Self-Aligning Satellite Dish in Rust: Servo

We'll use servo motors, specifically the SG90, to actuate our PTZ kit. Servo motors are driven by pulse width modulation(PWM) signals.

Table of Contents

• Servo


• 
  Implementation
  
  
  
  
  • 
    PWM
    
  
  
  
  
  
  



• Final Code

Servo

The SG90 is a microservo that can rotate through 180 degrees. It is operated by pulse width modulation (PWM) signals oscillating at 50 Hz. A pulse with a period of 1.5 ms (7.5% duty cycle) will position it to 0 degrees, 2 ms (10% duty cycle) to 90 degrees, and 1 ms (5% duty cycle) to -90 degrees. Feeding a signal with duty cycles ranging from 5 to 10% should therefore sweep the motor from 0 to 180 degrees.

Under Pins configure the pin.

// Configure GP20 as PWM channel 1A

pads_bank0.gpio(20).modify(|_, w| w

                            .pue().set_bit()// pull up enable

                            .pde().set_bit()// pull down enable

                            .od().clear_bit()// output disable

                            .ie().set_bit()// input enable

                            );



io_bank0.gpio(20).gpio_ctrl().modify(|_, w| w.funcsel().pwm());// connect to matching pwm

The programming parameters for the RP2040's PWM are set using the below formula.

Where:

• TOP is the upper limit count of the PWM's counter,


• CSR_PH_CORRECT is the phase correction selector that determines whether the PWM's counter counts backwards from TOP,


• and DIV_INT and DIV_FRAC are further used in dividing the counter to acquire a final PWM output.

For a wrap value of 50,000 and with phase correction enabled we can get the divider values below:

We will therefore set our DIV_INT as 25 and DIV_FRAC as 0.

// pwm2 set up

let pwm2 = dp.PWM.ch(2);// Acquire handle for pwm slice 2

resets.reset().modify(|_, w| w.pwm().clear_bit());// Deassert pwm



// Configuring pwm2

pwm2.csr().modify(|_, w| w

            .divmode().div()// free runnning counter determined by fractional divider

            .ph_correct().set_bit()// enable phase correction 

            );

pwm2.top().modify(|_, w| unsafe { w.bits(50_000) });// sets the wrap value: TOP

// For a fpwm of 50Hz w/ top of 50000 div need to be 25

pwm2.div().modify(|_, w| unsafe { w.int().bits(25).frac().bits(0) });

pwm2.csr().modify(|_, w| w.en().set_bit());// Enable pwm2

Final Code

Rearranging the code we get this final copy.

In the next part we shall run an example showing PWM in operation.