Articles from Techtravels Amiga Blog

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Sweet success, card works 100% in different DUT(Amiga)

So I think the Amiga 500 I was testing with has problems.  I’m not sure precisely what or why but

Today, I dropped my new A501 replacement card into a known-working Commodore Amiga 500.  It fired up immediately no issues with booting, everything worked, no wonkiness, memory tests passed 100%.  This amiga has an accelerator card, and I was able to run 300 tests of pretty much the entire memory space on the card in about 5 minutes.  Everything worked without error.

I’d like to get more data points here before I call it DONE, but it looks GREAT to me!

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Logic Analyzer probe loading seems to fix boot issue

While I had my logic analyzer connected to my memory card tonight, I noticed that I no longer had the white screen hang on bootup problem that I’ve been plagued with since this card came in.

As a matter of fact, it was booting up about 100% of the time.

I disconnected my HP logic analyzer probes, and all of a sudden, the Amiga won’t boot any longer.

It seems as if the impedance loading of the probes have been POSITIVELY affecting my circuit….slowing down the rise times and fixing whatever issue was in place.

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Stress testing the memory board

So I’ve written some assembly in the monitor software I’ve been using, and I’ve just created a large loop to see if I can get the card to fail.

I’m still having plenty of problems on bootup, but everything else is checking on.  So I’m not sure what the cause is, but I’m double checking that the memory system is reliable over a large number of reads/writes.

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RTC Progress

RTC appears to be working and keeping the date and time across cold-starts.

I picked the Epson RTC-72423 which is about a $6 chip, but is obviously directly compatible with the OKI Semiconductor MSM6242B.  The design, including the battery supervisor, had never been tested — not even as a prototype… and appears to have worked the first time!

I needed to do a “setclock reset”, set the date via “Date 28-Dec-13 02:26:00″ command, and then did a “setclock save”  Subsequent setclock loads brings it back!

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Found potential race condition on writes

After looking at datasheets for the 273 latches used and 74act74 flipflop used, the worst case propagation delay is something like 12.3ns.

But my NAND gate ACT00 logic which is triggered by the same CAS signals has a lower propagation delay of between 1-9ns.

So despite a 0ns setup time, it’s possible that I’m violating the SRAM setup times during a write by dropping /CE before the addresses have stabilized into the memory.

I haven’t come up with a solution yet, but it will likely involve adding an additional gate or two to increase the propagation delay.

 

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Populated PCB with fix for unneeded inverter

 

Lots of progress but little time to talk about it.

Here’s the fully populated card installed.  There were two issues identified so far with the PCB design.  One, the battery holder silkscreen was upside down.  The second fix involved an inverter that needed removed whenever we decided to migrates from a flip-flop(74LS74) design to a SR NAND latch made out of NAND(74LS00) gates.

The memory tests are passing at 100% now!@#

However, there is still a minor issue with how the card responds on bootup, and I’m investigating!

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PCBs have arrived!

Just in time for Christmas, the PCBs have arrived from China.  I used Seeed Studio (Fusion PCB Service) and they look pretty good.  Here’s the first look at them.

I’m just about to solder one up, so wish me luck!

am_pcb_front

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Found problems with incorrect footprints used for some chips

So I received all of the SMT parts today, and attempted to verify that I’ve used the right footprints for the parts.

This was a worthwhile effort because I found a few different problems.

Also, digi-key shipped me some mystery chip instead of my 3v supervisor….. they are fixing…..and I have the batteries and battery holders shipping soon as well.

Things are moving along…….

 

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