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Quadrature Decoder With Up/Down Counter — Use Case for CLB Using the PIC16F13276 Microcontroller With MCC Melody

This repository provides an MPLAB® X project that implements a quadrature decoder circuit using the Configurable Logic Block (CLB) peripheral.

The CLB peripheral is a collection of logic elements that can be programmed to perform a wide variety of digital logic functions. The logic function may be completely combinatorial, sequential, or a combination of the two, enabling users to incorporate hardware-based custom logic into their applications.

Quadrature signals are standard output waveforms of incremental encoders that come with motors. They provide information about the motor direction and shaft position. There are two types of encoders: linear and rotary, with two types of measurement each: absolute and incremental. The different technologies used for such encoders can be one of the following: magnetic, optical, inductive, capacitive, and laser.

A rotary absolute encoder outputs a unique code for each position of the motor's shaft. If the power of this encoder is lost and the shaft is rotated, the encoder can provide the correct position of the shaft. The rotary incremental encoder generates pulses while the shaft is rotated. If any pulse is missed, the correct position of the shaft cannot be determined any longer. This application uses a rotary incremental encoder to show how to decode the quadrature signals.

The following image highlights the Clockwise (CW) and Counterclockwise (CCW) conditions on the quadrature signals A and B. Signal A leading signal B (either on rising or falling edge) results in a clockwise detection, whereas signal B leading signal A (either on rising or falling edge) results in a counterclockwise detection.

Related Documentation

More details and code examples on the PIC16F13276 can be found at the following links:

• PIC16F13276 Product Page
• PIC16F13276 Code Examples on Discover
• PIC16F13276 Code Examples on GitHub

Software Used

• MPLAB X IDE v6.30 or newer or MPLAB® Tools for VS Code®
• MPLAB® XC8 v3.10 or newer
• PIC16F1xxxx_DFP v1.29.444 or newer

Hardware Used

• PIC16F13276 Curiosity Nano Development board
  

• Oscilloscope/Logic analyzer

• Rotary encoder
  

Note: The setup can be made with a rotary encoder or with a motor with encoder. In this example, the setup uses a rotary encoder.

• Motor with encoder
  

Operation

To program the Curiosity Nano board with this MPLAB X project, follow the steps provided in the How to Program the Curiosity Nano Board chapter.

Concept

This application showcases the capabilities of the CLB peripheral by creating a logic circuit that decodes the waveforms from a rotary encoder and a 4-bit up/down counter circuit used to track the ticks. The circuit comprises: two inputs, filtered phase signals from the encoder (the filtering circuit was implemented with a combination of the Configurable Logic Cell (CLC) modules), and four outputs corresponding to the 4-bit up/down counter. Additional logic is used to detect the overflow/underflow transitions of the up/down counter and to route them as interrupts.

To compute the total number of ticks, the counter value and the total number of overflow and underflow transitions are read in software. The total number of encoder increments is obtained using the following formula:

The decoder circuit implementation is displayed in the image below:

The phase signals from the rotary encoder have glitches when transitioning, so a debouncing circuit is used to avoid these false detections. The debouncing circuit is implemented using two CLC modules, as presented in the following figures:

Filtering the Signal from Phase A

Filtering the Signal from Phase B

To detect a clockwise turn of the rotary encoder, the phase A signal has to lead the phase B signal either on a rising or falling edge.

To detect a counterclowise turn of the rotary encoder, the phase B signal has to lead the phase A signal either on a rising or falling edge.

The logic mechanism used to decode the clockwise and counterclockwise ticks from the encoder consists of a 4-input Look-Up Table (LUT), with the following inputs: phase A, delayed phase A, phase B, and delayed phase B. The following diagrams present the CLB circuit implementation and the detection principle by using two graphs.

The implementation for the 4-bit up/down counter circuit can be found below:

To detect an overflow or an underflow event from the 4-bit up/down counter, the two Most Significant output bits (MSb) from the counter are used as input for a detector implemented within CLB. The overflow/underflow detector circuit outputs an interrupt everytime an overflow or an underflow event is detected. The implementation and working principle are presented in the figures below:

Setup

The following peripheral and clock configurations are set up using MPLAB® Code Configurator (MCC) Melody for the PIC16F13276:

1. Configuration Bits:

   • External Oscillator Selection bits: Oscillator not enabled
   • Reset Oscillator Selection bits: HFINTOSC (1 MHz)
   • Brown-out Reset Enable bits: Brown-out Reset disabled
   • WDT Operating Mode: WDT Disabled, SEN is ignored
     

