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Tail Wheel Assembly
[click photo for larger view]

Tail Wheel Assembly:

As shown in the photo above, Micro-Bot uses a Great Plains RC airplane tail wheel assembly, and a precision "press-to-fit" 3x10x4mm micro-bearing for the rear tail wheel. The bearing is pressed into the rear bumper as shown below in Photo #1. To level Micro-Bot or make adjustments, remove the top metal collar using the hex key wrench provided. Loosen the lower black metal collar, and slide the axle shaft up or down through the bearing to make the adjustment. Re-install the upper & lower metal shaft collars pressing them both firmly against the bearing while tightening the collar set-screws. The shaft & tail assembly should spin freely when both collars are installed properly.

The precision micro-bearing provides very smooth mechanical movements of the rear tail wheel axle and prevents binding. Keep both collars snug against the micro-bearing to prevent lateral movement, but do not over tighten.

Photo #1: Tail Wheel Assembly & Press-To-Fit Bearing

Servo Connections:

Connect the left servo motor connector to the 3-pin header marked "A". Connect the right servo motor connector to the 3-pin header marked "B". The silk-screen legends on the circuit board labeled W are the servo PWM control wires.  These PWM control wire on some servo brands will be white, while others may be orange. Check the servo datasheet before making the motor connections to avoid damaging the servo.

Grand Wing Servo Wire Colors: [Default servos shipped with each Micro-Bot]

Parallax & Futaba Servo Wire Colors:

Figure #1 shows the correct servo connections for Grand Wing brand servos. If you're using servo motors with white/red/black wire colors, the white wire will connect to the W position. Red to R, and black to B.

Figure #1: Grand Wing Servo Connections

The pin sockets marked A & B shown above in Figure #1 are for making the PIC I/O-pin connections to the modified servo motor PWM control lines.

Use the two motor wires provided with Micro-Bot to make the connections between the A & B pin sockets [shown in Figure #1a below], and the PIC I/O-pin female header socket [shown in Figure #1b below]. The bare motor wire ends plug into the A & B pin sockets. The other end of each motor wire has a 0.025 square male pin which plugs directly into the female header sockets [shown below in Figure #1b] to connect the PIC I/O-pins to each servo motor PWM control wire. Micro-Bots motor wires are the two wires with heat shrink on both ends.

Servo & Main Board Power Connections:

Figure #2 shows the 2-position screw terminals for connecting the 4 x AA, and 9-Volt batteries. The servo power wires are used to hold the servo PWM control wires in place by running the servo power wires up & under the circuit board, then around the servo control wires, and then back through the plastic wire guide. We found this handy for keeping the servo wires tight against the plastic wire holder.

Four AA batteries provide power for the servo motors. Connect the AA battery wires to the left screw terminal marked SERVO as show below in Figure #2.

The 9-volt battery provides power for the microcontroller and experimental circuits.

Figure #2: Power Connection Screw Terminals

Connect the leads from the 4 x AA battery pack to the 2-position screw terminal marked SERVO as shown above in Figure #2. The leads from the 9-Volt battery holder connect to the rear 2-position screw terminal on the right in Figure #2 above.

Important Note: When making battery connections be sure to observe polarity markings on the circuit board. Plug the red [positive] battery lead into the screw terminals marked +, and black battery leads into the screw terminal marked - [negative]. Reversing polarity on the battery connections may damage the servo motors. The +5V regulator for the microcontroller power section is diode protected to help prevent damage, but if polarity is reversed on the primary power input, the microcontroller section will not have power.

Figure #2 above shows the primary power switch in the OFF position. The center position supplies power to the microcontroller section. Slide this switch into the MIC position to program the PIC in-circuit without power to the motors. Slide it to the ALL position to supply power the motors & microcontroller. Figure #3 shows a close-up of the primary power switch. Be sure to return the power switch to the OFF position when you're not using Micro-Bot to conserve batteries.

