NU32: Using the dsPIC33FJ12MC201 QEI to SPI board

Using the dsPIC33FJ12MC201 breakout board for quadrature decoding.

The dsPIC33FJ12MC201 breakout board

Overview

Datasheet

The dsPIC series of PIC microcontrollers are inexpensive 16 bit microcontrollers, specializing in the ability do fast digital signal processing.

The dsPIC33FJ12MC201 is a low pin count chip that runs on 3.3V at 80MHz without an external oscillator. It has the ability to remap peripheral functionality to any pins (labeled RPx). It also has a special peripheral called QEI, or Quadrature Encoder Interface. The QEI module receives the quadrature square waves from an encoder and keeps track of the net motion of the encoder as a 16 bit unsigned integer.

Details

The breakout board has been pre-programmed to read a quadrature signal on pins RP7 and RP8 (pins 11 and 12). It will keep track of the net motion of the encoder as a 16 bit integer, initializing to 32767. The dsPIC will report the 16 bit unsigned number over SPI communication when a '1' is sent.

Library Functions

This code is an example of how to read the encoder count using the NU32.

The functions are:

• void NU32_dsPIC_initialize(void) - enables SPI4 as master at 8MHz
• int NU32_dsPIC_getCount(void) - sends a '1' to the dsPIC, returns the value returned by the dsPIC

NU32_dsPIC_Example.c in the .zip file above demonstrates how to use the breakout board.

Sample Code

To initialize the SPI communication, call NU32_dsPIC_initialize(), :

  NU32_dsPIC_initialize();

To read the encoder count, call NU32_dsPIC_getCount(), :

  int count = NU32_dsPIC_getCount();

More Information

The internal count of the encoder will "roll over" at 0 or 65535. It is up to you to note that if the encoder count has made an unreasonable change, it has probably rolled over!