Network rc car
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Network RC Car. Seth Schwiethale James Crosetto James Ellison. C ar’s I nput. square pulse of 1.0-2.0 ms, repeats every 20 ms It is the same for both steering and acceleration width of the pulse determines the direction/speed w / 1.5 ms as the nominal center

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Network rc car

Network RC Car

Seth Schwiethale

James Crosetto

James Ellison

C ar s i nput
Car’s Input

  • square pulse of 1.0-2.0 ms, repeats every 20 ms

  • It is the same for both steering and acceleration

  • width of the pulse determines the direction/speed w/ 1.5 ms as the nominal center

  • the amplitude of the pulse is from the reference level of the Vcc (5v)

Camera s output
Camera’s Output

  • 1 “alarm” output

  • max activation is 100 times/sec (increments of 10 ms)

The problem
The Problem

  • Camera’s output can’t produce a 20ms PWM signal that is highbetween 1-2ms

  • Car cannot be driven by Camera directly

  • need something that modifies the output signal of the camera so it can be used by the car

The solution
The Solution

  • Using a microprocessor (MC9S12DP256)

  • has a clock signal rate of 24MHz

  • has registers for a Pulse Width Modulator and an Enhanced Capture Timer

  • Signal from the camera can be varied between 10-110ms for speed and 120-220ms for steering

Pulse width modulation
Pulse Width Modulation

camera’s output


car’s input

Pulse width modulator
Pulse Width Modulator

  • used to create correct output signal

  • uses CPU clock signal to generate signal lengths

  • current signal is 24MHz and we need a 50Hz (20ms) signal (Factor of 480000!)

Determining the output signal
Determining the Output Signal

  • Initializing the Pulse Width Modulator

Modifying the c lock frequency
Modifying the Clock Frequency

  • The pulse width modulator uses registers to decrease the frequency of the clock signal before it is used as an output signal

  • Two output signals can be generated from the clock signal

Allows for finer control of signal

Setting the period
Setting the Period

  • The periods of both SA and SB need to be 20ms

Setting the duty cycle
Setting the Duty Cycle

  • The duty cycle (the part of the PWM signal that is high) needs to be 1-2ms in length

  • Initialized to 1.5 ms

Notice the difference!

Enhance capture timer
Enhance Capture Timer

  • Used to trigger interrupts

  • Set up to trigger an interrupt every time a signal goes high or low on an input port (TCTL4 and TIE)


  • Timer Register (TCNT) is incremented every clock cycle (24MHz clock = 24 million times per second)

  • Clock slowed to 187.5 KHz

  • TCNT is 16-bit register (0-65535)

  • Overflows about every 1/3 second (65535/187500)

  • Never overflows more than once per input signal from camera

I nterrupt