Chief Delphi - Updated Encoder Code Available

I've significantly updated my example quadrature encoder code with additional functionality and documentation. Here's an excerpt from the readme.txt:
Code:
This source code in this file implements an interface to two

quadrature output encoders with the assumption that they are

coupled to the robot's drive train, one on the left side, the

other on the right. Used with suitable PID control software,

encoders can be used to control the position and velocity of

of your robot. This software was tested with Grayhill 63R256

and 61K128 quadrature output optical encoders. Data sheets for

these devices are included.

                

This source code will work with the Robovation (A/K/A EDU-RC) 

robot controller and the FIRST Robotics robot controller.

On a 40MHz PIC18F8520, this software can track peak encoder 

count rates as high as a few thousand counts per second, which

should be more than adequate for most applications. To meet

your performance expectations, selecting the proper Counts Per 

Revolution (CPR) parameter of your encoder is very important.

If the CPR is too high, the robot controller will spend too

much time counting encoder "ticks" and not enough time on

other tasks. At the extreme, you will see very wacky behavior

in your robot controller including corrupted data, the red-

light-of-death or the controller may even think the robot is

traveling in a direction that it isn't. Selecting a CPR that

is too low will not give you the resolution you desire. The

CPR should be optimized to minimize the number of interrupts

your robot controller will have to service yet meet your

resolution expectations (yes, millimeter position resolution

to too much to ask for).

 

The included project files were built with MPLAB version 6.60.

If your version of MPLAB complains about the project version, 

the best thing to do is just create a new project with MPLAB's 

project wizard. Include every file except: FRC_alltimers.lib 

and ifi_alltimers.lib and you should be able to build the code.

This file is best viewed with tabs set to four characters.

 

****************************************************************

Here's a description of the functions in encoder.c:



Initialize_Encoders()

This function initializes the encoder software. It should be

called from user_routines.c/User_Initialization().

 

Get_Left_Encoder_Count()

Get_Right_Encoder_Count()

These functions will return the current number of encoder

counts or "ticks".



Set_Left_Encoder_Count()

Set_Right_Encoder_Count()

These functions are used to reset or change the encoder count.



Left_Encoder_Int_Handler()

Right_Encoder_Int_Handler()

When the phase-A signal of an encoder transitions from a zero

to a one, the microcontroller will automatically call these 

functions to sample the phase-B signal and increment/decrement

the encoder count variables. You shouldn't have to call these 

functions yourself.

 

****************************************************************

Five things must be done before this software will work 

correctly on the FRC-RC:

1) The left encoder's phase-A output is wired to digital input 

one and the phase-B output is wired to digital I/O six.

2) The right encoder's phase-A output is wired to digital input

two and the phase-B output is wired to digital I/O 8.

3) Digital I/O pins one, two, six and eight are configured as 

inputs in user_routines.c/User_Initialization(). If you notice

that the encoder only counts in one direction, you forgot to

do this step.

4) A #include statement for the encoder.h header file must be 

included at the beginning of each source file that calls the 

functions in this source file. The statement should look like 

this: #include "encoder.h".

5) Initialize_Encoders() must be called from user_routines.c/

User_Initialization().



Five things must be done before this software will work 

correctly on the EDU-RC:

1) The left encoder's phase-A output is wired to interrupt 

one and the phase-B output is wired to digital I/O six.

2) The right encoder's phase-A output is wired to interrupt 

two and the phase-B output is wired to digital I/O eight.

3) Digital I/O pins six and eight are configured as inputs in

user_routines.c/User_Initialization(). If you notice that the

encoder only counts in one direction, you forgot to do this 

step.

4) A #include statement for the encoder.h header file must be 

included at the beginning of each source file that calls the 

functions in this source file. The statement should look like 

this: #include "encoder.h".

5) Initialize_Encoders() must be called from user_routines.c/

User_Initialization().

 

Kevin Watson

kevinw@jpl.nasa.gov
The code can be found here: http://kevin.org/frc

-Kevin