7 Replies Latest reply on Jun 6, 2011 2:04 AM by gautamd_

    JR 8717 Servos


      I'm trying to design the drivers for a JR 8717 to make it control an arm, but it doesn't move. I used a PWM component with a clock at 1.28 khz, 8-bit resolution, UDB, one ouput, with a period of 255 and a cmp value of 22. What did I do wrong? Should the period be 20 with a cmp value of 2?

        • 1. Re: JR 8717 Servos

          Hi Myuugen,




          Can you let us know about the pulse width and pulse cycle requirement of JR8717?


          For the present configuration you have specified, the PWM output will be a pulse of 17.187ms with a frequency of 5.02Hz.


          If you set the period to 20 and compare value to 2, then the frequency is 64Hz and pulse width is 1562.5 micro sec.


          The right values can be chosen depending upon the requirement of your servo.

          • 2. Re: JR 8717 Servos

             I tried to contact JR and they said that any information not published on their website is proprietary and not given out to the public.




            That being said, I went to some forums and I've found this information



            Pulse Width:

            333-1520 µs




            Standard Signal centre +/- 0.4ms (1.1-1.9ms)


            I'm going to try to get a scope and measure the servo tomorrow.




            Also, is the method i'm using correct to control the servos with the chip?

            • 3. Re: JR 8717 Servos

              I noticed that you were able calculate the pulse width and cycle rate from the period and comp value. What is the calculation for that?


              I'm sorry if I'm asking a lot of dumb questions, but I've never worked w/ PSOC before and I don't have a very strong background in electrical engineering.

              • 4. Re: JR 8717 Servos

                Using a scope i found the period to be approximately 20 ms, with a center of  1.5 ms, a low of 1 ms, and a high of 2 ms.




                In the PWM component, I'm not using a 1 MHz clock, period of 20000, and a cmp of 1000.




                Everything builds correctly, and I have it attached to a digital out pin, however, the pin does not seem to be getting any more.




                I'm using a  CY8C55 PSoC 5 and i'm connecting my pins to: P0[2] (73), P0[1] 72, P0[0] 71, P4[0] 69, P12[3] 68, and P12[2] 67.

                • 5. Re: JR 8717 Servos

                  that third line should read at the end: does net any voltage

                  • 6. Re: JR 8717 Servos

                    Hi Myuugen,




                    I have created a project which uses a PWM component.




                    1) The clock frequency is 1MHz. Hence the clock period is 1 micro second.


                    2) The period is set to 19,999 to get a time period of 20ms (1 extra cycle is taken to roll over to zero). The resolution of PWM signal generated is 1 micro sec.


                    3) Now the Compare value can be varied from 333 to 1520 to get a pulse of 333micro sec to 1520 micro sec. The API PWM_WriteCompare ( ) is used to vary the compare value.




                    The waveform was checked on oscilloscope. The snapshots are as shown below:






                    Signal with a Period of 20ms and pulse of 333 micro sec.








                    Signal with a Period of 20ms and Pulse of 1520 micro sec.








                    The pulse of 332 micro sec.








                    The pulse of 1500 micro sec.




                    Find the project attached.

                    • 7. Re: JR 8717 Servos

                      The snapshot is attached in the folder below.


                      It shows pulses of duration 332 micro sec and 1500 micro sec.