On Sun, Sep 12, 2021 at 2:00 PM Gerrit Heitsch <gerrit_at_laosinh.s.bawue.de> wrote: > > > > > I've put the UM6502A and the suspect UM (chinese sanded and rebadged > > chip) on other Asteroids board and the "problem" follows these two CPU. > > R/W and phi2 signals just go directly to a 74LS244 input, so the CPU > > doesn't need to provide fanout for all the inputs. > > My board Vcc is 4.9V at the 6502 pins, so it shouldn't be an issue. > > With the oscilloscope I've seen the rising edge of phi2 is taking a bit > > more than 10nS. The UM6502 have the slowest rising edge of all CPUs I > could > > test. Of course the 65C02 has a nearly vertical rising edge (and works > > fine on Asteroids anyway). > > MOS/SY/R 6502 have a better phi2 rising edge than the UM, but the > > differences are marginal, maybe 2-5ns as far as my scope can tell. Also > > the "1" level > > on the MOS/SY/R are around 4.5V, while the UM goes to 4V (which > > shouldn't matter on TTL logic anyway). > > Have you tried to add a 3.3kohm or 4.7kohm pullup to PHI2 and maybe R/W > on the UM6502A? That would make the signal rise faster and get you a > higher HIGH. > > this is a very good suggestion! I'll try this "soon enough" when I finish repairing a few PETs that have been sent to me and they are now occupying the bench. I'll report the results. Frank IZ8DWFReceived on 2021-09-12 15:02:38
Archive generated by hypermail 2.3.0.