Articles from Techtravels Amiga Blog

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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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All SMT parts on order, rough idea of parts costs

So all the SMT parts have been ordered for the prototype PCB.  Once they come in, I’ll be testing the placement of the parts on top of a printed layout to see if the chips/parts match the layout — if there was a package screw up, and so on.

This board was never optimized for cost per se, but here’s what the major components cost:

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Sometimes the amiga recognizies my ram board, some very basic memory tests are passing

So I’m starting to make some progress here, but I still have some troubles with my expansion board prototype.

For the first time, the initial exec-rom-based check passed, and the memory auto-configured.  This happens occasionally on powerup.

Also, memtest is running a quick memory check and passing it with zero errors, but I really think it’s because it’s not doing a comprehensive test.

Troubleshooting is ongoing……….

 

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