Download TeamCommonGroundPhas2 - Department of Applied

California University of Pennsylvania
Department of Applied Engineering & Technology
Electrical Engineering Technology
CET 360: Microprocessor Engineering
Team: Common Ground
Smart Car Power Systems
Cody Morr
Kausthub Rao
Justin Winterhalter
Greg Zick
Detailed Task Description
To control and monitor the power consumption of a radio controlled
Smartcar.
Specific Requirements
To apply the correct voltage levels, using the LM2940 or L4941 5 volt
regulators, to the line sensing, steering, and drive systems, along with the Firebird
32 MCU. Also to use the MCP1702 3.3 volt regulator to monitor 3.3 volt radio
frequency communications systems. And finally, to monitor the 7.2 Volt NiCad
battery life.
Proposed Hardware Solution
On the MC33926 Motor Driver Carrier you want to find pin 1 that says Vin.
From Vin we will be getting all of the battery voltage, which can range from 0 to 10
volts. Since the battery can charge higher than what the processor is rated for there
needs to be a regulator. The ColdFire needs 5 volts which is the maximum number
of volts for it. By using an LM2940 LDO regulator it takes the 10 volts from the
battery and drops it down to 5 at a maximum current output of 1 amp. This
regulator will drive the ColdFire and also the steering servomotor. This regulator
has reverse battery protection and has a low dropout voltage at .5 volts. This means
the input voltage from the battery has to be 5.05 volts for it to operate. The
regulator has a very low voltage drop out. Most are not this low.
The 3.3 volt regulator input pin needs to be connected to the output pin of
the 5 volt regulator. This is an MCP1702 regulator that will take the 5 volts down to
3.3 volts with a maximum current output of 250mA. This will power the XBee which
is the serial communications receiver, but the regulator has a low dropout voltage of
2.8 volts.
Next comes the reading of the battery voltage. The battery can be 10 volts.
However, that’s too much for the ADC port pin of the ColdFire port PTD4 which was
selected for ADC conversion. The solution that the team had was to use a voltage
divider. This is a linear circuit that outputs a fraction of its input. By using two 10k
ohm resistors, the divider circuit will cut the voltage in half thereby making the
batteries’ voltage VBATT/2. The formula that was used to find out what resistors
𝑅2
were needed was (𝑅1+𝑅2) ∗ 𝑉𝑖𝑛. The way to wire a voltage divider is to set up a
resistor coming in from Vin, and then another resistor wired in series to ground. The
5 volt power supply is wired in the junction where the two resistors meet. And this
voltage is then sent to the port pin for the ADC and read by the ColdFire
MCF51JM128.
Proposed Software Solution
The software aspect was a challenge because of the manipulation of the ADC.
It was necessary to research the way in which we had previously manipulated the
ADC in our CET 335 course last semester. While looking through CET 335 notes, in
particular lab 7, there was a method which dealt with the issue of manipulating the
ADC. This was of course done in assembly language but we were able to gather
useful hints on how to address this problem in C++.
We also found hints to
manipulate the ADC in chapter 21 of the reference manual.
The first thing that needed to be done was to define VBATTMIN: This is a
variable we used to compare our voltages. Due to the hardware setup, VBATT is
actually VBATT/2, so we are only comparing half of the actual VBATT voltage in the
code. Here is the code that allows us to do this: #define VBATTMIN66.
The next thing that needed to be done was to initialize the ADC. We have
already initialized the ADC in the code but we still do need the following line of
code: APCTL2 |= APCTL2_ADPC11. This line of code allows PTD4 to be disabled as
GPIO.
Then we set the channel to the appropriate port.
Now the ADC is already
initialized, but it does need to know which port we are using. To do this, we used the
following code: ADCSC1 = 11.
By using this line, we set the ADC to channel 11
where PTD4 lives. We must also wait for the COCO flag which will tell the ADC that a
conversion
has
been
completed.
code: WAITFOR(ADCSC1_COCO).
Thus
we
now
require
this
line
of
We used the following line of code to determine the battery voltage:
VBATT/2 = ((ADC result register) * 500)/1024: This is an important line of code
that we researched and found by looking into lab 7 of CET 335. This is basically a
"scalability" equation used to get our resulting voltage in a form we can use. Of
course, the equation is open to change depending on the bit mode and if we want
our voltage in volts, millivolts, etc.
Finally, we used this line of code: VBATT/2 < VBATTMIN. This is our
comparison. It is also the only part of the code that runs through an if / else
statement. If VBATT/2 < VBATTMIN, then the battery is no longer supplying the car
with the proper voltage. When, and if this happens, we want to turn on a red LED,
turn off the motor, and also stop running the code. If VBATT/2 > VBATTMIN, then
we just want to turn on a green led. We used the following code to determine the
outcome if VBATT < VBATTMIN:
if (VBATT < VBATTMIN)
{
FB_RGB_BLUE = 1;
FB_RGB_GREEN = 1;
FB_RGB_RED = 0;
motorDC = 0;
FOREVER
{
}
}
else
{
FB_RGB_BLUE = 1;
FB_RGB_RED = 1;
FB_RGB_GREEN = 0;
}
The code that is used to turn off the motor is this: MotorDC=0. We also used
the forever loop to keep the processor from sending and transmitting data to the car
once the voltage drops below this threshold.
Resources and References
 "250 MA Low Quiescent Current LDO Regulator." Microchip Technology Inc.
Microchip Technology Inc., n.d. Web. 11 Mar. 2014.
 "ALIMENTAZIONE." ALIMENTAZIONE. N.p., n.d. Web. 11 Mar. 2014.
 "L4941 Very Low Drop 1A Regulator." STMicroelectronics, n.d. Web. 11 Mar.
2014.
 "LM2940/LM2940C 1A Low Dropout Regulator." Texas Instruments. Texas
Instruments, n.d. Web. 11 Mar. 2014.
 "Nickel–cadmium Battery." Wikipedia. Wikimedia Foundation, 28 Feb. 2014.
Web. 11 Mar. 2014.
 "PDIP | ChipKIT Development Platform." ChipKIT Development Platform. N.p.,
n.d. Web. 11 Mar. 2014.
 "Voltage Divider." Wikipedia. Wikimedia Foundation, 03 JulY2014. Web. 11
Mar. 2014.
 MCF51JM128 Reference Manual
 Notebook reference CET 335 lab 7 Analog to Digital Input