The main objective of this project was to create an FPGA-based system able to take advantage of the expanded input and processing capabilities of an external board. In our case, we found the Arduino Uno to be a good choice for a proof of concept. It has a large number of digital I/O pins, an inbuilt ADC, and an intuitive programming interface. We also decided to showcase the expanded I/O by creating an audio synthesizer which uses buttons attached to the Arduino as piano keys.

The original vision of the team was a full electronic keyboard built off of the Nexys DDR4 board, which took keyboard input as a way of determining which notes to play. This implementation never left the prototype stage of the project. We had trouble with two integral components of this project. First, the sinetables were difficult to generate on the FPGA board, so we generated them with python and tried to load them in. Unfortunately, static memory is difficult to manage in VHDL. While not insurmountable, this issue was compounded by the lack of a keyboard input driver for our Arduino.

This was replaced with the version of the project that we ended up implementing. We used a set of buttons connected to an Arduino to send digital output signals to the PMOD Ports of the Nexys DDR4 board. From there, an appropriate waveform would be generated and the output through another PMOD Port, into a DAC, and then into a speaker.

The system was designed to be easily extendable, and it is possible to expand the number of notes through increasing the number of PMOD Ports and buttons.

The PMOD I/O software was initially developed in isolation in order to speed up the overall development process. The construction of this software began with its constraint file. This is used by the NEXYS board to establish which hardware components will be referenced by the rest of the code. In this case, the team decided to use the first four pins of PMOD port JA (for additional information on PMOD ports, check out our contributions page). In the VHDL code the four pins were ported in as a STD_LOGIC_VECTOR of size 4 down to 1. This allowed us to represent the four pins as a single sequence of 1's and 0's.

In order to determine which pin is high, we would only need to check to see which bit in the binary sequence xxxx is a 1. For example, if the sequence is 0110, the second and third inputs are reading HIGH while the first and fourth are reading LOW. Once this is known, this software hands over control to the sound wave generation program, where a predetermined note is generated and output through the DAC.