Dallas Semiconductor DS1687, DS1685, DS12887, DS12885 Datasheet

Application Note 521

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lacement for the MC146818

OVERVIEW

The Motorola MC146818 is no longer in production. There is no direct replacement for this part.
However, systems that have been designed to use the MC146818 can be modified to operate with the
Dallas Semiconductor DS12885, DS12887, DS1685, or DS1687. This application note discusses the
differences between these parts and potential issues in converting from the MC146818 to the Dallas
Semiconductor Real Time Clocks.

DESCRIPTION

The MC146818 provided clock and calendar functions with 50 bytes of user RAM memory, and was
designed to work with either the Motorola or Intel processor timing. These features could be made
nonvolatile by providing a backup battery on the board to maintain the VDD supply. For timekeeping, the
MC146818 required a number of discrete components on the PCB along with a crystal to provide an input
frequency. The input frequencies could be 32.768 kHz, 1.048576 MHz, or 4.194304 MHz.

The DS12885 is designed to provide the same timekeeping functions with a total of 114 bytes of user
NV RAM. To reduce the part count on the PCB, the DS12885 will accept direct connection of a 32.768
kHz crystal with no additional timing components required. The DS12885 also has a separate input for a
backup battery with internal switching circuitry to avoid data corruption during a power fail condition.
The DS12885 is available in either 24-pin DIP or 28-pin PLCC packages.

The DS1685 includes an enhanced feature set including: century byte; 64-bit serial number; wake-up
alarm; kickstart; SMI recovery stack; 242 bytes of user NV RAM; and an auxiliary battery input. The
timekeeping algorithm in the DS1685 includes leap year compensation valid up to 2100. The DS1685 is
available in either 24-pin DIP or 28-pin PLCC packages.

The DS12887 and DS1687 are modules combining a RTC with a battery and crystal contained within the
package, so that no external parts are required. The DS12887 includes the DS12885 while the DS1687 is
built using the DS1685. The modules has the same foot print as the 600-mil DIP package, and can
therefore be placed on existing boards with a minimum of board level changes required.

If replacing the MC146818 using an existing 600-mil DIP footprint is desired, then the DS12887 or
DS1687 would be the preferred choice. This conversion will involve the least number of changes to the
board and will work without removing the existing timing circuitry from the board. Using the DS12885
or DS1685 will require replacing the timing circuitry with a 32.768 kHz crystal attached directly to the
crystal input pins. In order to make the DS12885 or DS1685 nonvolatile will also require the connection
of a 3-volt battery directly to pins that have other functions on the MC146818.

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APPLICATION NOTE 521

Replacing the MC146818

PIN CAPATIBILITY

There are some pin differences between the parts. Some of t hese may be addressed with soft ware, while
others may require PCB changes. Tables 1 gives the pin functions for the MC146818, DS12885,
DS1685, DS12887, and DS1687 in 24-pin, 600-mil DIP packages.

PIN NAMES - Table 1

Pin #

24-Pin

DIP

MC146818

DS12885

DS1685

DS12887

DS1687

1NCMOT
2OSC1X1 X1 NC NC

3OSC2X2 X2 NC NC
4 AD0 AD0 AD0 AD0 AD0
5 AD1 AD1 AD1 AD1 AD1
6 AD2 AD2 AD2 AD2 AD2
7 AD3 AD3 AD3 AD3 AD3
8 AD4 AD4 AD4 AD4 AD4

9 AD5 AD5 AD5 AD5 AD5
10 AD6 AD6 AD6 AD6 AD6
11 AD7 AD7 AD7 AD7 AD7
12 V

13
14 AS AS ALE AS ALE
15
16 NC Batt GND Batt GND NC NC
17 DS DS

18
19

20 CKFS V
21 CKOUT
22 PS NC V

23 SQW SQW SQW SQW SQW
24 V

SS

CE CS CS CS CS

R/

W R/W WR R/W WR

RESET RESET

IRQ IRQ IRQ IRQ IRQ

DD

GND GND GND GND

BAT

RCLR RCLR

V

CC

PWR

RD

KS RESET KS

V

BAT

BAUX

V

CC

MOT

DS

NC NC
NC
NC V

V

CC

PWR

RCLR

RD

BAUX

V

CC

FUNCTIONAL DIFFERENCES

Table 2 summarizes the functional changes by pin for the DS12885 and DS12887. Table 3 has the
changes for the DS1685 and DS1687. These changes will need to be comprehended and the appropriate
modifications made to the board in replacing the MC146818 with any of the Dallas Semiconductor parts.

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