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.
For those of you who might not get the significance, workbench loads a number of libraries into the extended fast memory on bootup. As you can see in this image below, more fast memory is actually being used than the chip, onboard memory immediately after bootup.
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!
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.
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!
I placed the order late last night for the PCB!
The PCB design is obviously finished, fully passes the kicad DRC checks, the problems with the footprints were solved.
I’ve decided to go with Seeed Studio’s Fusion PCB service. This is based on a recommendation from a knowledgeable friend that has been helping me.
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…….
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:
