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Forum:
Hardware
Beiträge im Thema:
7
Erster Beitrag:
vor 2 Jahren, 5 Monaten
Letzter Beitrag:
vor 2 Jahren, 5 Monaten
Beteiligte Autoren:
Michael Schoettner, Dennis Kuschel, William Buchholz

Startup problems - CPU & memory

Startbeitrag von Michael Schoettner am 12.09.2015 13:23

Dear all,

I have done a first test with the CPU and the Memory Base Board (6 PCBs in total) and run into problems. When powering up (5V on bus connector, PIN1 and 3) the four LEDs of the MemBoard go on and remain all on (So unfortunately, no moving lights). According to the post status table, 4 LEDs show address logic errors in the memory extension board, which is however not even connected in my case.

As recommended by Dennis I will first check whether the CPU is trying to start at all by looking at the /RD_ROM line which should show pulses whenever the CPU wants to read code from the EPROM. Before I will re-check all components and my soldering. Furthermore, I will replace all AC type ICs to HCs variantes to improve stability of the CPU and play with JP4 next week.

Any hints from people that experienced the same issues are more than welcome. Otherwise I will post my progress here in the forum to help other people that may face similiar bugs in the future.


Thank you & best regards,

Michael
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Antworten:

Re: Startup problems - CPU & memory

Hi Michael,

the initial state of the four LEDs is random. As long as the CPU is not working in any way the LEDs may show a static but random pattern. In your case all four LEDs are on. To ensure the LEDs really show an error code, you should observe the signal at IC1 pin 11 with an oscilloscope. Only when you observe a low-level pulse on the pin the register gets loaded and the LEDs are showing a correct value. It is most probably that the CPU is not working at all when all four LEDs are on but no memory extension board is installed.

von Dennis Kuschel - am 13.09.2015 13:27

Re: Startup problems - CPU & memory

Dear Dennis,

thank you very much for your reply. I have changed all AC type ICs to HCs types and "played" with JP4. The other jumper are in default configuration and I use one oscilator 20 Mhz. No luck so far.

I tried to observe some signals on the bus connector with a logic analyzer during startup, namely /RDC (A9), BUSCLK (A11) and /RD (C9) and /WR (C10).

The BUSCLK looks fine. But the other signals seem to be inverted, e.g. /RDC is low most of the time and goes up (in sync. with BUSCLK), which is not correct. I had a look at the schematics of "Microcode and common steering" and found IC16.C doing the RDC output. However, here it is not inverted to /RDC. Is this a typo or am I missing anything?

Furthermore, can you or anywone else give me an idea where to look for to find the origin of this inverted signal behaviour for /RDC. I hope this can help me out.

Many thanks in advance.


Best regards,

Michael
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von Michael Schoettner - am 19.09.2015 16:05

Re: Startup problems - CPU & memory

Just found one wrong resistor on thy myCPU interface board :mad:

R16: target: 22k, actual: 220k

And I saw this resistor is related to the reset curcuit. Could this cause my startup problems?

Many thanks again.

Michael
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von Michael Schoettner - am 19.09.2015 16:50

Re: Startup problems - CPU & memory

Those resistors are related to the set point of the comparator. It holds /RDC low until the power supply voltage stabilizes to its proper value. Notice there is feedback from the output of IC33A through R17 back to the input. So if the set point is incorrect then when /RDC goes high then it will immediately go low again, halting the CPU clock.

The issue that caused me the most headache when building was the crimp connectors for the backplane. My first one didn't press evenly so some connections were intermittent/high impedance. I found this before hooking everything up. My second attempt had a short between two lines of the address bus. Visual inspection showed no issue. Continuity was fine from each connector to the other connectors. Probing with my oscilloscope showed that the signal was only reaching ~2.5V on two of the address lines. It didn't occur to me to check for shorts between adjacent wires.

von William Buchholz - am 19.09.2015 20:04

Re: Startup problems - CPU & memory

Hi Michael,

Quote
Michael Schoettner
The BUSCLK looks fine. But the other signals seem to be inverted, e.g. /RDC is low most of the time and goes up (in sync. with BUSCLK), which is not correct. I had a look at the schematics of "Microcode and common steering" and found IC16.C doing the RDC output. However, here it is not inverted to /RDC. Is this a typo or am I missing anything?

Furthermore, can you or anywone else give me an idea where to look for to find the origin of this inverted signal behaviour for /RDC. I hope this can help me out.
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It is a typo, the signal RDCODE has negated logic, it should be named /RDCODE. And it is not generated by IC16.C, but by IC20, output pin 14 (OECOD). IC16.C adds only a bit logic to allow fetching the next OP-code one cycle earlier.

I am not sure why the RDCODE signal looks inverted to you - my assumption is that your MyCPU never gets reset (your reset logic has a bug), and the CPU starts executing the code anywhere in the ROM, leading it immediatly to crash. The CPU may hang in an endless loop that generates this pattern on the RDCODE wire.

von Dennis Kuschel - am 20.09.2015 08:54

Re: Startup problems - CPU & memory

Good news, the CPU and the memBaseBord work now :xcool:

(As described by William and Dennis the wrong R16 prevented my CPU from starting up). I also checked all my cables (including backplane) with a function generator and oscilloscope - everything OK.

Due to the 50 KB max. file attachement size I can post only a photo (but I have the moving lights).

(The LEDs are connected using short cable as I plan to later place the PCBs in a case).


Michael
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[attachment 63 mine.jpg]

von Michael Schoettner - am 21.09.2015 10:39
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