Issue #5 - Baseboard & Enclosure

The Digital QRP Breadboard project has come a long way in four issues! As chronicled so far, we’ve presented modules for the “brains” (microcontroller), the “eyes & ears” (LCD and keyboard), and the “voice” (precision frequency source). We’ve even created a special reflectometer to measure SWR and a software voltmeter to get the readings into the system. Now it’s time to pull it all together to create the first instance of our full-blown, high-performance Digital Breadboard system. Many readers have asked about the “big picture” for this adventure ... well, here it is!

We'll start out by discussing the enclosure and form factor, and then go through a functional inventory of the whole project. Software is then overviewed, showing how all components are made to dance in unison to create an extremely flexible and powerful device for your ham shack.

Enclosure & Form Factor

A project's “form factor” is the way the hardware modules are physically constructed and integrated as a whole. As the Digital Breadboard evolved over the last 12 months, I described that the various functional modules would be provided as small plug-in daughtercards to help with re-use and upgrade of the specific functions like the DDS, reflectometer, audio amplifier and others. I started laying out the baseboard pc board containing these daughtercards, but it quickly became apparent that this approach would be more expensive as compared to designing a single, integrated PCB containing most of the functionality.

In order to keep the cost of this project as low as reasonably possible, I therefore opted to design most modules directly into the layout for the baseboard, except for the HC908 microcontroller and the DDS chip. The 68HC908AB32 will continue to be provided as a daughtercard – in part for those who have already purchased it along the way, as well as because the microcontroller surface mount package cannot easily be attached by homebrewers. It’s also a very useful module that can be used other projects when kept in this small daughtercard format.

The DSP chip (AD9850 from Analog Devices) is also provided as a small plug-in daughtercard. The card provides the DDS chip, clock circuitry, a 5th-order elliptical low pass filter, an 8dB MAR-3 RF amplifier, and onboard voltage regulation. This board is provided as a daughterboard plug-in to allow the user to change or upgrade to using a different DDS in the future, if desired. Further, the DDS Daughtercard may easily be used in other projects controlled by other microcontrollers or even by a PC over its parallel printer port. (The DDS Daughtercard will be available in January 2003.)

Thus, the Breadboard now consists of a 3" x 5" baseboard containing most of the circuits, connectors/controls, LCD, keypad, and plug-in modules for the HC908 Daughtercard and the DSP daughtercard. This isn’t too much of a deviation from our original course, and I think most will appreciate the lower cost of the ultimate system. To retain the flexibility of being able to replace or upgrade any given module, even though it’s integral with the baseboard, I’ve layed out each module such that its I/O signals are conveniently located in one spot. This way, should you decide to upgrade your audio amp (for example) to a bigger/quieter version, all you’d need to do is slice some traces at the indicated points, insert a pinheader at the module’s I/O pads, and plug your own custom daughtercard onto the baseboard above the existing audio amplifier circuit.

As mentioned on the Home Page, we're trying to make this whole project even more immediately and ergonomically useful, and we've decided to make the first instance of the Digital Breadboard in the form of the "Antenna Analyzer II". The PC board is being layed out to fit into a 3.5" x 7.5" HPL-9VB plastic enclosure from PacTel which is a much better size for a handheld portable measurement instrument. The only Breadboard features we're sacrificing at first is some board space for the extra connectors, prototyping space and the spot for the DSP daughterboard. See the dedicated AntAnal web pages for complete functional description of the capabilities being provided in the first form of the Digital Breadboard kit.

Breadboard Functional Blocks

Referring to the schematics, let’s go through a module-by-module inventory of the Digital Breadboard and see what functionality we have in place as a system.

HC908 Daughtercard – The brains of the Breadboard is contained in this removable microcontroller daughterboard which holds the powerful-yet-inexpensive 68HC908AB32 microcontroller by Motorola, with lots of memory and I/O, and peripherals like counter/timers, asynchronous serial ports, and A/D converters. The HC908 daughterboard also contains the clock, reset pushbutton, voltage regulator and RS-232 drivers. As previously discussed in detail, the software supplied with the project allows for easy self-programming of the chip – just download new software programs to the chip and it burns the code into its flash memory. No need for special, expensive or complicated programming hardware with this project! By the way, you can find the HC908 Daughtercard schematic in the second installment of this column (QQ for January 2002). It’s also available online at the project’s website – see the References section at the end of this column.

DDS -- The Analog Devices AD9850 Direct Digital Synthesis (DDS) chip is used to generate frequencies for stimulus, analysis and measurement. Signal generation is possible from the sub-Hertz region to over 30 MHz, providing the Digital Breadboard with a superbly-precise, accurate and software-controllable source of stable signals for use in a variety of experiments. The DDS module includes a 100 MHz oscillator to drive the clock input of the Analog Devices AD9850, thus enabling the device to generate a maximum usable frequency of 30 MHz. A 5^th-order elliptic filter is used on the output of the DDS to ensure that a clean sinusoidal signal of about 750 mV is produced. This signal can be configured to go directly to the BNC at the edge of the board or go on for additional processing.

