SPI project

Noah Smith

The goal of this project is to use a simulated “sensor” input analog voltage, convert it to digital, and transmit to via SPI to Arduino and display on the serial monitor.

Our work is to design the digital circuitry and code for the SOC that this would be implemented on.

Step 1:

ADC

The first step is to get the analog to digital conversion working. To do this, I used the example code provided in the project description. I chopped everything out of the code except for the LED indication, as it was useful for debugging and fun to play with. It is important to note that you do need the addressing code, either hardcode in the address or don’t delete it. I deleted it and was very confused why nothing worked.

Step 2:

Top Module

Next up is building a top module to combine the ADC and SPI transmitter. In this project the Arduino will be the master, and the FGPA will be the slave. So we need to use SPI slave code for it. I chose to not use the SPI code out of the textbook and instead wrote my own, taking heavy inspiration from this GitHub. suoglu/Simple-SPI: Set of simple modules to communicate using SPI protocol. (github.com) This solved some issues I was having and is also just some well written code to learn from. (I also used this clock divider to eliminate errors)

Another important thing to note in the top module design is that the Arduino is expecting 8 bits of data per transmission. In the example code and in my code the 4 MSBs of the ADC output code are all that is used. So when you send the data to the SPI slave module, you either have to pad it with 0’s or pack 2 data bits into this transmission. You can see in line 84 of the above picture that I am passing SPIdata to SPIdata1 every clk cycle. This is then concatenated with the SPIdata to form an 8 bit transmission in the below code. We will use this packing later in the master side of the code.

Step 3:

Hardware

For the hardware portion of this project I wired up everything according to the block diagram. The picture can be seen below. The SPI cable colors are as follows. SCK-yellow, COPI-green (unnecessary), CIPO-Blue, CS-orange.

Step 4:

Arduino

In the Arduino code I used the SPI.h library to eliminate errors. I have the clock set to 3.125 MHZ to match the SPI slave module. I am receiving data LSB first. The other thing happening here is using bitmasking to unpack the combined data bit transmitted by the slave, and averaging the two 4 bit pieces of data.

Below is a working demonstration.