Back to: Build a 6502 based computer
One of my goals for this project was to push the clock speed that the computer could work at, possibly overclock it!
The way that I plan on doing this is by replacing the current crystal oscillator with one with a significantly higher frequency. I will then feed this into a circuit that divides the frequency of the crystal oscillator into fractions, which can then be tapped at different points. I want to design the frequency divider circuit so that it has a range of values that include a known working frequency (1MHz) and higher, including some that definitely won’t work at the momeent. This way I have stable frquencies to work with and frequencies that I can make adjustments to, to see if I can get the computer to work out. As well as making modifications to the hardware, there will need to be software modifications.
To keep with the remit of using era appropriate parts, I will look to use 74 or 40 series ICs. There are a number of possibilities but I want to use a 4040, which is a 12-stage binary ripple counter. This IC contains 12 master slace flip flops. The output of each flip flop feeds the next and the frequency of each output is half of the previous.
There is a reset pin but as I want this running all of the time, I have no plans to use it. RESET is active-high, so I will tie it to ground
The frequency of the crystal oscillator that I want to use is 10MHz. There are a couple of reasons as to why I have chosen this frequency. One if that this will include frquencies close to what is currently working and also some that are high enough to experiement with. The second is that my VGA graphics card that I am designing will use a 10MHz crystal oscillator! My thinking is that I can omit the crystal oscillator from the card and it can connect directly to the crystal oscillator. This way I have used one less crystal oscillator!
As there are 12 stages in the 4040, I will use a switch with 12 selections, so I will use a 1-pole 12-selection rotary switch.
| 10 MHz |
| 5 MHz |
| 2.5 MHz |
| 1.25 MHz |
| 625 KHz |
| 312.5 KHz |
| 156.25 KHz |
| 78.125 KHz |
| 39.0625 KHz |
| 19.53125 KHz |