The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Clocks and Calendars
Figure 10-1. Pinout of the DS1286 Watchdog Timekeeper.
The DS1286 contains a series of registers that store time, date, alarm, and configuration
information. You can read the current time and date from the DS1286 in hundredths of
seconds, seconds, minutes, hours, day of the week, date of the month, and year. Months of
different lengths and even leap years are handled automatically. Clock accuracy is better
than ±1 minute per month at 25 degrees Celsius.
Figure 10-2 shows the pin connections for a DS1286 in a BASIC-52 system. To accommodate
its crystal and power source, the DS1286 a 28-pin encapsulated DIP, just like the one
used by Dallas’ NVRAMs.
The pinout and wiring are similar to that for static RAM. The chip’s access time is 150
nanoseconds, which is well within the 8052-BASIC’s timing requirements. The eight data
lines (DQ0-DQ7) connect to the data bus. The chip has just six address inputs (A0-A5), which
are all it needs to access its 64 bytes. The clock/calendar uses 14 bytes, and 50 bytes of
nonvolatile RAM are available for any use. The chip is shown addressed at A000h, but you
can use any unused chip-enable. The chip’s WE and OE inputs are driven by RDANY and
WRITE.
The DS1286 has two interrupt outputs, INTA and INTB. You can program one of these to
toggle or pulse whenever the time and/or day match stored values. The other can generate
an interrupt if the DS1286’s watchdog register isn’t accessed periodically. You can use this
feature to automatically reset a system if a program crash causes the program to stop
The Microcontroller Idea Book 175