Rhococo prototype hardware debugging
Unfortunately most new hardware won be 100% functional on first try, Rhococo Rev 1.8 is no exception. I already noticed a few non optimal things in the layout. For one, I noticed that all the ceramic radial caps I had bought had wires bent to a 2/10 inch pitch, while the PCB used 1/10 inch, and 3/10 inch, that is already a reason for a new (rev 1.9) redesign.
But during first use, I also measured some voltages that were no exactly right. For one the supply voltage for the SRAM was not half a volt or so lower than 5V, but much less, the voltage drop over the D9 diode was just a few hundred millovolt (0.25V or so, not the expected 0.6V) meaning the current the SRAM draws is very low, a 1N4148 needs a good mA for a 0.6mA drop. Now is this more a theoretical than a practical problem, because Iḿ sure that when the SRAM is accessed (chip enable going low) the current consumption will immediately rise, and the diode will do its job, still I would prefer it when the voltage would be a stable 4.5V. I think the simple solution would be to place a resistor across C11 that draws 0.5mA from 4.5V, so an R = V / I = 4.5 / 0,001 = 4500 Ohm, or 4K7. I tried it by soldering 4K7 across C11, on the back of the PCB, and it worked, the supply voltage across the SRAM did indeed drop by 0.5V, to 4,7V. that should work.
a more difficult problem to solve is that I measured 3.9V across the 3V3 supply! somehow the 3V3 is pushed up, but I had expected it to! the reason is that 5V signals from the Z80 end up on inputs of the Propeller chip through the current limiting 1K resistors in the data-lines, and the Z80 control output signals /RD /WR and /IORQ, unfortunately I also found the first real mistake, in that I forgot to insert a limiting resistor in /IORQ, but moved it to the /BUSACK signal where it isn't really needed.
So I have to find a solution for that.
For R30 to R37 (data signals) and for R25, R26 and R27, I also plan to remove these resistors, and use round female socket pins instead, so that I can quickly replace these resistors with different values. A first attempt to remove the resistors failed, I have to get better solder wick, and so I choose to take the board to my day-job, where I have better equipment. I cannot afford to damage the PCB when pulling out these resistors. So more about this problem later.