RF Amp – Since the DDS puts out a relatively small signal all by itself, a Mini-Circuits MAR-3 amplifier is optionally provided in the path to boost the signal and make it more useful in a reflectometer application. This tiny amplifier provides 8 dB of gain, as set by the bias resistor and inductors on the common output/bias port, and enables the reflectometer in the following stage to achieve better low signal performance. The builder may configure this amplified RF signal to be routed to the BNC connector or go on for yet additional processing.

Reflectometer – The reflectometer, or SW bridge, was presented in the last issue of this column. The middle 1N34 diode samples and rectifies the AC imbalance in the Wheatstone bridge to produce a DC representation of the signal reflected back from the output circuit on the BNC connector (e.g., an antenna system.) The other two diodes sample and rectify the forward path and the load side to produce voltages representing the forward signal and the impedance, respectively. The three DC voltages are presented to the next stage for buffering and amplification.

Compensation & Buffer Amplifiers – This two-stage module utilizes true rail-to-rail op-amps for better low-signal performance. The first stage in each signal path employs another 1N34 diode as its feedback element of a unity gain amplifier to compensate for the nonlinearities of the diodes in the previous stage. This enables the component measurements to be much more accurate at QRP levels. The second stage boosts the voltage to better match the 5V full scale range of the A/D converter of the HC908. It also transforms the low impedance of the first stage to about 100K in order to present a better condition to the 10K-ohm input impedance of the A/D.

Liquid Crystal Display (LCD) – LCDs have become commonplace in our microcontroller projects. The Digital Breadboard uses an inexpensive 4-line by 20-character/line device to display status and measurement information to the user. The software driver for this display assumes that a common HD44780 controller-based LCD is used, so one could actually use a larger or smaller LCDs fairly easily instead of the specified one. One could also upgrade the Breadboard’s capabilities to use a graphic LCD with an appropriate software driver in place.

Keypad – A 12-button keypad is provided on the Digital Breadboard to give the user an ability to perform direct numeric input. This is useful in VFO frequency-setting applications, or for configuration and data entry situations. The keypad also serves as a splendid set of general purpose pushbuttons that can be assigned whatever function desired by the user. Hence there are no other separate pushbuttons provided in this project. Whenever a key is pressed, an interrupt is issued in the HC908 and the software scans the 4 row x 3 column switch matrix to determine which key has been actuated. This key code is returned to the software routine that is expecting the input.

Keyboard – A standard PS2-style keyboard, similar to many keyboards used on PCs these days, can be used with the Breadboard. The importance and utility of this input device will become more apparent in the near future when we introduce the final module, the DSP Daughtercard. This combination of DSP co-processing and fast alpha-numeric input by the operator is the basis for PSK31 and other digital modes intended to be supported by this project. The Dauphin keyboard used with the prototype Breadboard thus far, as pictured in previous columns and on the website, is a small PS2-style keyboard that is ideal for portable use with the project. The NJQRP has acquired a number of these neat little keyboards and will provide them with the Breadboard system when we begin production.

DSP Daughtercard – Perhaps the final major hardware module currently being designed for the project is a daughtercard containing a DSP intended for audio processing. One of the many goals for the Breadboard continues to have it perform as a stand-alone digital mode controller, allowing the user to communicate using PSK31 (et al) without the need for a completely dedicated PC. This DSP card, initially containing an Analog Devices ADSP-2189 digital signal processor and mating codec (integrated A/D and D/A converter), is fast enough to demodulate the audio signals coming from your transceiver’s speaker, and then simultaneously modulate the data you type on the alpha keyboard and send that audio out for input to your SSB rig. The DSP Daughtercard is being designed right now and will be presented in 2003.

Prototyping Area – Although difficult to show on a schematic, the Breadboard PCB is being layed out to provide a small area approximately 2” x 2” that is populated with plated through holes. Looking like perforated breadboard, this area can be used for personal experiments involving components not already provided on the Breadboard PCB. Further, all of the extra/unused signals are brought to the edge of this prototyping area so the homebrewer may wire-in any of the 13 available HC908 I/O pins for specialized purposes. Alternatively, or in addition to using hard-wired components in this proto area, the homebrewer could fabricate a small board to fit into those signal pads at the edge of the area, effectively producing a replaceable prototyping daughterboard that could be added/removed as the application warrants. Does this Digital Breadboard have flexibility? You bet!

