About the Dallas family (not the Ewings
) :
The origin are the Motorola MC146818 which will be very popular when started using in the IBM/AT.
It is have a total 64 bytes, 10 are the time/date, 4 control registers, 50 free for the user (PC store the Setup variables here).
Then many company produce compatible chips, Dallas Semiconductor company called their type as DS1285.
Then come the idea: why needed lot of additional components for the RTC chip? Then bend up some pins of the DS1285, and put to the IC top the crystal oscillator and lithium battery. The complete module called as DS1287. For use this in a system, only address decoding logic needed.
For the future types, also the ...85 ending the normal IC, and ...87 the complete module.
Another letter used, A at ending, for example DS1287A, this means it is have a RCLR input pin, which is clear the user memory bytes, but don't the time/date. This is the CMOS Clear jumper in PC motherboards.
At the next type, the memory size doubled, 128 totaly bytes, 114 free for user. These called as DS12885/DS12887/DS12885A/DS12887A.
Them coming the Y2K panic and Dallas make a hw hack for bad sw
this serie got a C letter...
The PC BIOS store the century byte at 32h address. DS12C885/DS12C887/DS12C885A/DS12C887A also use this byte as century, and load 20 to this location after 99/12/31 23:59:59... probably this function only interested at this moment
the datasheet don't talk about it what will happen after 2099/12/31...
The next secret in the naming the + signal, for example DS12C887A+
This is not a any interesting function, just mean ROHS compatible.
Latest serie the R series.
DS12R885 can charge rechargeable battery. But the DS12R887 only exist in a BGA version
In the DIP version use same battery as the older series, the charge circuit disabled, then this type called as DS12CR887... ...also a C letter, but different than the previous C, crazy naming
This serie have a RCLR pin at default, then no more ...A types.
