The development of HEC is behind schedule... - HEC News 3

Despite my best effort, the development of HEC is still behind schedule. The main cause is an unstable UM62256 RAM chip (that somehow became stable after a few days...).
Currently the plan for the HEC is:
Write a small serial monitor program to load program/flash the ROM via serial port. (so that I don't have to take the EEPROM out every time I want to update the code)
Enable interrupt and system calls.
PS/2 keyboard support (so that the HEC can operate as a standalone computer)
Full protected mode support

[@CarlosFBCruz]

Great project. I'be been following since the first video here from Brazil. Success!!

[@haroldjayhoover5370]

Happy New Year! I hope you're doing ok. I look forward to seeing more of your project.

[@ericwazhung]

You might be on to something regarding noise... I found in a project that most all my signals switch at roughly the same time (e.g. the data bus), it causes a LOT of electrical noise. But, they don't cause problems with each other (e.g. if they were all bundled together) because they're sampled quite a bit later, after their switching noise has stabilized. However, the gigantic amount of noise caused by their all switching simultaneously *does* couple into *other* signals (especially if their wires run parallel) like clock signals and, say, RD/WR, INTs, etc. which can change at any time. One easy fix, I found, was to use coaxial cable (thin stuff, like used in laptops to their wifi antennas, or even from things like Composite-video cables) for sensitive signals like those, (grounding the shield). Especially for the system clock. Another trick is to just run signals like that roughly perpendicularly over the bus wires. Don't let them run near each other in parallel! This sort of problem is not only hard to see on an oscilloscope because it's rare, but also because attaching the scope probe to the noisy signal can cause the noise to dissipate in the probe, itself. Heisenberg Uncertainty Principle of electronic noise! Also, do you have a solid ground/power topology? And decoupling capacitors?

[@andyhu9542]

To your final questions, the answer is no. The board is just thrown together with a giant bunch of wires and almost all decoupling caps are in the video circuit (to reduce noise in the analog signal). I think the ‘eisenberg Uncertainty Principle of electronic noise’ is probably why the RAM stablized itself (meaning it may not behave like this when I stop debugging it!) If that is the case, I will add a few decoupling caps.

[@hbox21]

this sounds like a problem with the ULA chip not getting priority over the z80 ?

[@andyhu9542]

Nope, it turns out that there was a problem with software...