Network RC CarSeth Schwiethale

James CrosettoJames Ellison

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)

Car’s Input

1 “alarm” output

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

Camera’s Output

Camera’s output can’t produce a 20ms PWM signal that is high between 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 Problem

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

The Solution

Pulse Width Modulation

car’s input

camera’s output

microprocessor

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!)

Pulse Width Modulator

Initializing the Pulse Width Modulator

Determining the Output Signal

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

Modifying the Clock Frequency

Allows for finer control of signal

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

Setting the Period

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

Initialized to 1.5 ms

Setting the Duty Cycle

Notice the difference!

Pulse Width Modulation Code

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)

Enhance Capture Timer

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

Counting

Interrupt

Conclusion