Lab 7: Serial Peripheral Interface (SPI) Communication

Hardware

• Analog Discovery Studio
• Arduino UNO, or other LINX compatible microcontroller

Software

• WaveForms
• LabVIEW Community
• Digilent WaveForms VIs
• Digilent LINX

As shown in the image to the right, the communication method generally uses four digital lines. The slave select on the master is connected to the slave select on the slave and the master informs the slave of when communication starts and stops. Unlike the UART protocol you learned in the previous lab, SPI is a synchronous protocol and the master and slave are synchronized using the same Clock line.

The two other digital lines are the Master Out, Slave In (MOSI) and the Master In, Slave Out (MISO). The MOSI is used to send data from the master to the slave. The MISO is used to send data back from the slave to the master.

For SPI communication, when the slave select is set to low, the master outputs a clock signal as seen in the diagram to the right. The slave then reads the current MOSI state on each rising edge of the clock.

Design a VI in LabVIEW that will send an SPI signal on one of the microcontroller's pins using LINX and a VI that reads in a serial signal, displays the digital waveform and decodes it. You can build your own VIs, following the steps below, or you can download the VIs used in this guide from here: SPI.zip.

The first VI, SPI_send.vi, is used to send out data. The Front Panel of this VI contains control elements for setting the COM port to which the microcontroller is connected, setting the digital pins used to send data, setting the signal parameters (clock frequency, endian-ness, SPI mode, CS logic), entering the data to be sent and two buttons to send the data and to stop the program.

The Block Diagram contains, except from the control and indicator elements, the blocks needed for communication with the microcontroller and data processing.

The SPI_receive VI's Front Panel contains controls for selecting the used Test and Measurement device, setting the digital IO channels used for receiving, the signal parameters and an indicator to display the decoded data. The main element of this front panel is the Digital Waveform Graph used to display the received signal. A stop button is also present, allowing the user to stop the reception when they want.

The Block Diagram of this VI contains the blocks needed to control the Mixed Signal Oscilloscope and to decode the signal.

Connect the ground of the microcontroller to the ground of the Analog Discovery Studio (black wire) and connect the SPI bus of the microcontroller to digital pins of the Analog Discovery Studio as follows:

In this lab, the MISO pin of the Arduino UNO will not be connected to any DIO lines, as the slave device won't send data to the master. Also, connect the microcontroller to the PC with a USB cable.

You can download the wiring diagram here: wiring_diagram_spi.zip

Follow the instructions below to set up your microcontroller in LabVIEW and send out the SPI signal for this experiment. In LabVIEW navigate to Tools → MakerHub → LINX → LINX Firmware Wizard. Set the Device Family to the brand of microcontroller you are using, the Device Type to the microcontroller type, and the Firmware Upload Method to Serial / USB.

In the next window, select the COM port connected to your microcontroller, then hit Next. Click Next again and click Finish. This allows the LINX Toolkit to automatically configure the firmware to work with LabVIEW.

Note: The COM port can be identified by going to Windows Device Manager → USB Serial Port.

As a first step, the control and indicator elements should be placed by right-clicking on the Front Panel and selecting the required element. Add a Send and a Stop button from the Boolean palette and a Numeric control to enter data. All the other control elements can be added by right-clicking on the respective inputs of the subvis from the LINX tab.

Arrange everything on the Front Panel. Rename the placed elements by double-clicking on their name.

In the Block Diagram, initialize the microcontroller connected to a serial port with the Open.vi from the LINX tab, then open and configure the SPI bus. Don't forget to add the necessary control element to the Front Panel.

In a while loop add an event structure and configure the triggering event to Mouse Up on the Send button. VIs in this structure will only be accessed when the send button is pressed. In the structure send the data using the respective VI from the LINX tab.

Add one more event to the structure, configured to be triggered by the Stop button. This event should stop the while loop.

When the program is finished, don't forget to close the MCU and handle the errors.

As a first step, the control and indicator elements should be placed by right-clicking on the Front Panel and selecting the required element. In this VI we need two Combo Boxes and two String indicators to set the Test and Measurement device and the DIO lines used. You can copy the Combo Boxes from the VI created in Lab 6: UART Serial Communications.

We need four control elements for setting the SPI signal parameters: a Numeric control for the clock frequency and three Rings or Enumerations for setting the CS logic level, the bit order, and the SPI mode.

Place a Digital Waveform Graph on the Front Panel, then turn off the auto-scaling for both axes. Right-click on the graph, click on Properties then select Cursors. Add 8 vertical cursors and set a visible style for them. Double click on the Front Panel anywhere to create a note. Create labels for the CS, MOSI, and SCK lines, then arrange these labels above the graph.

Create a numeric indicator to display the decoded data and a Stop button to interrupt the program if needed.

Arrange everything on the Front Panel. Rename the placed elements by double-clicking on their name.

In the Block Diagram create property nodes for the digital graph's YScale.Minimum, YScale.Maximum, XScale.Minimum and XScale.Maximum properties. Set the Y-axis range according to the selected digital channels (DIO 0 is on position 15, DIO 1 is on position 14, etc.). By default, the graph displays the whole buffer and the buffer size is 1000, so we can set the X-axis range from 0 to $1000t_{sample}$, where $t_{sample}=\frac{1}{f_{clock}\frac{1000}{2*nr_{bit}}}$, $f_{clock}$ being the clock frequency and $nr_{bit}$ the number of data bits (8).

Display the DIO lines used as MOSI and SCK with the respective indicators.

Open the Mixed Signal Oscilloscope (MSO), and configure it to acquire digital signals from the lines set as CS, MOSI, and SCK, with the sampling frequency of $f_{sample}=\frac{1}{t_{sample}}$. Enable the used digital channels, then set the Scope to be triggered by the rising/falling edge of the CS signal. Start the instrument.

In a while loop, read the Scope. Compare the results to the ones from the previous iteration (to a blank array in the first iteration). If the results are the same as the previous ones, or the instrument was auto-triggered, restart it. If the instrument was triggered normally, convert the waveform to a boolean matrix, then extract the columns which contain MOSI and SCK data.

The SPI mode determines, when should we read the incoming data. There are four modes, with the corresponding CPOL and CPHA values:

By SPI_timing_diagram.svg: en:User:Cburnett derivative work: Jordsan (talk) - SPI_timing_diagram.svg, CC BY-SA 3.0, Link

To decode the signal, find the indexes of all the rising/falling edges of the SCK signal, then save in an array the MOSI values at these indexes. To get the transmitted number, convert the binary array to a number.

You can use the indexes of the SCK signal edges and the ActCrs and Cursor.PosX property nodes of the digital waveform graph to set vertical cursors at these points, showing the moments when data is read.

If an error appears, or the Stop button is pressed, exit the loop and close the used instrument, disconnect the device, then handle the errors.