What is a "Phantom Clock" RTC, like the Maxim DS1315 and how is it different to "regular" RTCs?

Question

I am looking for some RTC on the Maxim website and noticed a few rather expensive chips that are labeled as "NV SRAM with Phantom Clock" and there is a dedicated "Phantom Time Chip", the DS1315. The Interface is just called "Phantom Clock".

After a short look at the datasheet, nothing caught my eye that would indicate a difference to a regular RTC.

Does anyone know what they are and makes these special and expensive?

Answer 1

The normal RTC chips have fixed addressing and are normally separate structures within a solution. A Phantom RTC, works in the same address space as an existing SRAM chip, and has an on board control that allows either normal SRAM R/W or the special serial location R/W required to get/set the clock data.

You would only use a phantom RTC device where you had severe MCU configuration constraints.