Wednesday, March 13, 2024

Halcyon clone v2 repair log

  •  Wrote test ROM, then hooked up logic analyzer to Z80 to see how far it got.  Was surprised to see that RESET' was held low.  Using logic probe, looked at U45 which is a flip flop that controls the reset circuit and saw that VRESET' was low despite WRESET' being high.  Hooked up logic analyzer to U45 and saw that pin 13 (flip flop reset) pulses low shortly after WRESET' goes high, which seems like it shouldn't be happening.
  • Hooked up logic analyzer to U44 and saw that pin 8 (which is supposed to be an output) is mirroring pins 12 (an input that takes in a 4 MHz clock) and 11 (an output that takes the 4Mhz clock to the MC6803P microcontroller).  Pin 8 seems to be completely ignoring its inputs, pins 9 and 10.  The Halcyon uses a 74HC08 chip for U44 instead of a 74LS08 (which I am using for my prototype) so I'm going to try the 74HC08 to see if it makes a difference.
  • Replacing U44 with 74HC08 fixed this problem!