I think you might be a bit confused about the SET bit. As you are probably aware, time is kept in a double buffered fashion in the DS12887. The purpose of the SET bit is to prevent the internal copy of the time from being written into the external, user-accessible copy of the time on each update cycle. When the SET bit is 0, the user-accessible copy of the time updates normally; when it is 1, the user copy of the time does not update, although interally, the '12887 continues to keep time.

Use of the SET bit is one way to guarantee that the you do not read a time value that is changing, potentially leading to the retreival of incorrect data. Similarly, it could also be used in setting of the time. However, you might find even more application-specific uses for it.



Erik is right; you need to clear up some fundamental issues relating to the connection of peripherals to the 8052's memory bus. Once you have that taken care of, you can work more with the '12887.

Also, I wrote extensively on the '12887 just a few days ago; you might want to take a look at it. If I recall correctly, setting the SET bit with my driver would be done something like this:

RTC_OPT_T RTC_opts;

RTC_opts = get_RTC_opts();
BIT_RECONF(RTC_opts, SET_MASK, SET_MASK);
set_RTC_opts(RTC_opts);

Then, if you later wanted to clear the SET bit, assuming that variable is still in scope and the contents of RTC_opts is still accurate:

BIT_RECONF(RTC_opts, SET_MASK, ~SET_MASK);
set_RTC_opts(RTC_opts);

If you find yourself doing this a lot, you might want to add a function to the driver that does everything necessary to set/clear the SET bit so as to avoid the needing several lines of RTC manipulation code in the higher-level calling code as well as reduce the overhead associated with the number of function calls. Code size was a greater concern when I wrote this driver, and thus, the interface is not very rich.

--Sasha Jevtic