Figure #3: Close-up of Switches, Jumpers, Etc..

XT/IO Switch:

As shown above in Figure #3, the Micro-Bot circuit board has a switch that’s marked XT IO. This switch is used to select the PIC internal oscillator or external oscillator. It also connects or disconnects the PIC16F628 PortA.6 & PortA.7 oscillator pins from the female headers used for I/O connections as shown in Figure #3 above.

With this switch in the IO position, you’ll need to use the 16F628 internal oscillator, and replace the @ DEVICE HS_OSC entries with @ DEVICE INTRC_OSC to use the PIC16F628 series internal 4MHz oscillator..

Using the internal oscillator frees up pins RA.6 & RA.7 for use as normal I/O-pins. When using an 18-pin PIC without an internal oscillator, this switch should always remain in the XT position to ensure the external crystal is connected to the PIC.

Note: All of the code examples provided with Micro-Bot are written for use with a 20MHz crystal. Using 20MHz keeps timing compatible with projects you'll find for the BASIC Stamp, including the BOE or Board of Education BASIC Stamp robot. If you use the PIC16F628 internal oscillator, or prefer to use an external 4MHz crystal with Micro-Bot, you will need to make adjustments to the example code provided with Micro-Bot.

PGM/RUN Switch & In-Circuit Programming

The PGM/RUN switch is used to switch between in-circuit programming, and run mode. In the PGM position pins RB.6 & RB.7 of the PIC microcontroller connect to the in-circuit programming header allowing the PIC installed in Micro-Bot to be programmed in-circuit. The EPIC Plus, Warp-13, and several other device programmers are capable of in-circuit programming. If you plan to use a device programmer other than the EPIC Plus or Warp-13, check with the manufacturer of your programmer to be sure it supports in-circuit programming.

After in-circuit programming, simply move the switch back to the RUN position to use pins RB.6 & RB.7 for general purpose I/O. This switch allows programming the PIC microcontroller "in-circuit" without disturbing external circuits connected to the PICs program data & clock pins RB.6 & RB.7. Without this switch, we would need to remove the PIC from the socket for programming or disconnect external circuits from pins RB.6 & RB.7 prior to in-circuit programming. Loads on these pins will cause in-circuit programming errors, and possibly damage some types of external circuits. Refer to the Micro-Bot schematics for further details.

Figure #4 shows the EPIC Plus programmer connected to Micro-Bot, the PGM/RUN switch, and the in-circuit programming headers. The EPIC Plus programmer has a dual 10-pin header [dual row, five pins in each row] that's used for connecting Zero-Insertion-Force programming adapters to the EPIC. These adapters are for programming different package styles such as 8-18-20 pin DIP, 28-40-pin DIP, and many different surface mount package styles. This same 10-pin header also serves to connect the EPIC Plus to the Micro-Bot main board for in-circuit programming as shown below in Figure #4.

Notice how the 5-pin Warp-13 in-circuit programming header pins are labeled RB6, RB7, VPP, GND, and VDD. This 5-pin header corresponds to the Warp-13a programmer in-circuit programming header.

Important Note: VDD from the Warp-13a & EPIC programmers is not used to power the Micro-Bot board during in-circuit programming. The  VDD silk screen legend on the Micro-Bot board programming headers is for in-circuit programming cable orientation purposes only. The VDD header pin does not connect VDD from either programmer to VDD on the Micro-Bot circuit board.

Power to the Micro-Bot board during in-circuit programming is provided by the 9-volt battery power supply installed in the Micro-Bot battery holder. The OFF/MIC/ALL power switch must be in the MIC position during in-circuit programming, and the EPIC or Warp-13a must be separately powered by their own power supply's.

