Later Dallas had versions with removable battery packs as well so wave soldering could be used.
you missed the point or misunderstood : wave soldering batteries is a problem due to shorting them in the wave. it is perfectly acceptable to wave solder the dallas or mostke/st parts. the battery is not accessible. the reason for snaphat on the surface mounted parts is because of reflow. the batteries do not like reflow. wave soldering takes 5 seconds tops. a reflow cycle takes 2 to 3 minutes in a chain oven. also inspection and cleaning is problematic as the pins are covered by the hat.
the hat is not designed to be removable. even though you can pry it off : that is not intended usage.
I conflated wave soldering with reflow soldering but my point still stands:
The two-piece assembly protects the lithium battery, contained in the PowerCap (upper half) from the high temperatures of reflow soldering. The module base (lower half) is a small PCB with surface-mount leads, SRAM, and controller ICs. After the module base is surface-mounted, the PowerCap is snapped onto the base to complete the module.I might accept these reasons if many existing working designs which used a low power SRAM and battery and write protect circuit were not changed to use NVSRAMs. In some cases the existing board was not even changed. I considered altering my 2440 back to the original SRAM plus battery configuration but found a better option.
not sure what you mean here but traditional designs use bug 2/3 a cell packs or packs with rechargeables because even low power rams like the 4316 (6116 compatible) or 4364 (6164 compatible) draw too much idle current. leaking batteires are notorious for destroying boards. with the dip or snaphat leakage cannot occur (it is contained in the device)
The cell used in the snaphat or zeropowers is a small 237 type cell. adding a realtime clock was a bonus.
I mean existing designs which used a primary lithium battery which was hardly likely to leak and low power SRAM were changed. The parts for write protecting the SRAM and monitoring the battery condition were removed and the integrated NVSRAM replaced the SRAM
without changing the printed circuit board. I was considering reverting the design on a Tektronix oscilloscope but found a better solution in the form of an EEPROM backed up SRAM.
The data is also lost even while powered externally if the internal battery voltage drops far enough to disable the supervisory circuits.
that's not the point. the factory restore sequence was to be used to be able to re-shelve parts or destroy data. (sanitizing) critical data.
My point is that there is an additional failure mode where the data is
not lost but becomes inaccessible because of low voltage to the supervisor circuit from the internal battery. If you disassemble the package and change the battery or apply power, it then becomes possible to access the data again but only applying external power is not sufficient.
I prefer EEPROMs.
these things were around way before eeproms became avaialble. 2816 are very expensive.. plus they need a dedicated wipe operation before a byte can be written and they are slow to erase and have limited cycles.
only later when serial eeproms were made did they become more robust.
They did not call them that but what were effectively serial EEPROMs go back to at least the mid or late 1980s. General Instruments was making them for their TV tuners in the form of the ER1400 1400 Bit Serial Electrically Alterable Read Only Memory and Tektronix used them for a while before replacing them with battery backed up SRAM which they later replaced with integrated NVSRAM without changing the circuit boards. There was some question about the reliability of the ER1400 and it was difficult to use but it was fully specified and in retrospect it looks pretty good compared to the 100% failure rate of its NVSRAM contemporaries.
I agree that EEPROM was problematical then. Of all of the solutions at the time, I would have gone with discrete battery backed up SRAM unless there was some other reason to use a parallel EEPROM and that is still my preference unless a modern serial EEPROM is more appropriate. I have never found the needed write protect circuits to be challenging. I have had too much hardware fail do to SRAM with embedded lithium batteries to ever trust them again.