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A small firmware was developed to communicate with the user application and to adjust the
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offset. It uses the "Lightweight USB Framework for AVRs" (LUFA) [1] to interface the built-in USB
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2.0 controller. The device registers itself as CDC (ACM) device. This enables the opposing
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application to treat the amplifier as a serial device. Platform independent serial communication
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libraries for Java are available and still supported. Most operating systems further provide
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CDC ACM drivers and hence make installation of the signal generator easy. Communicating
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directly through USB endpoints would have required a platform independent USB library. Such
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solutions exist, but as native libraries and therefore require a Java wrapper. It would also make
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programming more complicated and the hardware more expensive since it would require every
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device to have a USB controller.
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The structure of the firmware and its modules is depicted in figure 1 below. As mentioned
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before, the LUFA library handles the communication with the PC. Via the hardware serial peripheral
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interface (SPI) of the microcontroller, the DAC is controlled. It is used to generate the
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unshifted DC offset between 0V and 5V. The SPI transmission logic
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is located in the _SPI_ module. In the _Main_ module, the actual program logic is housed, and is
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described in the next section.
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image::https://es.technikum-wien.at/openlab/openlab_wiki/wikis/img/OpenLab_siggen_atmel_imp/Siggen_atmel_firmware.PNG[caption="Figure 1: ",title="Firmware structure",height=280,align="center"]
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_CRC_ holds the checksum implementation on the firmware side, and _Protocol_ contains the
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header and payload definitions for the OpenLab protocol. The _Util_ module contains just helper
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functions or preprocessor macros. By turning off all not required features in LUFA, and by
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compiling for space(-Os), the firmware requires about 4700 bytes.
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== Bibliography +
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The content of his chapter is taken from the master thesis "_Development of a Sound Card Based Arbitrary Waveform Generator_" by Jakob Kriegbaum, MSc, May 2017.
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. Title
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. [Online] fourwalledcubicle.com/LUFA.php
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