The AVR Beacon Super Keyer

By Murray Greenman ZL1BPU

Description of an HF beacon keyer with telemetry. What makes the keyer rather different is its versatility - it is a multi-mode unit, with ASK and FSK modulation, sending Feld-Hell and Morse on command.


Specifications     Keyer Schematic     Keyer Commands     Purchasing Details

What the Keyer Does

The Super Keyer prototype Morse code is an obvious choice for a beacon, since is is universally understood, provides good performance at low power, is simple to transmit and to understand. But why Hellschreiber (Feld-Hell)? Well, Hell is also a very simple mode, since it is also on-off keyed like Morse. Hell is also "human readable", but by eye rather than by ear.

With Hellschreiber it is also possible to monitor the signal for long periods without effort, and record the data as a graphics file. Hellschreiber is also very robust and works well in noise at relatively low power. The transmissions are only about 250 Hz wide.

The keyer will send both "Normal" and "DX mode" Feld-Hell, at the standard 122.5 bits/second, as well as half-speed, quarter-speed and eigth-speed. The slower speeds are less affected by multi-path, and are also less affected by noise, which is helpful when low power transmissions are involved. Slow speeds are no problem for normal beacon use, since little data is sent. PC software is available to support reception of all these modes.

Morse code mode also has four speeds, about 20 WPM, 10, 5 and 2.5 WPM. These speeds were chosen for an important reason - they are "Hell compatible", in that the time of one dot is the same as the time for one column of Hell data. This means that the Morse can be "read" off the screen visually (it looks like a bar code), and it also means that the timing of the Hell mode is preserved, so the phase of the Hell text either side of a Morse sequence remains the same. The lower speeds can be read with great sensitivity by using a spectrogram technique, ideal for LF operation.


Hell compatible Morse code telemetry
(it says DE ZL1BPU BCN 1/3FF 2/13F AR)

The single message can have a mix of all these modes, and telemetry can be sent in Morse as well as in Hellschreiber, and modes changed during the message. Outputs can also be controlled by the message, and used to change transmitter power or perhaps a choice of antennas or operating frequencies during the message. The message can be any length up to 255 characters (including commands), which is about three lines of text and data, or the "quick brown fox" sent four times! With such a large message memory, and the multiple mode message flexibility, it is easy to create several messages to be sent in a sequence in different modes.

The message can be changed at any time, by removing a link and rebooting the beacon processor (momentarily removing power). Once the new message has been "learned", which takes just a few seconds, the keyer will start again, and once the link is restored, will remember the message forever, even if power is lost.

The telemetry is transmitted in any format the user wishes, so for example only some channels can be used, they can be reported in any order, and with any associated text, for example: "BATTERY 13D HEATSINK 0A8 AMBIENT 054". High resolution is provided, almost as good as a typical digital voltmeter (0 - 1023 rather than 0 - 1999). The readings are reported in hexadecimal, so the three digits need to be converted to decimal, using the formula:

Decimal reading = D1 x 256 + D2 x 16 + D3
where Dn are the three HEX "digits" from left to right. Of course HEX counts "0123456789ABCDEF", so in the above formula use A=10, B=11 and so on.

The message can contain a mixture of text and commands, which are two character combinations starting with "$", for example "$1" is Hell mode (the default mode), $B sends the least significant "digit" of the data for A-D channel 0, and $7 turns on a digital output. And yes, "$$" sends the "$" character! Here's a typical message:

$1 TELEMETRY 0: $@$A$B 1: $D$E$F 2: $H$I$J 3: $L$M$N 4: $P$Q$R 5: $T$U$V
D: $C$G$K$O $4 73 DE ZL1BPU $0 DE ZL1BPU BCN 1/$@$A$B 2/$D$E$F + ~
which sends a message like the one shown further down the page.

Specifications

Check out the specifications.

Block Diagram

The keyer is really simple. As shown in the diagram below, it consists of a micro controller, a regulator chip, input and output connectors and some simple interface circuits to protect the micro. The crystal oscillator in the micro uses a readily available 3.58 MHz crystal, which could also (with suitable buffering) act as the reference for an 80m QRP transmitter. This frequency was chosen because it allows accurate timing of Hellschreiber elements.

The Analog and Digital Inputs can be to screw terminals, or whatever is convenient. The Radio Port can also be screw terminals, although most transceivers will require some extra circuitry, such as a pot to attenuate the audio, a receiver audio detector to inhibit transmit, and so on. The simple RS232 port is only used to change the message, and consists of two bipolar transistors. The picture below shows a typical telemetry message mostly in Hellschreiber mode. Can you read the Morse code at the end? (Hint - enlarge the image!)


A typical telemetry message

Keyer Applications

The keyer has been designed primarily as an LF - HF telemetry beacon primarily for the bands 160 - 30 metres. It is already set up to allow flexible messaging and data reporting, and to provide switchable antennas, frequencies or transmitter power levels. It has also be used quite satisfactorily with VHF beacons including one with awitched power levels. With some adaptation (different software) it could be used for:

General Description

The keyer is based on an AVR AT90S4433 processor running at 3.58 MHz. Take a look at the Schematic, and see the Specifications for more detail.

Port C of the micro is used for analog inputs which are reported as telemetry in the beacon message, and can be configured for a range of sensors.

