exxos's Atari, Amiga & retro forum

terriblefire wrote: 14 Jun 2024 22:24

Kepler wrote: 14 Jun 2024 18:07 I wish I had missed that and it was the cause of this grief...but it was the first IC I replaced. 😭

Is there any way you can capture /AS low and ROMOE at gary at the same time on your scope? trigger on the falling edge of /AS once .. if you could hold the machine in reset with the power on beforehand that would be even better.

Do you mean put the positive lead on _AS and _ROMEN of Gary at the same time or do you mean two different positives to capute two readings?
Also, I have never used a trigger on the scope before, although I have just watched a few tutorials...
It may also be helpful to explain what this test is for.

Re: Trigger.

Your 'scope should be able to set a voltage level to trigger the display when a condition arises. You should have a knob which sets the voltage threshold that causes the trigger to fire. There should also be a setting to say whether this is when the voltage is rising or falling.

In this case set the trigger to use channel 1 and a falling voltage. Set the voltage "trigger level" to be around 2 volts.

Connect the ground leads of both probes to a ground somewhere convenient and the connect channel 1 to the /AS pin on the CPU and channel 2 to the ROM /OE pin.

Hold down reset in some way and press the "Run Once" button on the 'scope. Release the reset.

The oscilloscope should then capture a trace on both channel 1 and channel 2 when the /AS (Address Select) line goes low and then stop leaving the traces on the screen.

If the CPU is working then this is always going to happen as it's a signal to the rest of the computer that the CPU has put an address onto the bus and it's valid.

The CPU will be initially asking for the data from address 0x00000000 and the system should be triggered to activate the /OE (Output Enable) signal to the ROM so that it puts the data onto the data bus.

Excellent post, thanks.
I have done as requested but can't seem to select a trigger voltage higher than +800mV and that is by selecting Mode "Normal" under the Trigger section.
Here is my scope.

Your vertical scale is 200mV per division which means that you need five vertical lines to get to 1 volt. You need to decrease the vertical scaling down to 2 volts per division to see the signals on the screen and adjust the trigger setting.

You will need to do this individually for each channel by rotating the bigger, bottom knob for each channel probably anti-clockwise.

P.S. I notice that channel 2 is set at 2.5V per division so that's fine.

stephen_usher wrote: 15 Jun 2024 13:57 @terriblefire given that the OP is reporting different symptoms depending upon which ROM is installed, do you think it may be a good idea to perform this test without either ROM installed, just in case the fault is being compounded by the ROM chip(s)?

I'm thinking it eventually hits some code in rom.. I'd like to see ROMOE and AS in the same time base coming out of reset.

stephen_usher wrote: 15 Jun 2024 19:12 Your vertical scale is 200mV per division which means that you need five vertical lines to get to 1 volt. You need to decrease the vertical scaling down to 2 volts per division to see the signals on the screen and adjust the trigger setting.

You will need to do this individually for each channel by rotating the bigger, bottom knob for each channel probably anti-clockwise.

P.S. I notice that channel 2 is set at 2.5V per division so that's fine.

I think I have done that now but neither on start-up nor on reset does anything register on the scope:

Kepler wrote: 15 Jun 2024 20:09 I think I have done that now but neither on start-up nor on reset does anything register on the scope:
IMG_20240615_200833.jpg

You had said before /AS would wiggle. If thats not the case anymore go back to checking the clocks and make sure reset really goes high.

When you get this right you will see AS in the middle of your scope screen transition from high to low. I want to see what ROMOE is as that happens.

A really dumb guide to triggers...

Kepler wrote: 13 Jun 2024 07:06 By "wiggle", I assume you mean fluctuate high and low?
I have just checked and both 1 and 19 on both ICs remain high.

I think there may have been some misunderstanding. The only time you have asked me if something "wiggles" is concerning the two logic ICs 74LS245.
I'm afraid _AS on the CPU doesn't fluctuate. It goes straight to high on start up.
Reset does go high and the clock is stable at 7.09Mhz.

If /HALT goes high as well as /RESET this suggests a bad CPU as the first thing that the 68000 does when coming out of reset is put 0x00000000 on the address bus and lower /AS so that it can read the vector at address zero. It's not running code at this point, it's an autonomic function of the chip.

_HLT doesn't go high. It stays low with a very minor fluctuation to high in between. But it primarily stays high. I have tested the CPU and it is fine.