Miscellaneous Circuits – A shaft encoder provides ultimate flexibility to the operator as a continuous rotation menu selector, numeric dial setting, frequency tuning, and so on. An input conditioning circuit serves as a front end for an electronic voltmeter and RF probe by presenting a buffered AC or DC signal to an A/D input on the HC908 card. A frequency counter function is provided by having a transistor shape the sinusoidal waveform input to a pin on the AUX connector before presenting the signal to an edge-sensitive counting input of the HC908. Software determines the period of the applied waveform and the frequency is then calculated. A Morse paddle may be connected to an input jack and software on the HC908 performs as an iambic keyer, which in turn drives an external transmitter through the Keyline output jack. A tone is also sounded, under control of the HC908, and is output through an LM386 audio amplifier. This audio tone can be the sidetone for the keyer, the output for an Audio Voltmeter (see the N2CX article concerning such a project elsewhere in this issue), or mode confirmation beeps. Finally, room is provided within the Digital Breadboard enclosure for a ten AA cell pack, thus providing portable power for the field use of the unit.

Software: The Master Program, HCmon Debug Monitor and the Exerciser

Master Program – As mentioned at the start, software running in the HC908 microcontroller makes all these hardware modules dance in unison to perform the various functions we want in the radio shack. The Master Program performs as a “real time executive” upon which all other functionality and software operations are inserted. As more features and capabilities are added to the Digital Breadboard, the software to control them will be merely added to this existing framework to provide an ever-growing, ever-capable list of “menu items” from which the operator can select.

Exerciser – A Main Menu is presented to the user on the LCD and all menu items are presented by rotating the Select control (shaft encoder). When the desired menu item is displayed, the user depresses the Select control or the asterisk button on the keypad to select that item. Oftentimes a menu item has sub-menus to allow the user to select various options for the chosen operation. This user interface is quite intuitive, easy to learn and quick to use.

The following functions and capabilities are present in version 1.0 of the Exerciser software: VFO, Voltmeter, Freq Counter, SWR Bridge, Freq Sweep, Impedance, Keyer, Keypad Test, Keyboard Test, LCD Test, A/D Calibrate, Serial Port Test and Program Load. This software comes pre-programmed on the HC908 Daughtercard, and it may be re-programmed locally by downloading the file from the Breadboard’s website and using the “Program Load” menu item with your PC connected to the RS-232 serial port. (This self-programming feature was described in previous columns and is on the website.)

Debug Monitor "HCmon" – This program is a simple, low-level debug monitor developed to support any project based on the HC908 Daughtercard. The operator interfaces to the HCmon by means of a dumb terminal connected to the RS-232 serial port of the MPU daughtercard. Through the Monitor's command/response structure, the operator may edit memory and MPU registers, set and reset breakpoints, "go" or single step from any executable location in the loaded program, load S record files sent by the terminal, program Flash memory from the downloaded S records, and read input ports and set output ports.

HCmon is programmed into all Daughtercards after manufacture. In some instances, other software programs may also be programmed in the HC908 Daughtercard at time of manufacture, providing the user with a growing number of software applications for this product (e.g., the Antenna Analyzer, HC908 Commander, the HC908 Digital VFO and others).

HCmon provides the user with an ability to program the permanent and nonvolatile "flash" memory directly from downloaded "S records". An S record file is a common format for the assembled or compiled 68HC908 MPU binary code, as used in most Motorola-based processors and tools. Thus, when a user wishes to load a program other than what arrived on the HC908 Daughtercard, he may download any of the available S record files from the project's Internet website and "burn" the program into the MPU's flash memory, thus making the program permanent and available for use. In this way the user is able to change the HC908 Daughtercard's "personality" only by using a terminal program connected to the Daughtercard on its serial port.

Most "dumb terminals" may be used to communicate with the HC908 Debug Monitor. Examples of such programs include HyperTerminal, Red Ryder, ProComm and PCPlus. However, a useful, public domain (freeware) terminal program called "Tera Term" is also available to run on Microsoft Windows platforms. Tera Term has a convenient scripting ability that can be invoked to send an S record file (like a new software program) to the HC908 Daughtercard for flash programming by HCmon. The Tera Term terminal program and its S record transfer script are provided on the HC908 project website for users to download and use on their systems.

What’s Next?

Well, there you have it – the completed first version of the hardware! It took five issues to bring the project to this point but there was plenty to chew on, absorb and try out along the way. We need some time now for the hardware availability to catch up with the prototype system developed to this point, so the next several columns will be devoted to software – what makes this Breadboard tick and how you can use what we have so far. This will give the development team some time to catch up on making the baseboard PCB available and smoothing out the software so everyone’s experience with this project will be very positive. After all, the end goal is to have a single box like this in the shack that can perform a multitude of functions merely by selecting the right menu item or by loading new software. In order to make it easy for the user, lots of work is needed behind the scenes … and we’re dedicated to doing it right!