1. Line Sensing 101 TEAM SpeedRacers _______ Team Leader
_______ Hardware Specialist
_______ Software Specialist
_______ Assistant

2. Overview Problem statement Hardware Software Q&A
requirements / constraints
interface
Software
MCU module(s) needed
code for initialization, operation
Q&A

3. Problem Statement
Line Sensing: to determine relative position of track line beneath car.
Line is ¾" wide black on white background
electrical tape
sample 8.5"x11" on website
Line
Sensor
MCU

4. Hardware Pololu QTR-8RC Reflectance Sensor Array
module of 8 IR LED/phototransistor element pairs

5. QTR-8RC Reflectance Sensor Array
Specifications:
Dimensions: 2.95" x 0.5" x 0.125" (without header pins installed)
Operating voltage:  VDC
Supply current: 100 mA (all IRLEDs on)
Output format: digital I/O compatible
Optimal sensing distance: 0.125" (3 mm)
Max recommended sensing distance: 0.375" (9.5 mm)
Weight without header pins: 0.11 oz (3.09 g)

6. QTR-8RC Reflectance Sensor Array
Operation:
each element has an R-C circuit
capacitor discharge rate is a function of reflected IR light
greater reflectance = shorter discharge time
algorithm:
C is 1st charged by driving it with a high output from an MCU I/O pin (~16us, use 20 to be safe)
I/O pin is switched to input and time to HL transition is measured
or simply wait for time 'x' and read H or L input

7. Scope Shots*
Output (yellow) 1/8" above white surface and MCU timing (blue)
Output (yellow) 1/8" above black line and MCU timing (blue)
* see data sheet for link to hi-res image

8. Interface
QTR-8RC is interfaced to an 8-bit MCU I/O port via a ribbon cable and breadboard adapter

9. Software: Algorithm Turn on IR LEDs (if off)
Set 8-bit I/O port to all outputs
Set output port to all highs
First Delay (20us)
Set I/O port to all inputs
Second Delay (?)
Read 8-bit H/L input value
Turn off IR LEDs (opt.)

10. Software Implementation
to achieve us delays, we will need to employ the MCU's timer peripheral (TPM1)
and also speed up the MCU from 4 MHz default to 32 MHz (using the MCG)
QTR-8RC is interfaced to I/O portB

11. Software: Initialization
// Initialize MCG module for high-speed operation void MCGinit_fll() { // IREFS=1 (default), also select IREFSTEN MCGC1 = MCGC1_IREFS | MCGC1_IREFSTEN; MCGC2 = 0; // switch to BDIV=1 MCGC4 = 0x01; // select mid DCO range in FLL (FLL factor=1024) while (!MCGSC_LOCK) {} // wait till FLL acquires lock // MCGOUT=CPUclk=32Mhz, Busclk=16MHz (with proper trim value) } // MCU-specific initialization void initializeMCU() SOPT1_COPT = 0; // disable COP SOPT2_CLKOUT_EN = 1; // enable BUSCLK output on PTF4 MCGinit_fll(); // activate MCG for 32 MHz clock // I/O port initialization FB_YLED_ENABLE; // set PTE6 to be output (to yellow LED) // Real-Time Counter initialization RTCSC = 0x08; // enable RTC, select 1ms period from 1kHz internal clock RTCMOD = 9; // RTIF every 10ms // TPM1 initialization TPM1SC = TPM1SC_CLKSA_MASK // select bus clock for timer clock + ?; // divby-16 prescaler (16MHz/16 = 1MHz Timer clock) EnableInterrupts;

12. Software: Timing Support
usDelay() is similar to previous msDelay() except much higher clock source is used
1 MHz timer clock instead of 1 kHz LPO
void usDelay(unsigned short n) {
word startcnt = TPM1CNT;
// wait for time to elapse
while ((word)(TPM1CNT-startcnt) < n) {} }

13. Software: Line Sensing
byte read_irarray() // read & return line sensor data {
PTBD = 0xff; // make Port B all highs
PTBDD = 0xff; // make Port B all outputs
usDelay(20); // allow sensor caps to charge
PTBDD = 0; // switch to all inputs
usDelay(???); // delay for discharge
return PTBD; // read & return sensor array results }
when line position data is needed, a call to "read_irarray" is made
ex: byte irdata; ∶ irdata = read_irarray();

14. Performance Issue
Considering amount of power required to drive the IRLEDs and the sensor sample rate, a huge power savings would be achieved if IRLEDs are turned off between readings.
Ex: if LEDs are on 1ms and sensor is read at 10Hz, the D.C. of the LEDs is 1ms/100ms = 1%
applied to the 100 mA!

15. Summary Problem statement Hardware Software requirements / constraints
interface
Software
MCU module(s) needed
code for initialization, operation

16. References
Huang, Han-Way. HCS12/9S12: An Introduction to Software and Hardware Interfacing. 2nd ed. Delmar, Cengage Learning, 2010.
MCF51JM128 ColdFire Integrated Microcontroller Reference Manual. 2. Freescale Semiconductor, Inc., Web.
QTR-8RC Reflectance Sensor Array." Pololu -. N.p., n.d. Web. 18 Feb
Sumey, Jeff. “CET Microprocessor Engineering.” California University of Pennsylvania. Web. 18 Feb

17. QUESTIONS?