Port B is used to program the micro, and to drive the Radio Port (connects to the rig). U3 is the PTT driver. PTT and start of message can be inhibited by an external input, and then latched low by the micro. The audio output at the Radio Port is AC coupled from the micro, and has two resistors holding an average DC level. When the audio tone stops the output is set tristate, and so no click or thump happens.

Port D is used for the serial control (programming) port, provides two general purpose outputs which can be script controlled, as well as four digital inputs which can be read and report via the telemetry message. U4 and U5 form a simple host-powered RS232 interface.

The message script is stored in EEPROM, and is independent of the program. The message can be up to 255 characters long, including text and commands, and can be changed any time using an RS232 serial cable and a terminal program. Messages can easily be hand typed, or can also be stored on disc and downloaded.

Any mixture of Morse and Hell modes can be used in the message. The format of the telemetry is completely free form. This is achieved through the use of a "command interpreter" which checks each character of the message for commands.

While the message is being transmitted, the same message appears at the RS232 port. Since the message on this port is terminated by <CR><LF>, the RS232 cable can be connected to a Packet Radio TNC so that the telemetry message is relayed by packet on VHF as well!

The font used is the ZL1BPU MOSAIC II 7 x 5 font, as can be seen in the following example. The slant is due to the 1% error in the micro reference oscillator frequency at the time of recording.


The ZL1BPU AVR Keyer in action

Keyer Commands

A simple yet comprehensive set of text-based KEYER COMMANDS allows the mode to be changed, telemetry and text to be transmitted.

Character Sets

The following characters are supported by the keyer:

Hell Mode
ASCII characters 0x20 (32) to 0x5F (95) comprising the numbers, upper case letters, and the following punctuation:

(space) ! " # $ % & ' ( ) * + , - . / : ; < = > ? [ \ ] ^ _

Lower case letters and punctuation above 0x5F (if placed in the message memory) will be converted to upper case and the corresponding symbols by subtracting 0x20 (32) from the ASCII value. The Tilde "~" cannot be sent, as it is used to define the end of the message in record mode.

Morse Mode
Letters and numbers, plus the following punctuation and prosigns:

(space) " $ ' ( ) + , - . / : ; = ? _

The prosigns are + (end of message AR), - (Dash DU), = (Pause BT), and _ (Underline IQ). As with Hell, letters and punctuation above 0x5F will be converted within range where possible. Unsupported characters do not send anything, but will incur a two dot-element delay.

Message Programming

The message is made up using text and commands, (see KEYER COMMANDS) and stored as a file. The programming mode has no error correction, so if something goes wrong, or a bad command is sent, the process will need to be restarted. The file can be sent to the keyer using Windows Terminal or almost any other terminal program that supports 300 baud, no parity, 8 data bits (300N81).

There is no handshaking between the computer and the keyer. Communication is slow so that each byte can be programmed before the next byte is received. This avoids the need for a message buffer and the complex timing of programming and serial communication at the same time. Programming is reliable when the message is hand sent, or if it is preassembled and sent from the terminal program at full speed.

The PB5 input to the micro (Port B, bit 5, pin 19) has been selected to indicate when message recording is required. (This is one of the pins in the programming header). Normally this pin is held low by a jumper. If the micro is powered up with the link to ground in place, the micro interprets this as normal mode, and will start as a beacon when power is applied. If the PB5 pin is left open when power is applied, the beacon goes into record mode, and stays there until it receives a "tilde" command (~), when it reverts to normal mode. If a mistake is made during programming, remove power from the keyer to allow the micro to reset. If you end up in programming mode inadvertently, the first character of the message will be erased, so you will need to send the message again.

As each character is sent and recorded, it is echoed back to the computer. Control characters will be stored and echoed to the computer during record, but are not sent by the beacon. Thus you can embed sequences which will not be transmitted. When the message is complete and tilde (~) has been detected, the keyer will send "73" as it reverts to normal mode. If the "73" is not received, the tilde was not detected. The tilde is stored as an end of message marker, but is not transmitted with the message.

Once the message is complete and the tilde received, the beacon will start. At this point, check that the message is correct, and if you are happy with it, place a jumper from PB5 to ground, then remove and reapply power to check that the beacon operates correctly and has remembered the message.

Message recording only affects the user message, which is stored in electrically eraseable reprogrammable memory (EEPROM) in the micro. It has no effect on the microprocessor instructions, which are stored in the microprocessor program memory, and can only be changed with a special programmer. The Hell and Morse character sets are also stored in the program memory, in a 512 byte table.

Application Examples

The Analog Inputs are quite versatile, so can be connected to many different voltage or resistive transducers. Here are a very few examples.

Software Development

If you are interested in developing your own beacon, consider adapting this design. The AT90S4433 processor is now difficult to find, but the ATMEGA8 is code and pin compatible, and the design is easily adapted to similar devices. The development tools for the AVR family are very good, and are mostly FREE! Make sure you have the latest versions for use with the AT90S4433 or ATMega8.

Where to get the Firmware

It is possible to purchase just the executable code, or both the executables and source code. See the Micro Page for details. The Keyer Schematic can be download here.


Copyright Murray Greenman 1997-2005. All rights reserved. Contact the author before using any of this material.