphone output are left unconnected, but a small header (1 mm pitch) is provided for prototyping.

The sound card works with a Raspbian stock image (see below).

DC / DC Converter

Since the transceiver will be powered from an external power supply anyway, it makes sense to derive the supply for the Pi from that same power supply. The board is therefore equipped with a RECOM RPM-2.0 2 A DC/DC converter that can be powered from a wide range of input voltages (3 V to 17 V). The output voltage can be trimmed with external resistors (R2 and R3). In Rev. 1.0 of the board, the voltage is slightly incrased to 5.1 V. Rev 1.1 further increases the voltage to 5.25 V. Make sure to read the errata below if you have Rev. 1.0. Input power is provided with a JST XH connector; a crowbar circuit takes out the 2 A SMD fuse in a reverse polarity condition. However, it is critical not to power the Raspberry Pi via the ordinary USB supply when the Hat is plugged in. Starting from Rev 1.1, the Pi can be USB-powered with the Hat inserted, but it is critical not to provide power via the DC/DC converter and the USB power supply simultaneously.

Note that the package of the RECOM part may be difficult to solder by hand, thus in case you decide to order PCBs from a board manufacturers, yu may want to order a stencil from them as well.

I/O circuitry

The PTT line is controlled with GPIO25 on the Raspberry Pi and is active high. It used to be possible to echo 25 to /sys/class/gpio/export and to then assert PTT manually using /sys/class/gpio/gpio25/value. You can still do that on recent versions of linux, but you have to add 25 to the base of gpiochip0, which turns out to be 512+25=537:

$ echo 537 > /sys/class/gpio/export
$ cd /sys/class/gpio/gpio537
$ echo 1 > value # Assert PTT
$ echo 0 > value # Clear PTT

However, this is not recommended anymore. Instead, you should use gpiod:

$ gpioset 0 25=1 # Assert PTT
$ gpioset 0 25=0 # Clear PTT

This only works as long as the pin is not exported using the deprecated sysfs interface. gpioinfo returns a summary of the current state of all gpio pins.

Asserting PTT activates a transistor configured in open-drain mode, thus the PTT pin on the JST XH connector is grounded when PTT is asserted, and Hi-Z otherwise. A 100 Ω series resistor and a diode are used for protection.

Quasselsperre is used to limit the duration for which PTT can remain active. This duration is set with the potentiometer in 15 minutes increments. Values between 15 minutes and 2 hours can be programmed. The timer is set during startup and the LED flashes once for each 15 minutes. (I.e. two flashes amount to a timer of 30 minutes). The timer is only set during startup. The LED starts to blink when the timer trips, and transmit is disabled until the Raspberry Pi clears PTT. Starting from Rev. 1.1: In case this facility is not used, the PTT input can be connected directly to the switching transistor by populating R5.

Please refer to the README file of our Quasselsperre project for further information on customizing, programming etc. In case this feature is undesired, the microcontroller can be omitted. In this case, you have to bypass the microcontroller with a jumper bridge.

The board also features a TOS line to report Tone Operated Squelch or Carrier Operated Squelch (TOS/COS) back to the Raspberry Pi. This is routed to GPIO24. There is some amount of input protection circuitry that can be used to reduce the voltage to 3.3 V. Remember that the Raspberry Pi is not 5 V tolerant.

Schematic and BOM

Here is an image of the schematic. A pdf version is available in the Release section of the github page.

An interactive BOM is available here for Rev 1.1, and here for Rev 1.0. You can use it to populate the board and copy the BOM to the clipboard to order parts from DigiKey. Note that we keep a local stock of general purpose parts, thus we only added part numbers for specialized parts.

Setup

To enable the sound card, add the following entries to /boot/config.txt and reboot the Raspberry Pi:

dtparam=i2c_arm=on
dtparam=i2s=on
dtoverlay=i2s-mmap
dtoverlay=rpi-proto

Test

For the first test do not solder the connector sockets to the PCB.

Here is an image of the 40pin Pi-GPIO Connector.

(1) Connect IN to 12VDC (limited to 10mA). Measure with DMM at Pin2 on 40pin Pi-GPIO Connector:

• There should be 5.25VDC
• Blue PGOOD/D2 LED should be constant on
• If measured voltage is lower (eg. 4.9VDC), look at R3
• If there is 0VDC, look at FB1

(2) Connect a second 3.3VDC voltage (limited to 10mA) to Pin17 on 40pin Pi-GPIO Connector:

• Red PTT/TOT/D3 LED blinks. Disconnect 3.3VDC voltage, change Poti position, connect 3.3VDC voltage again to Pin17.
• Red PTT/TOT/D3 LED blinks different times. The red PTT/TOT/D3 LED blinks once per 15 minutes Quassesperre. Threshold can be set here with this Poti.

(3) If everything OK, solder all connecors and 40pin Pi-GPIO Connector

(4) Start Pi with 12VDC via ElPiHat (1A limit) Caution! Do not power Pi via USB simultaneously!

(5) As soon Pi has booted: a) Test COS/PTT functionality with GPIO The PTT line is controlled with GPIO25 on the Raspberry Pi and is active high:

$ echo 25 > /sys/class/gpio/export
$ echo out > direction
$ cd /sys/class/gpio/gpio25
$ echo 1 > value # Assert PTT
$ echo 0 > value # Clear PTT

Asserting PTT activates a transistor configured in open-drain mode, thus the PTT pin on the JST XH connector is grounded when PTT is asserted, and Hi-Z otherwise. A 100 Ω series resistor and a diode are used for protection.

b) Enable sound card by adding the following entries to /boot/config.txt and reboot the Raspberry Pi:

dtparam=i2c_arm=on
dtparam=i2s=on
dtoverlay=i2s-mmap
dtoverlay=rpi-proto

c) Record and (re)play samples to test audio.

1. Start alsamixer, hit F6 and activate sound card 1 (snd_rpi_proto). Activate the Output Mixer HiFi item and set CAPTURE for the Line item.
2. Run arecord -V stereo -fdat -D 'hw:CARD=sndrpiproto,DEV=0' > /dev/null and observe the VU meter while injecting a signal into the left and the right channel. Once this works, arecord can be used to record a sample (e.g.arecord test.wav) for playback in the next step.
3. Play back the sample using aplay -D 'plughw:CARD=sndrpiproto,DEV=0' test.wav

License

This project is released under the terms of the CC-BY-SA license. © 2020 - 2021.