Micro-Bots main board has a Vpp blocking diode that isolates the programmers higher Vpp programming voltage from the main boards power supply rail during in-circuit programming. This diode is labeled D1 on the Micro-Bot Main Board Schematic

Figure #4: EPIC Plus Programmer In-Circuit Programming Connection

Easy Swap Oscillator:

Micro-Bots 20MHz oscillator is installed in pin sockets allowing quick changes to support different crystal oscillators. If you need to swap-out the crystal, simply un-plug the one that's installed & plug-in another one. Be sure to turn power off when replacing crystals. Some crystals have very long leads, and you may want to trim them down a bit to keep the crystal from extending too far above the crystal pin sockets. We've trimmed about 1/8" from the crystal leads as shown in the photos below.

Note: The Micro-Bot crystal oscillator circuit uses 22pF capacitors [yellow caps shown in photos below]. If you use another crystal with Micro-Bot, be sure to use a parallel cut crystal, and one that will function with the 22pF parallel capacitors in Micro-Bots oscillator circuit.

OFF/MIC/ALL Power Switch:

Micro-Bot has two separate power sources. One 9V battery for the microcontroller section, and four AA batteries providing 6-volts to the servo motors. The power switch [shown below] is marked OFF MIC ALL, and switches power to the microcontroller circuit when in the MIC position. In the ALL position power is switched ON to the microcontroller + servo motors. In the OFF position, power is switched OFF to both motors and primary controller. Turn power off before making any circuit connections, and when Micro-Bot is not in use.

Move this switch to the MIC position for in-circuit programming, and then move it to the ALL position to enable the servo motors. In the OFF position no power is supplied to motors or primary board, and batteries will be conserved. The green power indicator LED will be ON with this switch in the MIC or ALL positions indicating power is ON.

RESET Switch:

Shown above in Figure #3, the RESET switch connects ground to the PIC reset or /MCLR when pressed, and can only be used if you have /MCLR enabled. Using the @ DEVICE MCLR_ON option would allow you to use the RESET switch to force a hardware reset when pressed. Refer to the Micro-Bot schematic for details. It’s possible some folks may want to use the Micro-Bot board for PIC development with the in-circuit programming capabilities, so we added the external RESET switch to cover all possibilities. Reset can be used to force Micro-Bot to begin code execution from the beginning instead of resetting or cycling power with the power switch.

Pin RA5 is input only with /MCLR internally disabled, and is used during in-circuit programming. Installing jumper RA5/C connects RA5 to the prototyping headers to be used as an input pin.

WARNING: Vpp [programming voltage] is applied to RA5 by the programmer during the in-circuit programming process, and this may be potentially damaging to an external circuit or the programmer if the external circuit is connected to RA5 during in-circuit programming. Care should be taken if you use RA5 as an input pin together with in-circuit programming to avoid external circuit connections to RA5 during in-circuit programming.

If you do not need RA5 for an input pin, please leave the RA5/C jumper OFF. Micro-Bot is shipped without the RA5/C jumper installed, and the RA4/P pull-up jumper installed.

With jumper RA4/P installed, PORTA.4 is connected to the 10K pull-up resistor R1 as shown in the Micro-Bot schematic. RA.4 is a bi-directional I/O-pin, but can only output [sink] ground since it’s an open-drain type output pin. Connecting the 10K pull-up resistor to RA.4 with the RA4/P shorting jumper allows the pin to function normally. IE, it can output ground as any other I/O-pin, and +5V through the 10K pull-up resistor.

The photo in Figure #3 above shows RA4/P disabled, and RA5/C enabled. Be sure when you place the shorting jumpers on these headers to connect each jumper as shown on RA5/C in the Figure #3 photo. Each shorting jumper will be "in line" with the silk-screen legend on the circuit board when properly installed to enable the optional feature. You can remove a jumper or install it on one pin as shown in Figure #3 for the RA4/P disabled position. Leaving the jumper on a single pin when disabling a feature helps prevent loosing the jumper. These jumpers are small and easy to misplace.

Mechanical Drawings

Micro-Bot Main Circuit Board

Body & Wheel Assembly

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