Wednesday, February 4, 2009

Powers up and a solid signal!

And here is the top view. Note the RESET and HALT switches.

Coffee. You can sleep when you're dead.

I stayed up all night in late December 2008 and soldered for a few hours. The data lines are yellow, address lines are green. Power lines are red. Ground lines are black (ok, a couple are blue). You are looking at the wiring of the CPU, ROM, 74LS138 Address Decoder, and the 7404 inverter. With programmed ROM, this computer should be able to run and step through a program. Also wired are a HALT momentary switch, and a RESET momentary switch.

MC6847 Video Character Generator

This is the MC6847 character generator. If you look to the right side of the breadboard you can see the amps/transistors to generate the correct output signal (which I left on the breadboard for my own reference before soldering) The circuit frequency is generated with a 4 pin crystal oscillator, running at the industry standard 3.58 MHz frequency.
Below the breadboard is the soldered version of the amp/transistor signal generator. You can see the RCA jack that is plugged into a NTSC LCD monitor. That was the first time I have ever created my own soldered board, and it worked the first time! Sloppy, but it works! This circuit also works with a composite monitor -- in my case, an old Commodore 64 monitor.
This circuit will generate Black and White only, but I am planning on adding an MC1372 so color can be generated. Although I slightly modified the amp circuit by using 2N4400 amps instead of the 2N2222, the design is by Joseph Norman. Thanks Joseph!

Tuesday, February 3, 2009

Clock Frequency IN and OUT

The bottom image shows the square wave generated by the 1.0 MHz crystal oscillator before reaching the CPU. The top image shows the signal that is coming out of the 6802 CPU (pin 37 "E").
The signal shows that the frequency coming from pin 37 will be one-fourth the frequency that is sent into the CPU.

6802 CPU with 2 pin crystal oscillator

It is difficult to see the difference, but this is the breadboarded version using a 2 pin crystal instead of the 4 pin crystal oscillator. You can see the 2 capacitors next to the crystal.

6802 CPU with 4 pin crystal oscillator

This is a Motorola MC6802 CPU with a 1.0 MHz Crystal Oscillator (4 pin). I wanted to compare the signals generated by the 4 pin (which includes the oscillator) vs the 2 pin crystal. The 4 pin was a bit smoother.

Soldering CPU to ROM data lines

This was back sometime in December 2008, wiring up the data lines from the CPU to the ROM. Note the lines leading to a location on the board where more data lines can be added to the bus in the future (e.g. when RAM is added) -- instead of going directly from the CPU to the ROM chip.