DS17885/DS17887

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EXTENDED FUNCTIONS

The extended functions provided by the DS17885/DS17887 that are new to the RAMified RTC family

are accessed by a software-controlled bank-switching scheme, as illustrated in Figure 4. In bank 0, the

clock/calendar registers and 50 bytes of user RAM are in the same locations as for the DS1287. As a

result, existing routines implemented within BIOS, DOS, or application software packages can gain

access to the DS17885/DS17887 clock registers with no changes. Also in bank 0, an extra 64 bytes of

RAM are provided at addresses just above the original locations for a total of 114 directly addressable

bytes of user RAM.

When bank 1 is selected, the clock/calendar registers and the original 50 bytes of user RAM still appear

as bank 0. However, the Dallas registers that provide control and status for the extended functions are

accessed in place of the additional 64 bytes of user RAM. The major extended functions controlled by the

Dallas registers are listed below:

§ 64-bit Silicon Serial Number

§ Century Counter

§ RTC Write Counter

§ Date Alarm

§ Auxiliary Battery Control/Status

§ Wake-Up

§ Kickstart

§ RAM Clear Control/Status

§ 4kB Extended RAM Access

The bank selection is controlled by the state of the DV0 bit in register A. To access bank 0 the DV0 bit

should be written to a 0. To access bank 1, DV0 should be written to a 1. Register locations designated as

reserved in the bank 1 map are reserved for future use by Dallas Semiconductor. Bits in these locations

cannot be written and return a 0 if read.

Silicon Serial Number

A unique 64-bit lasered serial number is located in bank 1, registers 40h–47h. This serial number is

divided into three parts. The first byte in register 40h contains a model number to identify the device type

of the DS17885/DS17887. Registers 41h–46h contain a unique binary number. Register 47h contains a

CRC byte used to validate the data in registers 40h–46h. All 8 bytes of the serial number are read-only

registers.

The DS17885/DS17887 is manufactured such that no two devices contain an identical number in

locations 41h–47h.

Century Counter

A register has been added in bank 1, location 48H, to keep track of centuries. The value is read in either

binary or BCD according to the setting of the DM bit.

RTC Write Counter

An 8-bit counter located in extended register bank 1, 5Eh, counts the number of times the RTC is written

qualifies it. This counter is a read-only register and rolls over after 256 RTC write pulses. This counter