M48T86 datasheet(14/36 Pages) STMICROELECTRONICS

14 / 36 page

Clock operations

M48T86

14/36

Clock operations

Address map

The address map of the M48T86 is shown in Figure 8. It consists of 114 bytes of user RAM,

10 bytes of RAM that contain the RTC time, calendar and alarm data, and 4 bytes which are

used for control and status. All bytes can be read or written to except for the following:

Registers C & D are “Read only.”

Bit 7 of Register A is “Read only.”

The contents of the four Registers A, B, C, and D are described in the “Registers” section.

Time, calendar, and alarm locations

The time and calendar information is obtained by reading the appropriate memory bytes.

The time, calendar, and alarm registers are set or initialized by writing the appropriate RAM

bytes. The contents of the time, calendar, and alarm bytes can be either Binary or Binary-

Coded Decimal (BCD) format. Before writing the internal time, calendar, and alarm register,

the SET Bit (Register B; Bit 7) should be written to a logic '1.' This will prevent updates from

occurring while access is being attempted. In addition to writing the time, calendar, and

alarm registers in a selected format (binary or BCD), the Data Mode (DM) Bit (Register B;

Bit 2), must be set to the appropriate logic level ('1' signifies binary data; '0' signifies Binary

Coded Decimal (BCD data). All time, calendar, and alarm bytes must use the same data

mode. The SET Bit should be cleared after the Data Mode Bit has been written to allow the

Real Time Clock to update the time and calendar bytes. Once initialized, the Real Time

Clock makes all updates in the selected mode. The data mode cannot be changed without

reinitializing the ten data bytes. Table3 on page15 shows the binary and BCD formats of

the time, calendar, and alarm locations. The 24/12 Bit (Register B; Bit 1) cannot be changed

without reinitializing the hour locations. When the 12-hour format is selected, a logic '1' in

the high order bit of the hours byte represents PM. The time, calendar, and alarm bytes are

always accessible because they are double-buffered. Once per second the ten bytes are

advanced by one second and checked for an alarm condition. If a READ of the time and

calendar data occurs during an update, a problem exists where data such as seconds,

minutes, or hours may not correlate. However, the probability of reading incorrect time and

calendar data is low. Methods of avoiding possible incorrect time and calendar READs are

reviewed later in this text.

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