2. Clock Control:

   • Clock Source: HFINTOSC
   • HF Internal Clock: 32_MHz
   • Clock Divider: 1
     

3. Interrupt Manager:

   • Enable Single ISR Execution per entry: Enabled
     

4. CLB Synthesizer Library:

   • Clock Selection: LFINTOSC
   • Clock Divider: Divide clock source by 32
     

5. CLB1:

   • Enable CLB: Enabled
   • Generate ISR: Enabled
     

6. CRC:

   • Auto-configured by CLB

7. UART2:

   • Requested Baudrate: 115200
   • Data Size: 8
   • Redirect Printf to UART: Enabled
     

8. EUSART2:

   • Received Enable: Enabled
   • Serial Port Enable: Enabled
   • Transmit Enabled: Enabled
     

9. CLC1:

   • Enable CLC: Enabled
   • Logic Cell Mode bits: 1-input D flip-flop with S and R
   • Input 1: LFINTOSC
   • Input 2: CLCIN0 (CLCIN0PPS)
     

10. CLC2:

    • Enable CLC: Enabled
    • Logic Cell Mode bits: AND-OR
    • Input 1: CLCIN0 (CLCIN0PPS)
    • Input 2: CLC1_OUT
      

11. CLC3:

    • Enable CLC: Enabled
    • Logic Cell Mode bits: 1-input D flip-flop with S and R
    • Input 1: LFINTOSC
    • Input 2: CLCIN1 (CLCIN1PPS)
      

12. CLC4:

    • Enable CLC: Enabled
    • Logic Cell Mode bits: AND-OR
    • Input 1: CLCIN1 (CLCIN1PPS)
    • Input 2: CLC3_OUT
      

Note: For details on the CLC configurations, check the diagrams displayed in the MCC Melody figures.

11. Pin Grid View and Pins:

• EUSART2 RX2: RD7
• EUSART2 TX2: RD6
• CLBPPSOUT0: RC0 (4-bit counter bit 0)
• CLBPPSOUT1: RC1 (4-bit counter bit 1)
• CLBPPSOUT2: RC2 (4-bit counter bit 2)
• CLBPPSOUT3: RC3 (4-bit counter bit 3)
• CLC4 CLCIN1: RC6
• CLC3 CLCIN1: RC6
• CLC2 CLCIN0: RC7
• CLC1 CLCIN0: RC7
  
  

The following pin configurations must be made for this project:

Pin	Configuration	Function
RC7	Input	Encoder phase A (CLCIN0PPS)
RC6	Input	Encoder phase B (CLCIN1PPS)
RD6	Output	EUSART2 TX2
RD7	Input	EUSART2 RX2
RC0	Output	4-bit up/down counter bit 0 (CLBPPSOUT0)
RC1	Output	4-bit up/down counter bit 1 (CLBPPSOUT1)
RC2	Output	4-bit up/down counter bit 2 (CLBPPSOUT2)
RC3	Output	4-bit up/down counter bit 3 (CLBPPSOUT3)

Demo

The setup for the demo is presented in the diagram below.

In this demo, every clockwise or counterclockwise pulse detection results in a printed message on the serial terminal.

The figure below presents a logic analyzer capture that contains the waveforms corresponding to the messages sent to the terminal in the above figure.

Note: Follow the steps in the How to use MPLAB® Data Visualizer section to set up the Data Visualizer so that the text messages through UART2 are displayed correctly.

Summary

This application showcases how to create a quadrature decoder circuit that can detect and count clockwise and counterclockwise pulses on rising and falling edges of the quadrature signals using the CLB peripheral.

How to Use MPLAB® Data Visualizer

This section illustrates how to use the MPLAB® X Data Visualizer; this can be applied to any other projects, too.

Operation

1. Open the software terminal in MPLAB® X IDE. Click the Data Visualizer button.
   
   

2. Select the Input Source corresponding to the PIC16F13276 on the right side..
   

3. Identify the COMx port corresponding to the PIC16F13276 Curiosity Nano board and select it from the Connections tab on the left-side menu.
   

4. Set the specific Baud Rate, in this case 115200.
   

5. Press the Start streaming COMx button. The text messages will appear in the Terminal tab.
   

How to Program the Curiosity Nano Board

This chapter demonstrates how to use the MPLAB X IDE to program a PIC® device with an Example_Project.X. This is applicable to other projects, too.

1. Connect the board to the PC.

2. Open the Example_Project.X project in MPLAB X IDE.

3. Set the Example_Project.X project as main project.
   Right click the project in the Projects tab and click Set as Main Project.
   

4. Clean and build the Example_Project.X project.
   Right click the Example_Project.X project and select Clean and Build.
   

5. Select PICxxxxx Curiosity Nano in the Connected Hardware Tool section of the project settings:
   Right click the project and click Properties.
   Click the arrow under the Connected Hardware Tool.
   Select PICxxxxx Curiosity Nano (click the SN), click Apply and then click OK:
   

6. Program the project to the board.
   Right click the project and click Make and Program Device.
   

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