Wednesday, January 23, 2002  (Lecture)

    Today,  before the lecture, we quickly viewed some upgrades for our web page for the first time as a team.  We only had about 5 or 10 minutes, but it was fun and exciting to see all of the different free things you can download to add some color to a page.

    The lecture for the day consisted of an in-depth study of how the RCX microprocessor works.  The Power Point Presentation for the RCX Lecture contains pictorial information on the internal workings of the microprocessor as well as study notes.

The RCX Brick

The lecture began with informational facts about the Lego Brick:

• 16 million cycles/sec
• 16,000 bytes ROM (this stores basic system software and hardware BIOS)
• 32,000 bytes RAM (this stores firmware and programs)
• 8 bit processor
• 8--10 bit analog to digital converter (2^10-1 = 1023, the digital value that is displayed on the LCD screen)
• 3 input ports labeled "1", "2", and "3" (these ports activate different types of sensors)
• 3 output ports labeled "A", "B", and "C" (these ports run motors)

Firmware

The RCX also has an external part to its operating system called firmware.  This software is stored in RAM and can be continually upgraded for programming purposes.  This operating system allows the RCX to recognize program languages an takes up the majority of RAM leaving 6,000 bytes for programs by the user (approximately 1000 lines of instructions).  This is small in comparison to a personal computer but is large in comparison to the RCX microprocessor.

LCD Display

The LCD displays:

• the system clock
• the value on input and output ports
• the number of the program currently running
• the indicator for the catalog sensor (this sensor has 1 minute divided into 4 sections of 15 seconds each)

For a demonstration of the output port power levels, an oscilloscope was used.  This enabled us to see the value of voltage readouts at different power setting of the RCX.  However, even though the LCD displays 8, the highest programmable power level is 7 (this is because the levels start at 0 which is equal to 1).

Notes:

• If a set of sensors and motors were connected in parallel (i.e. on the same port) the same voltage is applied to everything.
• Inductors keep the power going where as resisters make the current discharge.
• Conductors take in the current
• The design of the sensor port prevents the RCX from shorting.
• The instructor enabled us to listen to a square wave, unfortunately it shorts the microprocessor
• Shorting causes a voltage to read 0
• Motors create the largest power drain on the RCX
• Polarity is determined by the way sensors are connected to the ports
• The microprocessor reads the voltage drop to configure output
• The microprocessor alternates quickly between task to make it appear that multiple task run simultaneously

• Touch sensor
• Temperature sensor

Active Loads

• Light sensor
• Rotation sensor

Input port readings with no load have a voltage drop of 5 volts equal to 1023 by the A/D converter.

• 5V=1023 display
• 2.5V=512 display
• 1.5V=307 display (equivalent to 1 AA battery)

• Open circuits prevent current flow (display reads 1023)
• Closed circuits allow current flow (display reads 0)
• Voltage loops equal 0
• When a load resistor equals the internal resister, the voltage is split between

Voltage Law: The sum of a voltage drop on a closed loop = 0
Ohm's Law: Voltage drop across a resistor = Resistance*Current = RI  (V=IR)
To determine resistance or current in the RCX:   -5 + IR +IRL = 0 or I= 5/(R + RL)
Therefore voltage across the load resistor of the RCX: VRCX = IRL = 5RL/(10,000 + RL)
The LCD readout: LCD = 1023RL/(10,000 + RL)

Homework:  Read chapter 4