College Board Information
An Assembled College Board:
Major Features
- Professional-quality double-sided printed circuit board
- Complete 5-volt, 1 Amp power supply (reverse-voltage protected)
- Innovative "breadboard-straddling" design allows for separating input and output circuitry
- Input voltage can be routed to breadboard, allowing for ADC voltage measurements for battery supplies
- Includes 28X1 reset circuitry and socket for ceramic resonator, simplifying change of external resonator
- Completely self-contained; serves as a bug-free and reliable platform for project development
- Provides an inexpensive, safe and flexible solution for classroom applications
Breadboard-Straddling
The easiest way to set up the College Board for breadboard use is to connect two 830 tie-point breadboards as shown below. Notice that the College Board silk-screen labels identify the 28X1 I/O pin connections and that the 8 connections for Port C are duplicated along the top and bottom edges of the College Board. Although Port C pins are all inputs by default, they can be re-configured either as inputs or outputs, so their duplication allows you to install all your output circuitry on the top breadboard and all your input circuitry on the bottom breadboard, which simplifies the wiring involved - just be sure to make your connections accordingly, and to not connect the same pin on both edges of the College Board! Also note the small amount of space between the breadboards. For some reason, when two breadboards are directly attached to each other the row spacing between them is not an exact multiple of 0.1 inches (IMHO, it should be)!
If you would like even more flexibility in circuitry layout, take a third 830 tie-point breadboard, remove the power rails, and cut the main section in half along its center-line. Attach one half to each side of the College Board as shown in the following photo. This arrangement allows for additional flexibility in wiring your projects because connections can be routed to the left or right of the breadboards without using up space in the main breadboard area.
Accessing the Input Voltage
The 3-pin male header located between the two electrolytic capacitors and labeled "Vcc Out" (see top photo on this page) is used to enable access to the input voltage if desired. For example, if your project is battery-powered you can place the 2-pin shunt in the "ON" position and access the real-time battery voltage level (less the drop across the 1N4001 diode) at either of the Vcc pins on the edges of the College Board. By setting up an appropriate voltage divider, you can monitor the real-time Vcc voltage level via any ADC input pin. Warning: if you aren't certain of how do do this, it is best to leave the "Vcc Out" shunt in the "OFF" position. Applying more than 5 volts to any PICAXE pin can damage or destroy the pin, and possibly the PICAXE processor as well!
A second reason to access Vcc on the breadboard would be whenever you have peripheral circuitry that requires voltage levels greater than 5 volts in order to operate. Whenever you enable Vcc access, it is most important to route it carefully on the breadboard. Failure to do so may result in damage or destruction of mis-wired components!
Ceramic Resonators
The PICAXE-28X1 does not require an external ceramic resonator to operate. Whenever there is no external resonator present, it defaults to its internal 4 MHz resonator. In fact, with a program running with (for example) a 20 MHz external resonator in place, the resonator can be physically pulled out of the College Board's 3-pin resonator socket and the program will continue to run at a speed of 4 MHz. To verify this, just write a simple LED-blinking program, run it, yank out the resonator and watch what happens!
The downside of the 28X1's flexible internal-external resonator features is that it is possible for your project to be running at a speed other than 4 MHz when you attempt to download a new file. If so, the download will fail to happen because it requires 4 MHz to operate. The solution to this problem is to do the following: Hold down the College Board's reset button, start the download and then release the reset button. This procedure works because the 28X1 always defaults to its 4 MHz resonator on startup or reset.
One Final Note
The two 2-pin headers located next to the 5x2 header and the power connector (see the top photo on this page) are not connected to any circuitry on the College Board; their only function is to physically support the end of the board. Therefore, it's OK to use that area of the breadboards for your I/O circuitry.