I am experimenting with the old CPUs. Got some from eBay. They are the MC68000 and the MC68008, both operating at 5V.
I basically wire them up on a breadboard by simply hooking up the Vcc/GND pins with 0.1uF decoupling capacitors. I source the +5V from the the USB port of my laptop.
I see that the USB power source gives from +5.0V to +5.11V. BUT when I measure the voltage the CPU's Vcc/GND pins, I get 3V or so. Why the voltage drop?
Is there something about these old CPUs. Of course, I could be getting dead CPUs, as buying such things from eBay is not 100% certain. The same for the Intel 8086 and the MC6809 CPUs.
BUT, when I try the modern, if you will, w65c816s CPU, WDC (Western Design Center's) 6502 compatible CPU that is still made today, it shows a strong 5V. Same for modern micro-controllers like the ATMEGA328, etc.
Does this have to do with TTL that is with old CPUs????