In this video, we will show you the difference between a correctly functioning MOS 6510 CPU and a broken one. We will first analyze the clock signal coming out of the VIC and then those of the CPU.
Analyzing the Clock Signal
Connect the oscilloscope to the VIC: Attach the oscilloscope's probe to the clock pin of the VIC chip.
Set the oscilloscope settings: Adjust the oscilloscope's settings to display a stable waveform. Typically, you'll want to set the time scale to around 2 microseconds per division and the voltage scale to 2 volts per division.
Observe the clock signal: If the VIC is working properly, you should see a clean, square-wave signal with a consistent frequency (around 1.5 MHz for the Commodore 64). Any distortions, irregularities, or missing pulses in the waveform could indicate a problem with the VIC or its associated circuitry.
Analyzing CPU Signals
Connect the oscilloscope to the CPU: Connect the oscilloscope's probes to appropriate address, data, or control lines of the CPU. Refer to the Commodore 64 schematics or service manual for specific connection points.
Trigger on a known signal: Set the oscilloscope to trigger on a known signal, such as the PHI2 clock signal. This will ensure that the waveform display is synchronized with the CPU's clock cycle.
Observe CPU signal patterns: If the CPU is functioning correctly, you should see specific signal patterns corresponding to the CPU's instructions and data manipulation. These patterns can vary depending on the instruction being executed. Deviations from expected patterns could indicate issues within the CPU itself.
Remember, analyzing CPU signals requires a good understanding of the CPU's architecture, instruction set, and timing diagrams. If you're not familiar with these concepts, it's recommended to seek assistance from someone with electronics or computer repair expertise.
An oscilloscope is a valuable tool for diagnosing issues with a CPU, but it's important to understand its limitations. Here's why it can be useful:
Visualize Signal Integrity: CPUs operate with very fast electrical signals. An oscilloscope allows you to see the shape (waveform) of these signals over time. This helps identify problems like:
Distortion: The signal should be clean and square-like. Distortions like rounding or ringing could indicate issues with the CPU itself or surrounding circuitry.
Noise: Excessive electrical noise can interfere with proper operation. The oscilloscope can reveal unexpected spikes or variations in the signal.
Timing Issues: While not ideal for precise timing analysis, an oscilloscope can show gross timing problems. For example, a delayed or missing clock signal could point towards a fault.
Compare to Reference Signals: If you have access to a known-good CPU or reference data, you can compare its signal characteristics on the oscilloscope with the one you're troubleshooting. This helps pinpoint deviations that might be causing malfunctions.
Chapters:
0:00 Wrong CPU Signal
0:25 Mounting a working CPU
1:07 Testing VIC clock signal
1:14 Testing clock on CPU pin
1:37 The PERFECT signal on PIN 3
13 июл 2024
