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RC SERVO CONTROL VIA TPU

by Jeff Loeliger

Start ý The Answer Isý ý Designing Functions ý Demonstration ý Youýre in Control ý Sources and PDF

DEMONSTRATION

To give a working example, I wrote a sample program for using this servo TPU function. The program, which reads one input knob and controls two output servos, was written for the Motorola MPC555 Evaluation Board as shown in Photo 1. The program configures the TPU, inputs custom microcode, and sets up the TPU to run the servo function on two channels.

Photo 1ýThe MPC 555 EVB runs demo code using a potentiometer to control two servos (one in normal mode and one in reverse mode).

The queued analog-to-digital converter (QADC) is configured to convert one channel continuously with the QADC value from the input knob and is then used to drive two servos in a continuous loop. One servo is run in normal mode while the other is run in reverse mode to complement each other as the knob is moved.

The MPC555 is a complex device with hundreds of registers. To make the device easier to use, thereýs an MPC555 header file that defines all the registers and associated bit fields. The MPC555 also has a utilities header that provides macros to make using the on-chip TPU simpler.

In order to run the servo program, the user must first calculate the MIN_COUNT and MAX_COUNT servo values. Doing this depends on how the TPU is configured. When determining how fast to run the timebases, all functions running on every TPU channel must be taken into consideration. To get the best resolution on the output signal, the timebase should be run as fast as possible (constrained only by the fact that MAX_COUNT must be less than $8000 timer counts).

For this sample program, the MPC555 has been configured to run at 40 MHz with the timebase set to system clock divided by four, giving a resolution of 100 ns. For a typical servo with a MIN_COUNT of 1 ms and MAX_COUNT of 2 ms, MIN_COUNT and MAX_COUNT values of $2710 and $4E20 respectively will result. In addition to defining the MIN_COUNT and MAX_COUNT values, the frequency of repetition of the output signal must also be established. The REPEAT parameter has a resolution of $8000 counts which, with a timebase of 100 ns, equates to 3.2 ms. As servos should be refreshed every 20ý30 ms, and the formula for calculating the repeat value is [REPEAT ý $8000], setting REPEAT to seven gives a safe value of 22.6 ms. This is the value used in the given sample program.

The demonstration program for running the servo function can be downloaded CPU code, TPU code.

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