cbmeeks wrote:
Is it advisable to simply plug in a 1 MHz crystal oscillator (can) directly into the CPU? On my oscilloscope the signal appears pretty "square". Or would it be better to run it through some inverters to square it out a little more?
Most of these can oscillators produce ample drive strength and a clean output. For hobby usage at moderate Ø2 frequencies (10 MHz and under) directly driving the MPU (and 65Cxx I/O if used) from the can is fine. My recommendation is once you cross that 10 MHz threshold you should run the can's output through a flip-flop (e.g., 74AC74 or 74AHC74) to sharpen the waveform and assure true 50:50 symmetry. Naturally, the oscillator's frequency in such a setup would be twice the desired Ø2 frequency. You should do your best to keep the signal path from the Ø2 generator to the Ø2 input on the MPU as short and direct as possible.
Incidentally, Jeff Laughton (Dr. Jefyl) has developed animated forms of a WDC timing diagram that illustrate, among other things, the effects of an asymmetric Ø2 clock.
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My faster crystals start appearing more sine wave in shape.
As Ed noted, scope bandwidth limitations may be why this is happening. A rule of thumb is that the scope's bandwidth should be at least 10 times that of the frequency being sampled in order to see a reasonable representation of a square wave. Your probe will also have an effect. The probe should be operated in the ×10 mode to reduce capacitive loading on the circuit and the scope's ground connection to the circuit under test should be through the ground pigtail on the probe.
The quality of the probe is also a significant factor. There is no such a thing as a good, cheap oscilloscope probe.
ArnoldLayne wrote:
Hi, this is basically what the WDC 65c02's datasheet is stating. So, you are saying that this not only goes for WDC but for 65c02 in general?
I'm asking because I did not find anything about this in the Rockwell datasheet.
This is design advice from WDC, who designed the 65C02—Rockwell enhanced the original 65C02 design, mostly with added instructions. WDC states that the prop time from Ø2-in to Ø2-out and Ø1-out is neither tested or guaranteed. This advice becomes important at elevated Ø2 rates, as signal slew can cause obdurate timing issues. For that reason, as well as the fact that currently available clock generation hardware has greatly improved over the years, all devices slaved to the Ø2 clock should be driven by the clock generator, not the MPU.
The two clock outputs on the 65C02 are there primarily to allow the device to be used in old hardware that was equipped with an NMOS 6502. There is really no good reason to use them in a new design, as modern clock generation hardware can easily drive many loads, which wasn't the case back in the days of the NMOS parts.
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Our homebrew-65(c)02-computer ("Steckschwein")...
Snort! Oink-oink!
BigEd wrote:
I was just thinking that sometimes people let features creep in, and it can be another way to not get started. Even character graphics - or serial I/O - is enough to make for an exciting self-built machine.
You are so right! Creeping featurism can not only increase the likelihood of a DOA design, it can result in the project never getting done.
Both of my POC units were designed with serial I/O driving a dumb terminal. By going this route, I was able to concentrate on the core logic and the development of the firmware, and not have to spend many hours trying to figure out how to put a picture on a screen or scan a keyboard. Hence from the time I first sketched out my design until I powered it only about four months elapsed, and it "almost" worked on the first try.
A final point to consider, and one that Ed also mentioned: while construction methods can have a strong influence on the success or failure of your project, some sloppiness is tolerable if you don't intend to run the unit too fast.
You can get away with murder at 1 MHz. is something that has often been said around here. Construction can be wires on a perfboard and as long as the logic is right and the wiring is correct, it will work. Only when you decide to amp up the Ø2 rate will construction techniques become important, and that would be the time to do a tight wire-wrap unit, or get a PCB made.